Recent research advances on simulation modeling of temperature distribution in microwave ablation of lung tumors
Recent research advances on simulation modeling of temperature distribution in microwave ablation...
Liu, Ju; Gao, Hongjian; Wang, Jinying; He, Yuezheng; Lu, Xinyi; Cheng, Zhigang; Wu, Shuicai
2023-12-31 00:00:00
COMPUTER ASSISTED SURGERY 2023, VOL. 28, NO. 1, 2195078 https://doi.org/10.1080/24699322.2023.2195078 RESEARCH ARTICLE Recent research advances on simulation modeling of temperature distribution in microwave ablation of lung tumors a a a a a b a Ju Liu , Hongjian Gao , Jinying Wang , Yuezheng He , Xinyi Lu , Zhigang Cheng and Shuicai Wu a b Faculty of Environmental and Life Sciences, Beijing University of Technology, Beijing, China; Department of Interventional Ultrasound, Chinese PLA General Hospital, Beijing, China KEYWORDS ABSTRACT Lung tumor; microwave Lung tumor is the first malignant tumor with the highest mortality, but only no more than one- ablation; temperature third of patients can be treated by surgical resection. Microwave ablation (MWA) has become a distribution; simulation new adjuvant therapeutic mean for lung tumors because of its low trauma, short treatment modeling time, large ablation volume and wide application range. However, the treatment parameters of MWA, such as input power and ablation time, still depend on the doctors’ experience, which leads to the ineffectiveness of MWA. Therefore, the accurate modeling of temperature distribu- tion of lung tumor MWA has become a significant technical problem to be solved. Recent research was devoted to personalized characterization of lung tumor parameters, finite element analysis of temperature distribution in MWA and accurate ablation effect evaluation. In this paper, a review of the recently obtained results and data will be presented and discussed. 1. Introduction is easily affected by the heat sink effect of peripheral blood vessels [8,9] and tissue carbonization [10,11], and Lung tumor is one of the most serious threats to the electric field distribution is not easy to be uniformly human health. In 2020, primary lung tumor is the controlled, resulting in incomplete ablation [12]. second most common malignancy worldwide with Furthermore, it is seen from the past researches that approximately 2.2 million new cases and 1.8 million the RFA can be employed only if the tumor size is less deaths [1,2]. Surgical resection is still the main method than 3 mm. In some cases, the skin burning is also a for the treatment of lung tumors. But due to the lack of major disadvantage of the RFA [10,13]. MWA is not eas- typical clinical symptoms of lung tumors, many patients ily affected by heat sink effect [14,15] and has the have reached advanced stage at the time of diagnosis advantages of minimal trauma, good tolerance, repeat- and are unable to undergo surgical resection [3]. ability, large ablation area, fast heating speed and less Some adjuvant treatments are gradually emerging damage to the surrounding normal tissue, so it has for the treatment of lung tumors, including radiother- attracted more and more attention in the clinical treat- apy, chemotherapy and thermal ablation. Imaged- ment of lung tumors [16,17]. guided thermal ablation techniques, including radiofre- At the same time, MWA antenna can achieve quency ablation (RFA), microwave ablation (MWA) and deeper penetration [18], higher specific absorption cryoablation have been widely used in the treatment of rate (SAR), faster damage rate in high impedance tis- inoperable lung tumor [4]. Other techniques, including sue (such as lung) and can reach a higher temperature laser ablation and irreversible electroporation (IRE), are in a short time, which makes MWA of lung tumors not widely used in lung ablation due to lack of clinical more efficient [19]. Therefore, MWA has become a bet- data [5]. Compared with other thermal ablation meth- ter way to treat unresectable lung tumors, and its ods, cryoablation has the disadvantages of long thera- peutic period, increased bleeding risk and complex ablation process is shown in Figure 1. The doctor preparation process. It is still controversial regarding inserts the microwave antenna into the lung tumor by cryoablation’s superiority over RFA and MWA [6,7]. RFA means of some imaging equipment and sets the CONTACT Hongjian Gao gaohongjian@bjut.edu.cn; Shuicai Wu wushuicai@bjut.edu.cn Faculty of Environmental and Life Sciences, Beijing University of Technology, Pingleyuan No. 100, Chaoyang District, Beijing 100124, China 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 J. LIU ET AL. Figure 1. Schematic diagram of MWA treatment of lung tumor. microwave power and ablation time according to the literature search (up to January 2023). A total of 1393 size and shape of the tumor. The high-frequency elec- articles were collected, in which 890 were selected using the query on Scopus and 503 on PubMed. tromagnetic field emitted by the antenna produces Following the elimination of duplicates, the relevant local high temperature to cause coagulative necrosis experimental studies were identified. In order to col- of tumor cells. After the ablation, the imaging equip- lect the most relevant articles, we identified keywords ment is used to detect whether the conformal treat- and divided them into four areas of interest: lung ment is achieved. tumor, microwave ablation, temperature distribution However, it will take time before MWA treatment and simulation modeling. The time limit and language based on simulation modeling to mature enough to restriction were not used. To try to expand our search, be accepted for clinical applications. In particular, the references lists of the retrieved articles were also expected temperature distributions cannot be accur- screened to identify additional studies [20]. ately predicted and thus the most suitable MWA treat- ment planning cannot be set down. Only relying on doctors’ experience may require multiple ablations 2.2. Study selection and aggravate pains of the patient. Temperature simu- The articles founded by our search were filtered. In lation has become a very important tool for predicting this systematic review, the main screening criteria ablation results, which can assist doctors to input were the original papers reporting the simulation appropriate treatment parameters to achieve con- model and ex vivo experiments of lung tumor MWA. formal ablation. This paper intends to provide a review The main inclusion criteria were: about the modeling of MWA temperature distribution of lung tumors in recent years, including exact charac- 1. Timeliness. The references should be based on terization of individual tissue parameters, simulation recent papers. modeling technology of lung tumor MWA, experimen- 2. Relevance. The references should be closely tal verification and clinical evaluation of MWA simula- related to the review topic. tion accuracy, uncertainties and technical challenges 3. Representativeness. The references should include in the simulation of MWA temperature distribution of representative literatures at home and abroad and lung tumors. should not be pointlessly piled up in large numbers. 4. Reliability. The references must be real and reli- 2. Methodology able, complete description items, easy to search 2.1. Search strategy and verify, avoid wrong citations. Two investigators independently examined Scopus 5. Authority. Authority is reflected in the author of and PubMed online database for a comprehensive the reference and published journal. Citing the COMPUTER ASSISTED SURGERY 3 work of authoritative authors will make the ex vivo experiments (condition) are summarized, as research more valuable. shown in Table 1. Exclusion criteria were: 3.2. Dynamic tissue parameters In the MWA process, the increasing temperature of a. articles not within the field of interest of this the lung tissue and tumor will have an impact on the review; thermophysical and electrical parameters. The changes b. articles that were too long ago; of these parameters will significantly affect the model- c. research focusing on the details of clinical ing accuracy. Therefore, it is still a challenging work to surgery; precisely derive dynamic tissue parameters. Gabriel d. articles that were not in the English language; e. articles that could not be found for their full text. et al. [42] calculated the permittivity and conductivity of lung tissue under the action of 2450 MHz electro- Two researchers independently reviewed the titles magnetic wave using fourth-order Cole–Cole model. The model is described as follows: and abstracts of the retrieved articles by the above cri- teria. The four authors then independently reviewed De r n s 0 00 e ðxÞ¼ e je ¼ e þ j (1) the full-text of the remaining articles to determine r 1 r r 1a xe 1 þðjxs Þ 0 n¼1 their final inclusion. where e is the dielectric increment of living tissue at the nth relaxation time, e and e are the static per- 0 1 3. Characterization of tissue parameters of mittivity and the permittivity when the frequency lung parenchyma and lung tumors tends to infinity, respectively, and s is the relaxation time. The tissue electrical properties (electrical con- Tehrani et al. [43] proposed an extended model of ductivity/r, permittivity/e) directly affect the absorp- tissue electrical conductivity and permittivity varying tion of electromagnetic energy that produces heat, with temperature during MWA and pointed out that while thermal properties (thermal conductivity/k, the temperature dependence of the thermal conduct- density/q, specific heat capacity/c) and perfusion ivity (k) and blood perfusion rate (x ) of biological tis- (blood perfusion/x) affect heat transfer in the tissue. sue is based on linear equations, which provided a Therefore, the setting of biological tissue parameters reference for scholars to perform simulation of MWA is of critical importance in the simulation modeling of temperature distribution based on dynamic biological MWA [21]. The precise characterization of tissue tissue characteristic parameters. These dynamic tissue parameters is conducive to improve the simulation parameters included thermal and dielectric properties accuracy. Many researchers have previously demon- are conducive to derive more accurate simulation strated the impact of uncertainty in tissue properties results and are described as follows: and their temperature dependency on MWA model outcomes. They have also carried out in-depth explor- eðTÞ¼ s 1 (2) 1 þ exp ðs s TÞ 2 3 ation on the parameters setting of the lung tissue. Furthermore, the effect of respiratory movement on rðTÞ¼ r 1 (3) 1 þ exp ðr r TÞ 2 3 tissue characteristics is also taken into account to x ¼ 0:000021T þ 0:0035 (4) increase the accuracy of simulation results. b kðTÞ¼ k þ DkðT T Þ (5) 0 0 where T is the temperature of lung tissue, s , s , s 1 2 3 3.1. Constant tissue parameters are constants of permittivity(e), r , r , r are constants 1 2 3 In order to simplify the calculation, some research of electrical conductivity(r), k , Dk and T are the 0 0 teams set biological tissue parameters to fixed values base-line thermal conductivity, the change in k due to in the simulation process. Instead of tumor tissue, temperature and the baseline temperature at which lung parameters are often used in lung MWA simula- k , respectively. tion. However, the dielectric properties of malignant Singh et al. [37] considered that the electrical con- tissues were 10–20% larger than those of normal tis- ductivity of tissue increased linearly with temperature sues [22]. The electrical and thermal properties of lung (2% per C). A non-linear piecewise decreasing model tissue (tissue type) involved in model simulation and for blood perfusion was proposed. They are expressed 4 J. LIU ET AL. Table 1. Summary of characteristic parameters of the lung tissue. Threshold Thermal conductivity Density Specific heat Blood perfusion Electrical Permittivity temperature 3 1 Num. Refs. k (W/mK) q (kg/m ) capacity c (J/kgK) x(S ) conductivity r (S/m) e T ( C) Tissue type Condition 1 Gao, 2019 [16] 0.2 480 –– 0.423 33 60 Porcine lung Ex vivo 2 Radmilovic, 2021 0.39 385 3886 0.0036 0.804 20.5 50 Lung Simulation (average) [23–25] 3 Yang, 2020-inflated 0.16 240 2500 0.00361 0.80 20.47 – Lung Simulation Mediate 0.18 360 2500 0.00361 1.24 34.42 – Lung Simulation Deflated [26,27] 0.20 480 2500 0.00361 1.68 48.38 – Lung Simulation 4 Liu, 2019-flooded 0.52 1030 3886 0.00709 –– – Lung Simulation Higher inflated 0.39 400 3886 0.00708 –– – Lung Simulation Lower inflated [28–33] 0.39 700 3886 0.00709 –– – Lung Simulation a a a a 5 Avishek, 2021 [34] 0.33 –– 0.0026 0.804 20.5 – Lung Simulation b b b b 0.39 –– 0.0155 1.69 43 – Lung Simulation c c c c 0.53 –– 0.0674 2.43 57.2 – Lung Simulation 6 Keangin, 2019 [35] –– – – – 30.01 – Porcine lung Ex vivo 7 Brace, 2009(inflated) [13] 0.302 260 2500 0.00087 0.804 20.5 – Porcine lung Ex vivo 8 Hasgall, 2022-deflated 0.39 1050 3886 0.00702 1.68 48.4 – Lung – Inflated [36] 0.39 394 3886 0.00263 0.804 20.5 – Lung – 9 Singh, 2018 [37,38] 0.39 394 3886 0.00263 0.122 –– Lung Simulation b a a a a 10 Singh, 2019 [39] 0.39 394 3886 0.00263 0.804 20.5 – Lung Simulation 11 Hu, 2022-deflated 0.39 480 3886 – 0.804 33 60 Porcine lung Ex vivo Inflated [12] 0.16 394 2500 – 0.306 20.47 60 Porcine lung Ex vivo 12 Sebek, 2021 0.39 –– – 1.47 40.5 – Lung Simulation (average) [40,41] Note: a: low; b: medium; c: high; ‘–’ indicates that there is no reference to this term in the text. COMPUTER ASSISTED SURGERY 5 as follows: parameters were used to simulate the conditions of blocking ventilation, and the sinusoidal function was rðTÞ¼ r½ 1 þ 0:02ðT T Þ (6) 0 b used to express the physical parameters under period- > x XðtÞ 0 b, 0 hi ically normal ventilation conditions. The results x ¼ b x 1 þ 25XðtÞ 260XðtÞ 0 XðtÞ 0:1 b, 0 > showed that a larger ablation area can be produced x exp½ XðtÞ XðtÞ > 0:1 b, 0 using the dynamic physical parameters, but the (7) reverse heating effect of microwave antenna will also be more significant, thus resulting in greater damage where T ¼ 37 C, r and x are the baseline elec- b 0 b, 0 to surrounding normal tissue. trical conductivity and baseline blood perfusion rate In order to propose personalized treatments for dif- for the lung tissue, respectively, and XðtÞ is the ferent patients, Tacprasartsit et al. [48] supplemented induced thermal damage. They also pointed out that the radiomics features on the traditional biological in Equation (5), Dk ¼ 0:0013, T ¼ 37 C: Choi et al. heat transfer model. Nine regression convolution [44] considered that k ¼ 0:396, Dk ¼ 0:239: neural networks (CNN) specially trained for each fea- Bianchi et al. [45] summarized the specific heat of ture are used to evaluate nine different characteristics lung adenocarcinoma cells at different temperatures, i.e. 3640 J/(kgK) at 37 C, 3880 J/(kgK) at 43 C, of the tumor. The results are used as radiological fea- tures, and then the physical characteristics of the 3850 J/(kgK) at 50 C and 3790 J/(kgK) at 60 C. patients’ specific tumor and lung tissue are obtained Bonello et al. [46] studied the temperature depend- according to a certain transformation model. In this ence of dielectric properties of ex vivo sheep lung tis- sue in the temperature range of 25 Cto90 C. It was way, personalized treatment parameters can be found that the permittivity and electrical conductivity obtained, but this technique requires the measure- decreased linearly with the increase of temperature, ment knowledge of tissue characteristics related to medical imaging data. It is still challenging to deter- which is contrary to the study of Singh et al. [37]. And the temperature coefficients under 2.45 GHz are 0.87 mine the relationship between radiological features and 1.21, respectively. Bianchi et al. [47] studied the and tissue parameters. characterization of the temperature dependence of In the selection of tissue parameters, the simulation the thermal properties of lung tissue from room tem- model using the tissue characteristics under exhalation perature (21 C) to over 90 C. They found that the condition was more accurate. Some studies have also thermal diffusivity (a) and k of the lung tissue found that the ablation area obtained by using increased exponentially with temperature, while the dynamic tissue parameters is larger, but there is a volumetric heat capacity (c ) changed less noticeable, problem of obvious backward heating effect. To date, which was described by a linear equation: the static tissue parameters which do not change with temperature are widely used in the study, and the aðTÞ¼ 0:1815 þ 0:002224 exp ð0:0739TÞ (8) dynamic tissue parameters of lung tissue are still in kðTÞ¼ 0:2852 þ 0:001288 exp ð0:08196TÞ (9) the exploratory stage. c ðTÞ¼ 1:526 0:003253T (10) Furthermore, since the characteristic parameters of 4. Simulation modeling technology of lung lung varied with the respiratory process, Radmilovi tumor MWA et al. [23] studied the change trend of electrical con- ductivity and permittivity of inspiratory and expiratory Due to the lack of typical clinical data, computer simu- lungs at different microwave frequencies. It was found lation has become an important tool for predicting that the relative permittivity decreased while the elec- the temperature distribution in MWA procedure. The trical conductivity increased with the increase of fre- workflow diagram is shown in Figure 2, and some of quency. The electrical conductivity changed a little at the key technologies are summarized in this section. frequencies of lower than 1 GHz. But at higher fre- quencies, the increase of electrical conductivity is rela- 4.1. Geometric modeling tively large. The relative permittivity and electrical conductivity of the lung during deflation are about The geometric modeling of MWA includes three parts: twice as large as those when inflated. Yang et al. [26] microwave antenna modeling, normal lung tissue studied the temperature distribution of lung tissue modeling and lung tumor modeling. The geometric during MWA under blocking ventilation and normal models conforming to the true anatomical structure ventilation, respectively. The constant physical will produce more real simulation results. 6 J. LIU ET AL. Figure 2. Simulation process of lung tumor MWA. Figure 3. Lung tissue model. (a) Cylindrical lung tissue model. (b) Real lung tissue model. energy transmission loss. Different from the needle- 4.1.1. Construction of microwave antenna model Depending on therapeutic organ and the size and shaped rigid microwave antennas used in traditional shape of the tumor, a specific microwave antenna is percutaneous ablation, Pfannenstiel et al. [18] devel- pierced directly to the tumor site for emitting micro- oped a new type of flexible microwave transmitter, wave energy. Microwave antenna is usually composed which was transmitted to targeted tumor through of inner conductor, insulating medium, outer con- bronchoscope. This equipment can enhance the accur- ductor and catheter. Phairoh et al. [27] designed a tip- acy of transmitter placement, improve ablation effect, open coaxial antenna with a microwave frequency of reduce the risk of complications such as pneumo- 2450 MHz to obtain spherical energy deposition. More thorax and treat targets that can’ t be reached by a researchers use ring-open coaxial antennas to provide common antenna. symmetrical ring-coil heating zones [49]. The micro- wave antenna made of coaxial cable is commonly 4.1.2. Construction of the lung tissue model Because it is usually assumed that the lung is isotropic used, and a 1 mm-wide slot is cut on the outer con- ductor to emit electromagnetic waves into the tissue. uniform tissue, an ideal cylindrical model is con- structed. Additionally, the electromagnetic energy At the same time, the antenna is encapsulated in the PTFE catheter to prevent the microwave antenna from emitted by the microwave antenna is an axisymmetric adhering to the dry ablation tissue. In addition, a cir- region, so the cylindrical model can be simplified to a culate water cooling system needs to be added inside rectangle model, which is solved in a two-dimensional the antenna to avoid unnecessary thermal damage to coordinate frame. A model of idealized lung tissue patients [16]. Habert et al. [49] used full antenna water combined with a microwave antenna is shown in cooling technology, choke coil design at the front end Figure 3(a). TM of the antenna, and Antiphase technology to pre- In order to construct a more realistic lung model, vent heat from spreading backward, and to reduce Yang et al. [26] divided respiratory process into ten the central temperature of the ablation zone and the stages. They reconstructed lung tissue models COMPUTER ASSISTED SURGERY 7 Figure 4. Lung tumor model. (a) Whole lung tissue-level model. (b) Traditional spherical tumor model. (c) Real tumor model based on CT slice. according to the respiratory stage, in which 0 repre- In the construction of geometric model, the simple cylindrical lung model is widely used, but the model sented the end of inspiration and 50% or 60% repre- sented the end of expiratory. The model of lung tissue involving human body structure (heart, trachea, blood vessels and so on) is few, which needs further investi- after reconstruction is shown in Figure 3(b). gation in the future. 4.1.3. Construction of the lung tumor model Many scholars assumed that the tumor was spherical 4.2. Calculation of electromagnetic energy in MWA simulation, in this case it can be simplified deposition into a two-dimensional model to simplify the calcula- 4.2.1. Electromagnetic wave conduction equation tion, as shown in Figure 4(b). In the simulation of lung tumor MWA, the electric field In order to obtain a more accurate tumor model, and magnetic field are time-varying TEM (transverse some researchers used CT slices of the tumor [50]to electromagnetic) waves. In two-dimensional axisym- reconstruct real lung tumor model, as shown in Figure metric cylindrical coordinates, the time-varying electric 4(c). The reconstructed lung tumor model was intro- field and magnetic field are described as follows [52]. duced into the Freeform tactile design system for Electric field: smooth processing to make simulation results more jðxtkzÞ accurate. Because there are trachea and bronchus in E ¼ e e (11) lung tissue, some tissue parameters such as thermal jðxtkzÞ H ¼ e e (12) conductivity, electrical conductivity and density will u rZ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi change with the gas volume in the tissue. Tian et al. ZP in [51] placed a bronchus around the microwave antenna n ¼ (13) p ln ðr =r Þ outer inner based on the traditional model and set its airflow to 3.0 L/min, which is the same as human respiratory where e is the longitudinal component of electric jðxtkzÞ rate. Liu et al. [28] took the structure of trachea into field, e is the traveling wave factor, e is the consideration to simulate the ablation process in a transverse component of distribution function of elec- more real and complex anatomical environment. And tric field, Z is the wave impedance, P is the MWA in five different tumor models were generated, mainly input power, r is the outer radius of dielectric, r outer inner including the following: tumors near the main airway, is the inner radius of dielectric, x ¼ 2pf is the angular tumors located in the deep lung, tumors attached to frequency of microwave, f is the frequency of micro- 2p the bronchial wall and tumors adjacent to but not wave, k ¼ is the propagation constant, k is the attached to the bronchial wall. wavelength and n is the integral constant. According 8 J. LIU ET AL. to Maxwell equation, the electromagnetic wave con- 4.3. Biological heat transfer technology duction in tissue is characterized by Helmholtz equa- 4.3.1. Tissue heat transfer equation tion: In the simulation of MWA temperature distribution, jr 1 the heat transfer in biological tissue is calculated by r ðe Þ r H l k H ¼ 0 (14) r u r u xe 0 biological heat transfer models. It is found that the temperature near the emission slot is the highest in where e is the relative permittivity of lung tissue; e is r 0 12 general. With the increase of distance, the influence of the vacuum permittivity (e ¼ 8:854 10 F/m); r the heat source becomes weaker and the rate of the is the electrical conductivity of lung tissue (S/m); H is temperature rise decreases. For the lung tissue, the the magnetic field intensity (A/m); l is the relative commonly used models include Pennes equation and permeability (l ¼ 1) and k is the free space wave porous media model [53,54]. Compared with the trad- number (rad/m). The specific absorption rate (SAR) of itional Pennes equation, the porous media model will lung tissue can be obtained by solving the distribution produce the results closer to the experimental data, of electromagnetic field in lung tissue: but at the same time, the solving process is more SAR ¼ j E j (15) complicated [54]. 2q In order to improve the accuracy of temperature 4.3.1.1. Pennes equation. distribution simulation of MWA, an improved SAR @T method with thermal conductivity term can also be qc ¼r ðkrTÞþ q c x ðT TÞþ Q þ Q b b b met ext @t used [52]. The calculated SAR is used as the heat (19) source term of the model for simulation. Compared where q is the density of lung tissue (kg/m ); q is the with the simulation result obtained by traditional elec- density of blood (kg/m ); c is the specific heat capacity tromagnetic coupling method, this SAR method sim- of lung tissue [J/(kgK)]; c is the specific heat capacity plifies simulation process, has better consistency with of blood [J/(kgK)]; T is the temperature of lung tissue the experimental data and improves simulation accur- ( C); T is the temperature of blood ( C); k is thermal acy. In general, the SAR near the emission slot is the conductivity [W/(mK)]; x is blood perfusion rate maximum [26]. However, the disadvantage of this (s ); Q is the heat generated by new metabolism met method is that the SAR is a fixed value at a certain (W/m ); Q is the heat generated by microwave gen- ext temperature and should be accurately measured. erator (W/m ). 4.2.2. Electromagnetic wave boundary condition 4.3.1.2. Heat transfer equation of porous media setting model. Aiming at the Helmholtz electromagnetic wave equa- tion, Selmi et al. [24] pointed out that the z axis was a @T qc ¼r ðkrTÞþ W q c ðT TÞ q c v rT oE b b b z b b symmetry axis: E ðt, r ¼ 0, zÞ and ðt, r ¼ 0, zÞ, and r @t or there was continuity of the tangential component of þ D ðk rTÞþ q b m the electrical field at the interface between the tissue (20) and the tumor: where k represents the apparent increase of thermal nðE E Þ¼ 0 (16) 2 1 conductivity in tissue caused by hemoperfusion in small vessels and v represents the total blood perfu- Avishek et al. [34] adopted the first-order scattering sion vector in the tissue. Other parameters are boundary equation to reduce the boundary reflection described above. and calculated the electric field. The construction of tissue heat transfer models is a n E ¼ 0 (17) key technology in MWA temperature distribution n ðr EÞ jkn ðE nÞ¼ 0 (18) simulation. Different heat transfer models have their The electric field distribution can be obtained by own advantages and disadvantages, as shown in solving Equation (14) with the boundary conditions Table 2. given by Equations (17) and (18). According to the Pennes equation is the most commonly used in the study, and it has many advantages [55]. But it is over- electric field derived from these equations, the tem- perature distribution can be obtained by solving the simplified and ignores many problems. Thus, it has corresponding biological heat transfer equation. been further improved by many researchers. In the COMPUTER ASSISTED SURGERY 9 Table 2. Comparison of advantages and disadvantages of heat transfer models. Heat transfer model Advantages Disadvantages Pennes equation The calculation process is simple, easy to analyze and It is assumed that the blood flow is uniform. The arterial widely used, and the additional term used to show blood is kept at constant temperature. Different that the perfusion heat transfer is linear in directions of blood flow are not considered. The temperature. arteriovenous convection is ignored. Porous media model The division of biological tissue into solid and liquid The process of solving solid tissue and liquid tissue is overcomes the defect of Pennes equation. complex. process of MWA, due to the rapid increase of tem- relatively closer to the experimental data [59]. At the perature, the water will evaporate quickly and the car- same time, the solution is relatively more complex. bonization will also occur. Therefore, some scholars 4.3.2. Heat transfer boundary condition setting have introduced the phase transition heat transfer In order to solve the biological heat transfer equation, analysis of tissue moisture when solving the heat it is necessary to set the boundary conditions. In gen- transfer equation of biological tissue [56]. Additionally, eral, the heat transfer boundary conditions are set as the direct contact between the liquid and the tissue follows: will produce the heat source term. Therefore, adding the liquid heat transfer equation to the tissue heat 1. Z axis (see Figure 4(b)) is a symmetry axis, transfer equation can achieve the goal of expanding expressed as shown in Equation (22). It describes ablation area. In order to build a more accurate simu- that the heat flux is zero. lation model, Truong et al. [57] combined Bill’s law 2. The insulation condition is suitable for the sur- with the Pennes biological heat transfer equation to rounding lung tissue. In other words, the heat flux analyze the heat transfer in biological tissue. Tucci through the surrounding wall is equal to zero, et al. [54] added the effect of tissue vaporization to expressed as shown in Equation (23) [34]. the Pennes equation, so that the term (qc)is expressed as follows: Q ¼ 0 (22) ðq c Þ 0 C < T < 99 C > l t h c x, t n ðkrTÞ¼ 0 (23) fg ðqcÞ ¼ 99 C < T < 100 C (21) > DT b, t where n is the unit vector normal to the boundary q c T > 100 C g g t (z ¼ 0or z ¼ 80mm r ¼ 30mm). where q and c are density and specific heat of tissue at temperature below 100 C (liquid phase), q and c 1. The outer boundary adopts the Dirichlet condition are density and specific heat of tissue at temperature of constant temperature (37 C) [28]. above 100 C (gas phase), h is the product of water fg latent heat of vaporization and water density at 100 C Equation (23) was further specified for each bound- and c is the water content inside the lung tissue. x, t ary by Selmi et al. [24]: Because the lung tissue is filled with air, and the @T @T @T traditional cylindrical lung model does not take into k j ¼ 0; k j ¼ 0; j ¼ 0 (24) z¼0mm z¼80mm r¼30mm @z @z @r account the effect of air, a porous media lung tissue model has been proposed. Coupled with the electro- They also supposed that the heat flux is continuous magnetic field of MWA, the flow field of air in lung tis- at the interface between the tissue and the tumor, i.e. sue and the thermal field of porous media, the k rT ¼ k rT (25) lung lung tumor tumor temperature distribution of lung tissue can be derived. Convective boundary conditions are applied to the The simulation results are closer to the in vitro experi- mental results. The ablation area produced by porous surface of external emitter to indicate water cooling: media model is 29% smaller and the maximum tem- n krT ¼ hðT TÞ (26) perature is 36% lower [54]. Wang et al. [58] also where n is the outer normal vector, the convective pointed out that the increase of porosity will result in 1 1 a more uniform temperature distribution, which may heat transfer coefficient h is 1000 Wm K and the cooling water temperature T is set to 15 C. The lead to more effective hyperthermia treatment. In a word, the Pennes equation is widely used, but value of h depends on the antenna. Its value and the it is oversimplified. The porous media model is T will be different in different studies. 1 10 J. LIU ET AL. 4.4. Evaluation technique of thermal coagulation The score of necrotic tissue is expressed by h , and zone of lung tumors its value is related to a : h ¼ 1 exp ðaÞ (29) Ablation coagulation zone is an important index to evaluate the thermal ablation effect. By comparing the where X indicates the degree of damage to the tissue, coagulation area with the size of tumor, we can know T indicates the reference temperature and t indicates whether the conformal coverage achieved. The com- the time that exceeds the reference time. The param- mon evaluation indexes of ablation coagulation zone eter R (¼8.314 J/mol/K) represents the universal gas include coagulation zone shape and coagulation zone constant, A represents the frequency factor, E repre- volume. sents the reaction energy barrier, DE represents the activation energy of irreversible damage reaction 4.4.1. Shape evaluation of coagulation zone and h represents the fraction of necrotic tissue. 39 1 The ablation coagulation zone will increase with the For the lung tissue, A ¼ 7:39 10 s and E ¼ 5 66 1 increase of ablation frequency and ablation power, 2:577 10 J=mol [37], or A ¼ 1:46 10 s and E ¼ 5 45 1 but beyond a certain range, only the longitudinal 4:428 10 J=mol [65], or A ¼ 1:61 10 s and E ¼ diameter of the coagulation zone will continue to 3:06 10 J=mol [40], respectively. In the FEM model, increase with the increase of ablation power, thus the induced thermal damage value X ¼ 1, corre- forming an ellipsoidal shape. In addition, some sponding to 63% probability of cell death (i.e. researchers proposed the sphericity index (SI) [49]to h ¼ 63%), has been used as a critical threshold for evaluate the coagulation zone shape. The sphericity of calculating coagulation volume. 1 corresponds to a perfect sphere, the sphericity less Liu et al. [28] used the equivalent heating minutes than 1 corresponds to a slender shape elongated at 43 C (EM ) to calculate the equivalent thermal 43 C along the antenna direction, and the sphericity greater dose distribution and then evaluated the MWA effects than 1 corresponds to a coagulation zone extending by adding time intervals of different temperature in the direction of orthogonal to the antenna. exposures, as follows: 0:5 T > 43 C ð43T Þ 4.4.2. Volume evaluation of coagulation zone EM ¼ R t , R ¼ (30) 43 C i i¼1 0:25 T < 43 C The clinical goal of MWA is to heat the tumor zone to a cytotoxic temperature, while maximally sparing non- where R is the proportional constant of cell mortality targeted tissue outside of this zone [60]. Therefore, it dependent on temperature and T is the average tem- th is very important to accurately evaluate the extent of perature ( C) of the i time interval (minutes). This thermal coagulation zone. The volume of thermal concept of thermal dose originates from the Arrhenius coagulation zone depends on ablation power and model of cell damage accumulation, which is a mature time. The volume is usually evaluated by isotherm index to predict the thermal effect of tissue or the threshold (IT), Arrhenius model and thermal equivalent degree of injury, and is widely used in thermotherapy dose (TID) [61]. and thermal ablation monitoring and evaluation. A With regard to IT method, the tissue is considered thermal dose of 120–240 min at 43 C usually causes to be coagulative when the temperature exceeds a considerable tissue necrosis, but the sensitivity varies certain threshold. The usual thresholds involve 60 C between tissue types [66]. [62], 54 C[63,64] and 50 C[24]. The advantages and disadvantages of these three Many researchers [26,48] also calculate the thermal thermal damage assessment techniques are shown in damage volume based on Arrhenius model [54], which Table 3. is expressed as follows: In view of the fact that the Arrhenius damage s model only considers two states of biological tissue (alive cells and dead cells), Tehrani et al. [43] proposed XðsÞ¼ Ae dt (27) RTðtÞ a three-state cell death model to calculate the size of ablation coagulation zone. That is, the cell death pro- In addition, a is used to indicate the degree of tis- cess under the temperature gradient is described by sue damage: coupling the ordinary differential equation, and the tissue is divided into three states: alive cells, vulner- da DE ¼ A exp (28) able cells and dead cells. The expression is as follows: dt RT COMPUTER ASSISTED SURGERY 11 Table 3. Comparison of advantages and disadvantages of thermal damage assessment techniques. Thermal damage assessment model Advantages Disadvantages Isotherm threshold (IT) Straightforward and intuitive The correlation between target type and heating duration is not taken into account. Arrhenius model Simple, direct and widely used Only two states of biological tissue are considered, and the transition between these states is single and irreversible. Thermal equivalent dose (TID) Widely used in thermal damage models at lower Not suitable for thermal ablation processes with temperatures higher temperatures above 50 C. f included the longitudinal diameter and transverse A V!D (31) b diameter of coagulation zone, as well as the slot tem- perature of microwave antenna. The experiment for where A, V and D are alive cells, vulnerable cells and each combination of power and time is repeated five dead cells, respectively. The positive rate constant k times. Keangin et al. [35] applied microwave power indicates the transition from alive to vulnerable state, levels of 60 W, 80 W and 100 W to porcine lung tissue while the reverse rate constant k represents the self- for 360 s. The ablation diameter was measured and repair process from vulnerable state to fully functional the ablation volume was calculated. resurrection state. Once beyond the critical point, the Table 4 shows the ablation results of different treat- cell enters a state of death (D), after which the process ment parameter combinations (microwave power/P is irreversible. Cell survival rate (V þ D) was used to and ablation time/t) during lung tissue (tissue type) determine the size of lesion. The ablation results MWA in simulation and ex vivo experiments (condi- showed that the results of three-state cell death tion). The evaluation indexes of the thermal damage model were closer to the experimental data. zone include the long diameter (L) and short diameter In the evaluation of coagulation zone, IT and (s) along the cross section of the ablation antenna, the Arrhenius model are widely used in the study because volume (V) of the coagulation zone, the sphericity (SI), of its simplicity, intuition and wide range of applica- the maximum temperature (T) reached at the end of tion. The coagulation zone after lung tumor MWA is the ablation and the coagulation threshold (T ). generally concentrated near the tip of the antenna and the emission slot, which is oval on the whole [67]. And the coagulation zone of expiratory group is larger 5.2. Clinical evaluation than that of inspiratory group [12]. Therefore, in clin- The simplest geometric modeling is usually used in ical treatment, one-lung ventilation [26] (meaning that the model simulation, but in clinic, the lung tissue patients only use the lung on the non-operative side contains a substantial amount of air and blood vessels, for ventilation) may be used for ablation surgery. and is adjacent to heart, so the ablation effect is affected by many factors. For the lung tumor with a 5. Experimental verification and clinical diameter of 2–8.5 cm (average 4 cm), clinicians usually evaluation of simulation accuracy of lung choose the ablation power of 60 to 70 W and the abla- tumor temperature distribution tion time of 6–10 min, which are slightly higher than the ablation power and ablation time used in the ex 5.1. Experimental verification of simulation model vivo experiments [16]. In the MWA procedure, the choice of treatment In the thermal ablation surgery, it is necessary to parameters such as input power and duration is crit- combine image guidance technology for real-time ical to ensuring the success of the procedure, because monitoring to achieve conformal ablation. In clinical the improper parameter usage may lead to incom- application, there are mainly three kinds of image plete ablation or excessive ablation. Therefore, some guidance techniques used in thermal ablation: ultra- researchers have carried out different explorations, sound, computed tomography (CT) and magnetic res- including using simulation technology to simulate the onance imaging (MRI). Ultrasound-guided and CT- ablation results of different parameter combinations guided ablation is performed in most cases, while and verifying these results by experiments. The aim is MRI-guided ablation is rare. Shen et al. [70] evaluated to provide some guidance for clinical application. Gao the safety, practicability and effectiveness of MRI- et al. [16] studied the coagulation zones at three guided MWA in patients with the lung malignant power levels (30 W, 40 W, 50 W) under different heat- tumor. The results showed that compared with CT, ing time (2 min, 4 min, 6 min). The measured indexes MRI was easier to identify whether ablation was 12 J. LIU ET AL. Table 4. Ablation results for different power and time combinations. Transverse Longitudinal Maximum Power Time diameter diameter Volume Sphericity temperature Threshold Num. Refs P (W) t (s) s (mm) L (mm) V (cm ) SI T ( C) temperature T ( C) Tissue type Condition 1 Gao 30/40/50 120 13.4/19.1/22.0 16.3/22.9/28.5 –– 67.2/73.6/78.7 60 Porcine lung Ex vivo 2019 [16] 30/40/50 240 –– 71.5/81.7/99.3 60 Ex vivo 30/40/50 360 23.0/36.8/39.9 30.2/41.2/51.4 –– 75.6/85.3/106.7 60 Porcine lung Ex vivo 30/40/50 360 29.5/44.0/48.9 41.1/55.7/66.3 –– 183.2/231.9/280.7 60 Porcine lung Simulation 40.5/46.9/51.9 56.3/62.1/66.7 Lung 2 Pfannenstiel 60/80 300 11.5/14 31.5/24.5 – 0.37/0.65 –– Porcine lung Ex vivo 2017 [18] 60 300 16 37.75 –– – – Canine lung In vivo 3 Habert 50 600 – 36 17.1 0.65 –– Porcine lung In vivo 2021 [49] 75 600 – 40 17.9 0.54 –– Porcine lung In vivo 100 600 – 43 20.7 0.53 –– Porcine lung In vivo 100 300 – 43 11.0 0.56 –– Porcine lung In vivo 100 600 – 43 20.7 0.53 –– Porcine lung In vivo 100 720 – 52 30.2 0.35 –– Porcine lung In vivo 4 Gao 60 600 33.62 51.91 –– 123.6 – Lung Simulation 2017 [52] 60 600 34 55 –– – – Phantom Ex vivo 5 Keangin 60 360 20 – 4.19 –– 50 Porcine lung Ex vivo 2019 [35] 80 360 24 – 7.23 –– 50 Porcine lung Ex vivo 100 360 26 – 9.20 –– 50 Porcine lung Ex vivo 6 Radmilovic–Radjenovic2022[68] 10 600 –– – – 63.4 – Lung Simulation 7 Sebek 32 300 17.67 25.3 –– – – Porcine lung In vivo 2020 [69] 24 600 17.6 20 –– – – Porcine lung In vivo 8 Sebek2021 [40] 30/60 600 15/30 –/35 –– – – Porcine lung In vivo 30/60 600 16.8/30.6 27.2/37 –– – – Lung Simulation Note: ‘–’ indicates that there is no reference to this term in the text. COMPUTER ASSISTED SURGERY 13 complete or not. In addition, MRI could reduce or found that the microwave coagulation zone was big- eliminate the artifact caused by respiration movement ger obviously. However, the backward heating effect or heartbeat movement, could be performed without of the antenna was more significant, which would des- using contrast media and could avoid damage to pul- troy a larger range of healthy tissue, and couldn’t monary vessels. Thus, it may guide the antenna to effectively kill the tumor in the center of the puncture puncture the lesion more accurately and avoid point. repeated operations to expose patients to ionizing radiation. The whole ablation process can be moni- 6.2. The influence of patient-specific anatomical tored in real time and the therapeutic effect can also structure be evaluated in real time. 6.2.1. The influence of pulmonary trachea on abla- tion results 6. Uncertainty factors in the simulation of Phairoh et al. [27] and Sanpanich et al. [71] studied MWA temperature distribution of lung tumors the effect of endobronchial airflow on MWA by using a tip-open coaxial antenna and a slot-open coaxial Lungs are close to the heart, trachea, blood vessels and other structures, so the thermal ablation of lung antenna, respectively. They placed a catheter with air- tumor is affected by respiratory movement, the speed flow near the microwave antenna to represent a bron- chus. Their results showed that in the absence of and direction of air flow in the bronchi and the heat bronchi, the distribution of electromagnetic wave sink effect of blood vessels. energy and the shape of coagulation zone were sym- metrically spherical. The existence of airflow in the 6.1. The influence of respiratory movement on the bronchus would affect the heating mode and the simulation accuracy damaged zone would be affected by the thermal con- The lung is filled with a large amount of air, which ductivity and electrical conductivity of air fluid, which greatly affects the temperature distribution of MWA. resulted in the asymmetry of coagulation shape in the Therefore, the accurate prediction and control of abla- lung model. This study had some limitations because tion zone are still very challenging. The volume of air it used a simple lung model for simulation. in the lung changes with respiratory movement, so tis- Constructing a porous lung structure composed of sue parameters of the lung also change. Yang et al. many alveoli or air sacs and taking into account the [26] performed three-dimensional reconstruction of rich capillary network around the lung would be the lung models at different respiratory stages and studied follow-up work. the change trend of lung tissue parameters during res- Liu et al. [28] explored the feasibility of constructing piration and the effect of these tissue parameters on 3D lung models of specific patients in the lung tumor MWA coagulation zone. During the ablation process, MWA simulation. Lung tumor models were attached they divided the tissue parameters into dynamic and to or not attached to the trachea, around the main static to simulate respectively. The sinusoidal function bronchus or around the bronchioles located in the was used to represent periodic physical parameters of deep lung, respectively. The results showed that ther- lung tissue, and the fixed parameter value was used mocoagulation effects were different for lung tumors to represent a certain stage of the respiratory process, under different locations. thus running through the whole respiratory process Tian et al. [51] studied the influences of bronchi on including end expiratory, end inspiratory and inter- ablation results. When building the simulation model, mediate state. By setting different angular frequency they placed a bronchiole near the opening at the tip values in sinusoidal functions, the effect of respiratory of coaxial antenna and set the airflow velocity to frequency on the temperature distribution in the abla- 3 L/min that matched the velocity of human body. tion zone was also studied. The results showed that They found that the airflow would take away part of there was no significant difference between different the heat, thus forming a nonuniform temperature respiratory frequencies on the damage zone. At the distribution. end of exhalation, the coagulation zone formed by Anai et al. [72] conducted in vivo experiments to the antenna was more concentrated in the tip and verify the effects of bronchi on ablation results. They slot. The backward heating effect was weaker, and the compared ablation results between the experimental damage to healthy tissue around the antenna was group (occlusion of unilateral main bronchi) and the less. Using the dynamic physical parameters, it was control group (without any treatment) and found that 14 J. LIU ET AL. the diameter and volume of coagulation zone in the diameter and coagulation zone in the experimental experimental group were significantly larger than group were larger than those in the control group. those of the control group. The limitation was that the experiment caused serious complications. 6.2.2. The influence of blood vessels on ablation results 7. Discussion Wang et al. [73] studied the effects of blood flow MWA is a promising therapeutic technique in the parameters (e.g. vessel diameter and vessel-antenna treatment of malignant tumors. However, some thera- spacing) on the temperature distribution and coagula- peutic parameters cannot be accurately set before tion volume of MWA. It was found that the coagula- tion area on the side of vessels was smaller than that clinical ablation surgery. As a result, it will be useful to on the other side, and the distance between the develop numerical tools that can correctly predict the MWA temperature distribution. In this paper, the simu- antenna and vessel had a more significant effect on lation techniques of MWA for lung tumors in recent the ablation result than the diameter of vessel. However, if the blood vessel was located outside the years were reviewed in order to provide some help for predicted coagulation zone, the ablation results would the future research. Previous studies have shown that tissue properties, hardly be affected by the blood vessel. The blood frequency, power and tip temperature significantly temperature also affected the volume of coagulation affect hyperthermia, especially MWA [75–77]. The tis- zone, which was obvious near the blood vessel. The sue thermophysical and electrical parameters change closer the blood temperature was to the body tem- perature (37 C), the closer the volume of coagulation with the increase of temperature, which has a signifi- cant impact on the ablation effects [23]. With the zone was to the actual measured value. increase of blood perfusion rate, thermal conductivity Vaidya et al. [74] comprehensively considered vas- and permittivity, the coagulation zone will decrease. cular morphological parameters and the construction of ablation antennas and studied their effects on the The increase of electrical conductivity has a positive thermal damage zone. In order to evaluate the effects effect on the ablation results. When other parameters are kept constant, the coagulation zone will expand of geometric parameters on thermal damage, they with the increase of frequency and power. However, introduced three indicators: the average lesion bound- ary displacement D ð%Þ, the maximum Nusselt num- in order to avoid damage to normal tissue, the use of ber Nu and the relative ablation volume V : The low power for a long time is more reasonable [16]. max rel Compared with other parameters, the applied power, results showed that the most important contribution frequency and blood perfusion rate have relatively sig- to the directional effect measurement D was the par- ameter d (the vertical distance between the center of nificant effects on the coagulation zone volume [34]. It antenna slot and the vessel-antenna connection). The is worth noting that the coagulation zone under the most important factor affecting Nu was the param- dynamic tissue parameters is larger, but the backward max heating effect is also more significant [26]. Although eter r (blood vessel radius) and V decreased with the rel decrease of the distance between antenna and blood increasing power and frequency may improve ablation vessel. The directional effect of thermal damage efficiency, this approach may not be appropriate for occurred between 0.4 mm and 0.5 mm radius of blood all types of tumors. For tumors with small radius (r < 1 cm), increasing power has no significant effect vessels, and blood vessels were classified according to on the ablation results. For larger tumors (r 1.5 cm), the condition under which the directional effect occurred. However, this study only considers the influ- higher power will destroy more tumor cells. For slen- ence of a single blood vessel, does not consider der tumors, the use of lower frequency and double- slot antennas will obtain better results, however, for parameters except the blood vessel radius and lacks spherical and oblate tumors, the use of higher fre- the experimental verification of simulation conclusions. quency will produce better results [43]. Therefore, the Anai et al. [72] studied the effects of blood flow on optimal ablation parameters such as frequency, power, the thermal ablation coagulation zone by occluding time and antenna setting should be selected accord- the ipsilateral pulmonary artery of the tumor and set ing to the shape and size of the tumor. Some up the experimental group (occluded pulmonary researchers have also studied the relationship between artery) and the control group (without any treatment) ablation volumes and tip temperatures. The results for in vivo experiments. The results showed that the showed that the higher tip temperature could COMPUTER ASSISTED SURGERY 15 improve MWA results [78]. Additionally, in the process vessels and tumors still need to be further explored. of simulation, the modeling based on the real anatom- Pennes biological heat transfer equation is the most ical structure of the human body [28] will also make widely used, but the ablation results obtained by the the prediction results more accurate. porous media heat transfer model are closer to the Although many scholars have established various real situation. Although the relationship between bio- logical tissue parameters and temperature has been simulation models of lung tumors MWA and made studied, the biological tissue parameters in most certain achievements, there are still challenges in the microwave simulation models still choose fixed values. simulation of temperature field: (1) Most of the models The construction of internal structures of lung tissue are based on the results of ex vivo porcine lung such as blood vessels and tumors for modeling and experiments or tissue phantom experiments and do simulation analysis is still a problem worthy of in- not consider the individual differences. (2) The effects depth study. The solution of these problems will pro- of the large blood vessels, the heart and other organs vide some guidance for improving the prediction around lung tissue are ignored. In the reconstruction accuracy of MWA temperature distribution and coagu- of the lung tissue model, these organs need to be lation zone of lung tumors. reconstructed together to get a more realistic model [28]. (3) Although some scholars have studied the rela- tionship between biological tissue characteristic Acknowledgements parameters and temperature, most microwave simula- The authors would like to thank the anonymous reviewers tion models still use fixed values for biological tissue for their constructive comments and suggestions. parameters. (4) The main problem of lung tumor MWA is local recurrence [79], and pneumothorax is the most Author contributions common serious complication [80,81] after percutan- eous ablation. The main cause of pneumothorax Conceptualization, S.W. and H.G.; investigation, Y.H. and X.L.; seems to be associated with the insertion of the resources, Z.C.; writing—original draft preparation, J.L. and J.W.; writing—review and editing, S.W. and H.G.; funding antenna [82]. Therefore, some flexible MWA applica- acquisition, S.W. All authors have read and agreed to the tors are under development [83,84] and can offer the published version of the manuscript. possibility of reducing the risk of pneumothorax. The limitations of this review include the lack of in- Disclosure statement depth understanding of treatment parameters for clin- ical application. Surgical path planning, another key No potential conflict of interest was reported by the technique for lung tumor MWA, has not been author(s). reviewed. In addition, the studies related to Hyperbolic equation and Weinbaum-Jiji equation are Funding not involved in the comparison of heat transfer mod- This work was supported by the National Natural Science els. Because the achieved MWA results may be linked Foundation of China [Grant No. 61871005 and No. to the specific antenna, the summarized data are not 82171941]. necessarily general. References 8. Conclusions [1] Sung H, Ferlay J, Siegel RL, et al. Global cancer statis- MWA has been proved to be a safe and feasible alter- tics 2020: GLOBOCAN estimates of incidence and native to surgery with acceptable morbidity and mor- mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2021;71:209–249. tality in medically inoperable patients with lung [2] Siegel RL, Miller KD, Fuchs HE, et al. Cancer statistics, tumor. The model simulation techniques of lung 2022. CA Cancer J Clin. 2022;72(1):7–33. tumor MWA in recent years were mainly reviewed in [3] Alexander ES, Dupuy DE. 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http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.pngComputer Assisted SurgeryTaylor & Francishttp://www.deepdyve.com/lp/taylor-francis/recent-research-advances-on-simulation-modeling-of-temperature-ZzUs10EPsl
Recent research advances on simulation modeling of temperature distribution in microwave ablation of lung tumors
Recent research advances on simulation modeling of temperature distribution in microwave ablation of lung tumors
Abstract
Abstract Lung tumor is the first malignant tumor with the highest mortality, but only no more than one-third of patients can be treated by surgical resection. Microwave ablation (MWA) has become a new adjuvant therapeutic mean for lung tumors because of its low trauma, short treatment time, large ablation volume and wide application range. However, the treatment parameters of MWA, such as input power and ablation time, still depend on the doctors’ experience, which leads to the...
COMPUTER ASSISTED SURGERY 2023, VOL. 28, NO. 1, 2195078 https://doi.org/10.1080/24699322.2023.2195078 RESEARCH ARTICLE Recent research advances on simulation modeling of temperature distribution in microwave ablation of lung tumors a a a a a b a Ju Liu , Hongjian Gao , Jinying Wang , Yuezheng He , Xinyi Lu , Zhigang Cheng and Shuicai Wu a b Faculty of Environmental and Life Sciences, Beijing University of Technology, Beijing, China; Department of Interventional Ultrasound, Chinese PLA General Hospital, Beijing, China KEYWORDS ABSTRACT Lung tumor; microwave Lung tumor is the first malignant tumor with the highest mortality, but only no more than one- ablation; temperature third of patients can be treated by surgical resection. Microwave ablation (MWA) has become a distribution; simulation new adjuvant therapeutic mean for lung tumors because of its low trauma, short treatment modeling time, large ablation volume and wide application range. However, the treatment parameters of MWA, such as input power and ablation time, still depend on the doctors’ experience, which leads to the ineffectiveness of MWA. Therefore, the accurate modeling of temperature distribu- tion of lung tumor MWA has become a significant technical problem to be solved. Recent research was devoted to personalized characterization of lung tumor parameters, finite element analysis of temperature distribution in MWA and accurate ablation effect evaluation. In this paper, a review of the recently obtained results and data will be presented and discussed. 1. Introduction is easily affected by the heat sink effect of peripheral blood vessels [8,9] and tissue carbonization [10,11], and Lung tumor is one of the most serious threats to the electric field distribution is not easy to be uniformly human health. In 2020, primary lung tumor is the controlled, resulting in incomplete ablation [12]. second most common malignancy worldwide with Furthermore, it is seen from the past researches that approximately 2.2 million new cases and 1.8 million the RFA can be employed only if the tumor size is less deaths [1,2]. Surgical resection is still the main method than 3 mm. In some cases, the skin burning is also a for the treatment of lung tumors. But due to the lack of major disadvantage of the RFA [10,13]. MWA is not eas- typical clinical symptoms of lung tumors, many patients ily affected by heat sink effect [14,15] and has the have reached advanced stage at the time of diagnosis advantages of minimal trauma, good tolerance, repeat- and are unable to undergo surgical resection [3]. ability, large ablation area, fast heating speed and less Some adjuvant treatments are gradually emerging damage to the surrounding normal tissue, so it has for the treatment of lung tumors, including radiother- attracted more and more attention in the clinical treat- apy, chemotherapy and thermal ablation. Imaged- ment of lung tumors [16,17]. guided thermal ablation techniques, including radiofre- At the same time, MWA antenna can achieve quency ablation (RFA), microwave ablation (MWA) and deeper penetration [18], higher specific absorption cryoablation have been widely used in the treatment of rate (SAR), faster damage rate in high impedance tis- inoperable lung tumor [4]. Other techniques, including sue (such as lung) and can reach a higher temperature laser ablation and irreversible electroporation (IRE), are in a short time, which makes MWA of lung tumors not widely used in lung ablation due to lack of clinical more efficient [19]. Therefore, MWA has become a bet- data [5]. Compared with other thermal ablation meth- ter way to treat unresectable lung tumors, and its ods, cryoablation has the disadvantages of long thera- peutic period, increased bleeding risk and complex ablation process is shown in Figure 1. The doctor preparation process. It is still controversial regarding inserts the microwave antenna into the lung tumor by cryoablation’s superiority over RFA and MWA [6,7]. RFA means of some imaging equipment and sets the CONTACT Hongjian Gao gaohongjian@bjut.edu.cn; Shuicai Wu wushuicai@bjut.edu.cn Faculty of Environmental and Life Sciences, Beijing University of Technology, Pingleyuan No. 100, Chaoyang District, Beijing 100124, China 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 J. LIU ET AL. Figure 1. Schematic diagram of MWA treatment of lung tumor. microwave power and ablation time according to the literature search (up to January 2023). A total of 1393 size and shape of the tumor. The high-frequency elec- articles were collected, in which 890 were selected using the query on Scopus and 503 on PubMed. tromagnetic field emitted by the antenna produces Following the elimination of duplicates, the relevant local high temperature to cause coagulative necrosis experimental studies were identified. In order to col- of tumor cells. After the ablation, the imaging equip- lect the most relevant articles, we identified keywords ment is used to detect whether the conformal treat- and divided them into four areas of interest: lung ment is achieved. tumor, microwave ablation, temperature distribution However, it will take time before MWA treatment and simulation modeling. The time limit and language based on simulation modeling to mature enough to restriction were not used. To try to expand our search, be accepted for clinical applications. In particular, the references lists of the retrieved articles were also expected temperature distributions cannot be accur- screened to identify additional studies [20]. ately predicted and thus the most suitable MWA treat- ment planning cannot be set down. Only relying on doctors’ experience may require multiple ablations 2.2. Study selection and aggravate pains of the patient. Temperature simu- The articles founded by our search were filtered. In lation has become a very important tool for predicting this systematic review, the main screening criteria ablation results, which can assist doctors to input were the original papers reporting the simulation appropriate treatment parameters to achieve con- model and ex vivo experiments of lung tumor MWA. formal ablation. This paper intends to provide a review The main inclusion criteria were: about the modeling of MWA temperature distribution of lung tumors in recent years, including exact charac- 1. Timeliness. The references should be based on terization of individual tissue parameters, simulation recent papers. modeling technology of lung tumor MWA, experimen- 2. Relevance. The references should be closely tal verification and clinical evaluation of MWA simula- related to the review topic. tion accuracy, uncertainties and technical challenges 3. Representativeness. The references should include in the simulation of MWA temperature distribution of representative literatures at home and abroad and lung tumors. should not be pointlessly piled up in large numbers. 4. Reliability. The references must be real and reli- 2. Methodology able, complete description items, easy to search 2.1. Search strategy and verify, avoid wrong citations. Two investigators independently examined Scopus 5. Authority. Authority is reflected in the author of and PubMed online database for a comprehensive the reference and published journal. Citing the COMPUTER ASSISTED SURGERY 3 work of authoritative authors will make the ex vivo experiments (condition) are summarized, as research more valuable. shown in Table 1. Exclusion criteria were: 3.2. Dynamic tissue parameters In the MWA process, the increasing temperature of a. articles not within the field of interest of this the lung tissue and tumor will have an impact on the review; thermophysical and electrical parameters. The changes b. articles that were too long ago; of these parameters will significantly affect the model- c. research focusing on the details of clinical ing accuracy. Therefore, it is still a challenging work to surgery; precisely derive dynamic tissue parameters. Gabriel d. articles that were not in the English language; e. articles that could not be found for their full text. et al. [42] calculated the permittivity and conductivity of lung tissue under the action of 2450 MHz electro- Two researchers independently reviewed the titles magnetic wave using fourth-order Cole–Cole model. The model is described as follows: and abstracts of the retrieved articles by the above cri- teria. The four authors then independently reviewed De r n s 0 00 e ðxÞ¼ e je ¼ e þ j (1) the full-text of the remaining articles to determine r 1 r r 1a xe 1 þðjxs Þ 0 n¼1 their final inclusion. where e is the dielectric increment of living tissue at the nth relaxation time, e and e are the static per- 0 1 3. Characterization of tissue parameters of mittivity and the permittivity when the frequency lung parenchyma and lung tumors tends to infinity, respectively, and s is the relaxation time. The tissue electrical properties (electrical con- Tehrani et al. [43] proposed an extended model of ductivity/r, permittivity/e) directly affect the absorp- tissue electrical conductivity and permittivity varying tion of electromagnetic energy that produces heat, with temperature during MWA and pointed out that while thermal properties (thermal conductivity/k, the temperature dependence of the thermal conduct- density/q, specific heat capacity/c) and perfusion ivity (k) and blood perfusion rate (x ) of biological tis- (blood perfusion/x) affect heat transfer in the tissue. sue is based on linear equations, which provided a Therefore, the setting of biological tissue parameters reference for scholars to perform simulation of MWA is of critical importance in the simulation modeling of temperature distribution based on dynamic biological MWA [21]. The precise characterization of tissue tissue characteristic parameters. These dynamic tissue parameters is conducive to improve the simulation parameters included thermal and dielectric properties accuracy. Many researchers have previously demon- are conducive to derive more accurate simulation strated the impact of uncertainty in tissue properties results and are described as follows: and their temperature dependency on MWA model outcomes. They have also carried out in-depth explor- eðTÞ¼ s 1 (2) 1 þ exp ðs s TÞ 2 3 ation on the parameters setting of the lung tissue. Furthermore, the effect of respiratory movement on rðTÞ¼ r 1 (3) 1 þ exp ðr r TÞ 2 3 tissue characteristics is also taken into account to x ¼ 0:000021T þ 0:0035 (4) increase the accuracy of simulation results. b kðTÞ¼ k þ DkðT T Þ (5) 0 0 where T is the temperature of lung tissue, s , s , s 1 2 3 3.1. Constant tissue parameters are constants of permittivity(e), r , r , r are constants 1 2 3 In order to simplify the calculation, some research of electrical conductivity(r), k , Dk and T are the 0 0 teams set biological tissue parameters to fixed values base-line thermal conductivity, the change in k due to in the simulation process. Instead of tumor tissue, temperature and the baseline temperature at which lung parameters are often used in lung MWA simula- k , respectively. tion. However, the dielectric properties of malignant Singh et al. [37] considered that the electrical con- tissues were 10–20% larger than those of normal tis- ductivity of tissue increased linearly with temperature sues [22]. The electrical and thermal properties of lung (2% per C). A non-linear piecewise decreasing model tissue (tissue type) involved in model simulation and for blood perfusion was proposed. They are expressed 4 J. LIU ET AL. Table 1. Summary of characteristic parameters of the lung tissue. Threshold Thermal conductivity Density Specific heat Blood perfusion Electrical Permittivity temperature 3 1 Num. Refs. k (W/mK) q (kg/m ) capacity c (J/kgK) x(S ) conductivity r (S/m) e T ( C) Tissue type Condition 1 Gao, 2019 [16] 0.2 480 –– 0.423 33 60 Porcine lung Ex vivo 2 Radmilovic, 2021 0.39 385 3886 0.0036 0.804 20.5 50 Lung Simulation (average) [23–25] 3 Yang, 2020-inflated 0.16 240 2500 0.00361 0.80 20.47 – Lung Simulation Mediate 0.18 360 2500 0.00361 1.24 34.42 – Lung Simulation Deflated [26,27] 0.20 480 2500 0.00361 1.68 48.38 – Lung Simulation 4 Liu, 2019-flooded 0.52 1030 3886 0.00709 –– – Lung Simulation Higher inflated 0.39 400 3886 0.00708 –– – Lung Simulation Lower inflated [28–33] 0.39 700 3886 0.00709 –– – Lung Simulation a a a a 5 Avishek, 2021 [34] 0.33 –– 0.0026 0.804 20.5 – Lung Simulation b b b b 0.39 –– 0.0155 1.69 43 – Lung Simulation c c c c 0.53 –– 0.0674 2.43 57.2 – Lung Simulation 6 Keangin, 2019 [35] –– – – – 30.01 – Porcine lung Ex vivo 7 Brace, 2009(inflated) [13] 0.302 260 2500 0.00087 0.804 20.5 – Porcine lung Ex vivo 8 Hasgall, 2022-deflated 0.39 1050 3886 0.00702 1.68 48.4 – Lung – Inflated [36] 0.39 394 3886 0.00263 0.804 20.5 – Lung – 9 Singh, 2018 [37,38] 0.39 394 3886 0.00263 0.122 –– Lung Simulation b a a a a 10 Singh, 2019 [39] 0.39 394 3886 0.00263 0.804 20.5 – Lung Simulation 11 Hu, 2022-deflated 0.39 480 3886 – 0.804 33 60 Porcine lung Ex vivo Inflated [12] 0.16 394 2500 – 0.306 20.47 60 Porcine lung Ex vivo 12 Sebek, 2021 0.39 –– – 1.47 40.5 – Lung Simulation (average) [40,41] Note: a: low; b: medium; c: high; ‘–’ indicates that there is no reference to this term in the text. COMPUTER ASSISTED SURGERY 5 as follows: parameters were used to simulate the conditions of blocking ventilation, and the sinusoidal function was rðTÞ¼ r½ 1 þ 0:02ðT T Þ (6) 0 b used to express the physical parameters under period- > x XðtÞ 0 b, 0 hi ically normal ventilation conditions. The results x ¼ b x 1 þ 25XðtÞ 260XðtÞ 0 XðtÞ 0:1 b, 0 > showed that a larger ablation area can be produced x exp½ XðtÞ XðtÞ > 0:1 b, 0 using the dynamic physical parameters, but the (7) reverse heating effect of microwave antenna will also be more significant, thus resulting in greater damage where T ¼ 37 C, r and x are the baseline elec- b 0 b, 0 to surrounding normal tissue. trical conductivity and baseline blood perfusion rate In order to propose personalized treatments for dif- for the lung tissue, respectively, and XðtÞ is the ferent patients, Tacprasartsit et al. [48] supplemented induced thermal damage. They also pointed out that the radiomics features on the traditional biological in Equation (5), Dk ¼ 0:0013, T ¼ 37 C: Choi et al. heat transfer model. Nine regression convolution [44] considered that k ¼ 0:396, Dk ¼ 0:239: neural networks (CNN) specially trained for each fea- Bianchi et al. [45] summarized the specific heat of ture are used to evaluate nine different characteristics lung adenocarcinoma cells at different temperatures, i.e. 3640 J/(kgK) at 37 C, 3880 J/(kgK) at 43 C, of the tumor. The results are used as radiological fea- tures, and then the physical characteristics of the 3850 J/(kgK) at 50 C and 3790 J/(kgK) at 60 C. patients’ specific tumor and lung tissue are obtained Bonello et al. [46] studied the temperature depend- according to a certain transformation model. In this ence of dielectric properties of ex vivo sheep lung tis- sue in the temperature range of 25 Cto90 C. It was way, personalized treatment parameters can be found that the permittivity and electrical conductivity obtained, but this technique requires the measure- decreased linearly with the increase of temperature, ment knowledge of tissue characteristics related to medical imaging data. It is still challenging to deter- which is contrary to the study of Singh et al. [37]. And the temperature coefficients under 2.45 GHz are 0.87 mine the relationship between radiological features and 1.21, respectively. Bianchi et al. [47] studied the and tissue parameters. characterization of the temperature dependence of In the selection of tissue parameters, the simulation the thermal properties of lung tissue from room tem- model using the tissue characteristics under exhalation perature (21 C) to over 90 C. They found that the condition was more accurate. Some studies have also thermal diffusivity (a) and k of the lung tissue found that the ablation area obtained by using increased exponentially with temperature, while the dynamic tissue parameters is larger, but there is a volumetric heat capacity (c ) changed less noticeable, problem of obvious backward heating effect. To date, which was described by a linear equation: the static tissue parameters which do not change with temperature are widely used in the study, and the aðTÞ¼ 0:1815 þ 0:002224 exp ð0:0739TÞ (8) dynamic tissue parameters of lung tissue are still in kðTÞ¼ 0:2852 þ 0:001288 exp ð0:08196TÞ (9) the exploratory stage. c ðTÞ¼ 1:526 0:003253T (10) Furthermore, since the characteristic parameters of 4. Simulation modeling technology of lung lung varied with the respiratory process, Radmilovi tumor MWA et al. [23] studied the change trend of electrical con- ductivity and permittivity of inspiratory and expiratory Due to the lack of typical clinical data, computer simu- lungs at different microwave frequencies. It was found lation has become an important tool for predicting that the relative permittivity decreased while the elec- the temperature distribution in MWA procedure. The trical conductivity increased with the increase of fre- workflow diagram is shown in Figure 2, and some of quency. The electrical conductivity changed a little at the key technologies are summarized in this section. frequencies of lower than 1 GHz. But at higher fre- quencies, the increase of electrical conductivity is rela- 4.1. Geometric modeling tively large. The relative permittivity and electrical conductivity of the lung during deflation are about The geometric modeling of MWA includes three parts: twice as large as those when inflated. Yang et al. [26] microwave antenna modeling, normal lung tissue studied the temperature distribution of lung tissue modeling and lung tumor modeling. The geometric during MWA under blocking ventilation and normal models conforming to the true anatomical structure ventilation, respectively. The constant physical will produce more real simulation results. 6 J. LIU ET AL. Figure 2. Simulation process of lung tumor MWA. Figure 3. Lung tissue model. (a) Cylindrical lung tissue model. (b) Real lung tissue model. energy transmission loss. Different from the needle- 4.1.1. Construction of microwave antenna model Depending on therapeutic organ and the size and shaped rigid microwave antennas used in traditional shape of the tumor, a specific microwave antenna is percutaneous ablation, Pfannenstiel et al. [18] devel- pierced directly to the tumor site for emitting micro- oped a new type of flexible microwave transmitter, wave energy. Microwave antenna is usually composed which was transmitted to targeted tumor through of inner conductor, insulating medium, outer con- bronchoscope. This equipment can enhance the accur- ductor and catheter. Phairoh et al. [27] designed a tip- acy of transmitter placement, improve ablation effect, open coaxial antenna with a microwave frequency of reduce the risk of complications such as pneumo- 2450 MHz to obtain spherical energy deposition. More thorax and treat targets that can’ t be reached by a researchers use ring-open coaxial antennas to provide common antenna. symmetrical ring-coil heating zones [49]. The micro- wave antenna made of coaxial cable is commonly 4.1.2. Construction of the lung tissue model Because it is usually assumed that the lung is isotropic used, and a 1 mm-wide slot is cut on the outer con- ductor to emit electromagnetic waves into the tissue. uniform tissue, an ideal cylindrical model is con- structed. Additionally, the electromagnetic energy At the same time, the antenna is encapsulated in the PTFE catheter to prevent the microwave antenna from emitted by the microwave antenna is an axisymmetric adhering to the dry ablation tissue. In addition, a cir- region, so the cylindrical model can be simplified to a culate water cooling system needs to be added inside rectangle model, which is solved in a two-dimensional the antenna to avoid unnecessary thermal damage to coordinate frame. A model of idealized lung tissue patients [16]. Habert et al. [49] used full antenna water combined with a microwave antenna is shown in cooling technology, choke coil design at the front end Figure 3(a). TM of the antenna, and Antiphase technology to pre- In order to construct a more realistic lung model, vent heat from spreading backward, and to reduce Yang et al. [26] divided respiratory process into ten the central temperature of the ablation zone and the stages. They reconstructed lung tissue models COMPUTER ASSISTED SURGERY 7 Figure 4. Lung tumor model. (a) Whole lung tissue-level model. (b) Traditional spherical tumor model. (c) Real tumor model based on CT slice. according to the respiratory stage, in which 0 repre- In the construction of geometric model, the simple cylindrical lung model is widely used, but the model sented the end of inspiration and 50% or 60% repre- sented the end of expiratory. The model of lung tissue involving human body structure (heart, trachea, blood vessels and so on) is few, which needs further investi- after reconstruction is shown in Figure 3(b). gation in the future. 4.1.3. Construction of the lung tumor model Many scholars assumed that the tumor was spherical 4.2. Calculation of electromagnetic energy in MWA simulation, in this case it can be simplified deposition into a two-dimensional model to simplify the calcula- 4.2.1. Electromagnetic wave conduction equation tion, as shown in Figure 4(b). In the simulation of lung tumor MWA, the electric field In order to obtain a more accurate tumor model, and magnetic field are time-varying TEM (transverse some researchers used CT slices of the tumor [50]to electromagnetic) waves. In two-dimensional axisym- reconstruct real lung tumor model, as shown in Figure metric cylindrical coordinates, the time-varying electric 4(c). The reconstructed lung tumor model was intro- field and magnetic field are described as follows [52]. duced into the Freeform tactile design system for Electric field: smooth processing to make simulation results more jðxtkzÞ accurate. Because there are trachea and bronchus in E ¼ e e (11) lung tissue, some tissue parameters such as thermal jðxtkzÞ H ¼ e e (12) conductivity, electrical conductivity and density will u rZ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi change with the gas volume in the tissue. Tian et al. ZP in [51] placed a bronchus around the microwave antenna n ¼ (13) p ln ðr =r Þ outer inner based on the traditional model and set its airflow to 3.0 L/min, which is the same as human respiratory where e is the longitudinal component of electric jðxtkzÞ rate. Liu et al. [28] took the structure of trachea into field, e is the traveling wave factor, e is the consideration to simulate the ablation process in a transverse component of distribution function of elec- more real and complex anatomical environment. And tric field, Z is the wave impedance, P is the MWA in five different tumor models were generated, mainly input power, r is the outer radius of dielectric, r outer inner including the following: tumors near the main airway, is the inner radius of dielectric, x ¼ 2pf is the angular tumors located in the deep lung, tumors attached to frequency of microwave, f is the frequency of micro- 2p the bronchial wall and tumors adjacent to but not wave, k ¼ is the propagation constant, k is the attached to the bronchial wall. wavelength and n is the integral constant. According 8 J. LIU ET AL. to Maxwell equation, the electromagnetic wave con- 4.3. Biological heat transfer technology duction in tissue is characterized by Helmholtz equa- 4.3.1. Tissue heat transfer equation tion: In the simulation of MWA temperature distribution, jr 1 the heat transfer in biological tissue is calculated by r ðe Þ r H l k H ¼ 0 (14) r u r u xe 0 biological heat transfer models. It is found that the temperature near the emission slot is the highest in where e is the relative permittivity of lung tissue; e is r 0 12 general. With the increase of distance, the influence of the vacuum permittivity (e ¼ 8:854 10 F/m); r the heat source becomes weaker and the rate of the is the electrical conductivity of lung tissue (S/m); H is temperature rise decreases. For the lung tissue, the the magnetic field intensity (A/m); l is the relative commonly used models include Pennes equation and permeability (l ¼ 1) and k is the free space wave porous media model [53,54]. Compared with the trad- number (rad/m). The specific absorption rate (SAR) of itional Pennes equation, the porous media model will lung tissue can be obtained by solving the distribution produce the results closer to the experimental data, of electromagnetic field in lung tissue: but at the same time, the solving process is more SAR ¼ j E j (15) complicated [54]. 2q In order to improve the accuracy of temperature 4.3.1.1. Pennes equation. distribution simulation of MWA, an improved SAR @T method with thermal conductivity term can also be qc ¼r ðkrTÞþ q c x ðT TÞþ Q þ Q b b b met ext @t used [52]. The calculated SAR is used as the heat (19) source term of the model for simulation. Compared where q is the density of lung tissue (kg/m ); q is the with the simulation result obtained by traditional elec- density of blood (kg/m ); c is the specific heat capacity tromagnetic coupling method, this SAR method sim- of lung tissue [J/(kgK)]; c is the specific heat capacity plifies simulation process, has better consistency with of blood [J/(kgK)]; T is the temperature of lung tissue the experimental data and improves simulation accur- ( C); T is the temperature of blood ( C); k is thermal acy. In general, the SAR near the emission slot is the conductivity [W/(mK)]; x is blood perfusion rate maximum [26]. However, the disadvantage of this (s ); Q is the heat generated by new metabolism met method is that the SAR is a fixed value at a certain (W/m ); Q is the heat generated by microwave gen- ext temperature and should be accurately measured. erator (W/m ). 4.2.2. Electromagnetic wave boundary condition 4.3.1.2. Heat transfer equation of porous media setting model. Aiming at the Helmholtz electromagnetic wave equa- tion, Selmi et al. [24] pointed out that the z axis was a @T qc ¼r ðkrTÞþ W q c ðT TÞ q c v rT oE b b b z b b symmetry axis: E ðt, r ¼ 0, zÞ and ðt, r ¼ 0, zÞ, and r @t or there was continuity of the tangential component of þ D ðk rTÞþ q b m the electrical field at the interface between the tissue (20) and the tumor: where k represents the apparent increase of thermal nðE E Þ¼ 0 (16) 2 1 conductivity in tissue caused by hemoperfusion in small vessels and v represents the total blood perfu- Avishek et al. [34] adopted the first-order scattering sion vector in the tissue. Other parameters are boundary equation to reduce the boundary reflection described above. and calculated the electric field. The construction of tissue heat transfer models is a n E ¼ 0 (17) key technology in MWA temperature distribution n ðr EÞ jkn ðE nÞ¼ 0 (18) simulation. Different heat transfer models have their The electric field distribution can be obtained by own advantages and disadvantages, as shown in solving Equation (14) with the boundary conditions Table 2. given by Equations (17) and (18). According to the Pennes equation is the most commonly used in the study, and it has many advantages [55]. But it is over- electric field derived from these equations, the tem- perature distribution can be obtained by solving the simplified and ignores many problems. Thus, it has corresponding biological heat transfer equation. been further improved by many researchers. In the COMPUTER ASSISTED SURGERY 9 Table 2. Comparison of advantages and disadvantages of heat transfer models. Heat transfer model Advantages Disadvantages Pennes equation The calculation process is simple, easy to analyze and It is assumed that the blood flow is uniform. The arterial widely used, and the additional term used to show blood is kept at constant temperature. Different that the perfusion heat transfer is linear in directions of blood flow are not considered. The temperature. arteriovenous convection is ignored. Porous media model The division of biological tissue into solid and liquid The process of solving solid tissue and liquid tissue is overcomes the defect of Pennes equation. complex. process of MWA, due to the rapid increase of tem- relatively closer to the experimental data [59]. At the perature, the water will evaporate quickly and the car- same time, the solution is relatively more complex. bonization will also occur. Therefore, some scholars 4.3.2. Heat transfer boundary condition setting have introduced the phase transition heat transfer In order to solve the biological heat transfer equation, analysis of tissue moisture when solving the heat it is necessary to set the boundary conditions. In gen- transfer equation of biological tissue [56]. Additionally, eral, the heat transfer boundary conditions are set as the direct contact between the liquid and the tissue follows: will produce the heat source term. Therefore, adding the liquid heat transfer equation to the tissue heat 1. Z axis (see Figure 4(b)) is a symmetry axis, transfer equation can achieve the goal of expanding expressed as shown in Equation (22). It describes ablation area. In order to build a more accurate simu- that the heat flux is zero. lation model, Truong et al. [57] combined Bill’s law 2. The insulation condition is suitable for the sur- with the Pennes biological heat transfer equation to rounding lung tissue. In other words, the heat flux analyze the heat transfer in biological tissue. Tucci through the surrounding wall is equal to zero, et al. [54] added the effect of tissue vaporization to expressed as shown in Equation (23) [34]. the Pennes equation, so that the term (qc)is expressed as follows: Q ¼ 0 (22) ðq c Þ 0 C < T < 99 C > l t h c x, t n ðkrTÞ¼ 0 (23) fg ðqcÞ ¼ 99 C < T < 100 C (21) > DT b, t where n is the unit vector normal to the boundary q c T > 100 C g g t (z ¼ 0or z ¼ 80mm r ¼ 30mm). where q and c are density and specific heat of tissue at temperature below 100 C (liquid phase), q and c 1. The outer boundary adopts the Dirichlet condition are density and specific heat of tissue at temperature of constant temperature (37 C) [28]. above 100 C (gas phase), h is the product of water fg latent heat of vaporization and water density at 100 C Equation (23) was further specified for each bound- and c is the water content inside the lung tissue. x, t ary by Selmi et al. [24]: Because the lung tissue is filled with air, and the @T @T @T traditional cylindrical lung model does not take into k j ¼ 0; k j ¼ 0; j ¼ 0 (24) z¼0mm z¼80mm r¼30mm @z @z @r account the effect of air, a porous media lung tissue model has been proposed. Coupled with the electro- They also supposed that the heat flux is continuous magnetic field of MWA, the flow field of air in lung tis- at the interface between the tissue and the tumor, i.e. sue and the thermal field of porous media, the k rT ¼ k rT (25) lung lung tumor tumor temperature distribution of lung tissue can be derived. Convective boundary conditions are applied to the The simulation results are closer to the in vitro experi- mental results. The ablation area produced by porous surface of external emitter to indicate water cooling: media model is 29% smaller and the maximum tem- n krT ¼ hðT TÞ (26) perature is 36% lower [54]. Wang et al. [58] also where n is the outer normal vector, the convective pointed out that the increase of porosity will result in 1 1 a more uniform temperature distribution, which may heat transfer coefficient h is 1000 Wm K and the cooling water temperature T is set to 15 C. The lead to more effective hyperthermia treatment. In a word, the Pennes equation is widely used, but value of h depends on the antenna. Its value and the it is oversimplified. The porous media model is T will be different in different studies. 1 10 J. LIU ET AL. 4.4. Evaluation technique of thermal coagulation The score of necrotic tissue is expressed by h , and zone of lung tumors its value is related to a : h ¼ 1 exp ðaÞ (29) Ablation coagulation zone is an important index to evaluate the thermal ablation effect. By comparing the where X indicates the degree of damage to the tissue, coagulation area with the size of tumor, we can know T indicates the reference temperature and t indicates whether the conformal coverage achieved. The com- the time that exceeds the reference time. The param- mon evaluation indexes of ablation coagulation zone eter R (¼8.314 J/mol/K) represents the universal gas include coagulation zone shape and coagulation zone constant, A represents the frequency factor, E repre- volume. sents the reaction energy barrier, DE represents the activation energy of irreversible damage reaction 4.4.1. Shape evaluation of coagulation zone and h represents the fraction of necrotic tissue. 39 1 The ablation coagulation zone will increase with the For the lung tissue, A ¼ 7:39 10 s and E ¼ 5 66 1 increase of ablation frequency and ablation power, 2:577 10 J=mol [37], or A ¼ 1:46 10 s and E ¼ 5 45 1 but beyond a certain range, only the longitudinal 4:428 10 J=mol [65], or A ¼ 1:61 10 s and E ¼ diameter of the coagulation zone will continue to 3:06 10 J=mol [40], respectively. In the FEM model, increase with the increase of ablation power, thus the induced thermal damage value X ¼ 1, corre- forming an ellipsoidal shape. In addition, some sponding to 63% probability of cell death (i.e. researchers proposed the sphericity index (SI) [49]to h ¼ 63%), has been used as a critical threshold for evaluate the coagulation zone shape. The sphericity of calculating coagulation volume. 1 corresponds to a perfect sphere, the sphericity less Liu et al. [28] used the equivalent heating minutes than 1 corresponds to a slender shape elongated at 43 C (EM ) to calculate the equivalent thermal 43 C along the antenna direction, and the sphericity greater dose distribution and then evaluated the MWA effects than 1 corresponds to a coagulation zone extending by adding time intervals of different temperature in the direction of orthogonal to the antenna. exposures, as follows: 0:5 T > 43 C ð43T Þ 4.4.2. Volume evaluation of coagulation zone EM ¼ R t , R ¼ (30) 43 C i i¼1 0:25 T < 43 C The clinical goal of MWA is to heat the tumor zone to a cytotoxic temperature, while maximally sparing non- where R is the proportional constant of cell mortality targeted tissue outside of this zone [60]. Therefore, it dependent on temperature and T is the average tem- th is very important to accurately evaluate the extent of perature ( C) of the i time interval (minutes). This thermal coagulation zone. The volume of thermal concept of thermal dose originates from the Arrhenius coagulation zone depends on ablation power and model of cell damage accumulation, which is a mature time. The volume is usually evaluated by isotherm index to predict the thermal effect of tissue or the threshold (IT), Arrhenius model and thermal equivalent degree of injury, and is widely used in thermotherapy dose (TID) [61]. and thermal ablation monitoring and evaluation. A With regard to IT method, the tissue is considered thermal dose of 120–240 min at 43 C usually causes to be coagulative when the temperature exceeds a considerable tissue necrosis, but the sensitivity varies certain threshold. The usual thresholds involve 60 C between tissue types [66]. [62], 54 C[63,64] and 50 C[24]. The advantages and disadvantages of these three Many researchers [26,48] also calculate the thermal thermal damage assessment techniques are shown in damage volume based on Arrhenius model [54], which Table 3. is expressed as follows: In view of the fact that the Arrhenius damage s model only considers two states of biological tissue (alive cells and dead cells), Tehrani et al. [43] proposed XðsÞ¼ Ae dt (27) RTðtÞ a three-state cell death model to calculate the size of ablation coagulation zone. That is, the cell death pro- In addition, a is used to indicate the degree of tis- cess under the temperature gradient is described by sue damage: coupling the ordinary differential equation, and the tissue is divided into three states: alive cells, vulner- da DE ¼ A exp (28) able cells and dead cells. The expression is as follows: dt RT COMPUTER ASSISTED SURGERY 11 Table 3. Comparison of advantages and disadvantages of thermal damage assessment techniques. Thermal damage assessment model Advantages Disadvantages Isotherm threshold (IT) Straightforward and intuitive The correlation between target type and heating duration is not taken into account. Arrhenius model Simple, direct and widely used Only two states of biological tissue are considered, and the transition between these states is single and irreversible. Thermal equivalent dose (TID) Widely used in thermal damage models at lower Not suitable for thermal ablation processes with temperatures higher temperatures above 50 C. f included the longitudinal diameter and transverse A V!D (31) b diameter of coagulation zone, as well as the slot tem- perature of microwave antenna. The experiment for where A, V and D are alive cells, vulnerable cells and each combination of power and time is repeated five dead cells, respectively. The positive rate constant k times. Keangin et al. [35] applied microwave power indicates the transition from alive to vulnerable state, levels of 60 W, 80 W and 100 W to porcine lung tissue while the reverse rate constant k represents the self- for 360 s. The ablation diameter was measured and repair process from vulnerable state to fully functional the ablation volume was calculated. resurrection state. Once beyond the critical point, the Table 4 shows the ablation results of different treat- cell enters a state of death (D), after which the process ment parameter combinations (microwave power/P is irreversible. Cell survival rate (V þ D) was used to and ablation time/t) during lung tissue (tissue type) determine the size of lesion. The ablation results MWA in simulation and ex vivo experiments (condi- showed that the results of three-state cell death tion). The evaluation indexes of the thermal damage model were closer to the experimental data. zone include the long diameter (L) and short diameter In the evaluation of coagulation zone, IT and (s) along the cross section of the ablation antenna, the Arrhenius model are widely used in the study because volume (V) of the coagulation zone, the sphericity (SI), of its simplicity, intuition and wide range of applica- the maximum temperature (T) reached at the end of tion. The coagulation zone after lung tumor MWA is the ablation and the coagulation threshold (T ). generally concentrated near the tip of the antenna and the emission slot, which is oval on the whole [67]. And the coagulation zone of expiratory group is larger 5.2. Clinical evaluation than that of inspiratory group [12]. Therefore, in clin- The simplest geometric modeling is usually used in ical treatment, one-lung ventilation [26] (meaning that the model simulation, but in clinic, the lung tissue patients only use the lung on the non-operative side contains a substantial amount of air and blood vessels, for ventilation) may be used for ablation surgery. and is adjacent to heart, so the ablation effect is affected by many factors. For the lung tumor with a 5. Experimental verification and clinical diameter of 2–8.5 cm (average 4 cm), clinicians usually evaluation of simulation accuracy of lung choose the ablation power of 60 to 70 W and the abla- tumor temperature distribution tion time of 6–10 min, which are slightly higher than the ablation power and ablation time used in the ex 5.1. Experimental verification of simulation model vivo experiments [16]. In the MWA procedure, the choice of treatment In the thermal ablation surgery, it is necessary to parameters such as input power and duration is crit- combine image guidance technology for real-time ical to ensuring the success of the procedure, because monitoring to achieve conformal ablation. In clinical the improper parameter usage may lead to incom- application, there are mainly three kinds of image plete ablation or excessive ablation. Therefore, some guidance techniques used in thermal ablation: ultra- researchers have carried out different explorations, sound, computed tomography (CT) and magnetic res- including using simulation technology to simulate the onance imaging (MRI). Ultrasound-guided and CT- ablation results of different parameter combinations guided ablation is performed in most cases, while and verifying these results by experiments. The aim is MRI-guided ablation is rare. Shen et al. [70] evaluated to provide some guidance for clinical application. Gao the safety, practicability and effectiveness of MRI- et al. [16] studied the coagulation zones at three guided MWA in patients with the lung malignant power levels (30 W, 40 W, 50 W) under different heat- tumor. The results showed that compared with CT, ing time (2 min, 4 min, 6 min). The measured indexes MRI was easier to identify whether ablation was 12 J. LIU ET AL. Table 4. Ablation results for different power and time combinations. Transverse Longitudinal Maximum Power Time diameter diameter Volume Sphericity temperature Threshold Num. Refs P (W) t (s) s (mm) L (mm) V (cm ) SI T ( C) temperature T ( C) Tissue type Condition 1 Gao 30/40/50 120 13.4/19.1/22.0 16.3/22.9/28.5 –– 67.2/73.6/78.7 60 Porcine lung Ex vivo 2019 [16] 30/40/50 240 –– 71.5/81.7/99.3 60 Ex vivo 30/40/50 360 23.0/36.8/39.9 30.2/41.2/51.4 –– 75.6/85.3/106.7 60 Porcine lung Ex vivo 30/40/50 360 29.5/44.0/48.9 41.1/55.7/66.3 –– 183.2/231.9/280.7 60 Porcine lung Simulation 40.5/46.9/51.9 56.3/62.1/66.7 Lung 2 Pfannenstiel 60/80 300 11.5/14 31.5/24.5 – 0.37/0.65 –– Porcine lung Ex vivo 2017 [18] 60 300 16 37.75 –– – – Canine lung In vivo 3 Habert 50 600 – 36 17.1 0.65 –– Porcine lung In vivo 2021 [49] 75 600 – 40 17.9 0.54 –– Porcine lung In vivo 100 600 – 43 20.7 0.53 –– Porcine lung In vivo 100 300 – 43 11.0 0.56 –– Porcine lung In vivo 100 600 – 43 20.7 0.53 –– Porcine lung In vivo 100 720 – 52 30.2 0.35 –– Porcine lung In vivo 4 Gao 60 600 33.62 51.91 –– 123.6 – Lung Simulation 2017 [52] 60 600 34 55 –– – – Phantom Ex vivo 5 Keangin 60 360 20 – 4.19 –– 50 Porcine lung Ex vivo 2019 [35] 80 360 24 – 7.23 –– 50 Porcine lung Ex vivo 100 360 26 – 9.20 –– 50 Porcine lung Ex vivo 6 Radmilovic–Radjenovic2022[68] 10 600 –– – – 63.4 – Lung Simulation 7 Sebek 32 300 17.67 25.3 –– – – Porcine lung In vivo 2020 [69] 24 600 17.6 20 –– – – Porcine lung In vivo 8 Sebek2021 [40] 30/60 600 15/30 –/35 –– – – Porcine lung In vivo 30/60 600 16.8/30.6 27.2/37 –– – – Lung Simulation Note: ‘–’ indicates that there is no reference to this term in the text. COMPUTER ASSISTED SURGERY 13 complete or not. In addition, MRI could reduce or found that the microwave coagulation zone was big- eliminate the artifact caused by respiration movement ger obviously. However, the backward heating effect or heartbeat movement, could be performed without of the antenna was more significant, which would des- using contrast media and could avoid damage to pul- troy a larger range of healthy tissue, and couldn’t monary vessels. Thus, it may guide the antenna to effectively kill the tumor in the center of the puncture puncture the lesion more accurately and avoid point. repeated operations to expose patients to ionizing radiation. The whole ablation process can be moni- 6.2. The influence of patient-specific anatomical tored in real time and the therapeutic effect can also structure be evaluated in real time. 6.2.1. The influence of pulmonary trachea on abla- tion results 6. Uncertainty factors in the simulation of Phairoh et al. [27] and Sanpanich et al. [71] studied MWA temperature distribution of lung tumors the effect of endobronchial airflow on MWA by using a tip-open coaxial antenna and a slot-open coaxial Lungs are close to the heart, trachea, blood vessels and other structures, so the thermal ablation of lung antenna, respectively. They placed a catheter with air- tumor is affected by respiratory movement, the speed flow near the microwave antenna to represent a bron- chus. Their results showed that in the absence of and direction of air flow in the bronchi and the heat bronchi, the distribution of electromagnetic wave sink effect of blood vessels. energy and the shape of coagulation zone were sym- metrically spherical. The existence of airflow in the 6.1. The influence of respiratory movement on the bronchus would affect the heating mode and the simulation accuracy damaged zone would be affected by the thermal con- The lung is filled with a large amount of air, which ductivity and electrical conductivity of air fluid, which greatly affects the temperature distribution of MWA. resulted in the asymmetry of coagulation shape in the Therefore, the accurate prediction and control of abla- lung model. This study had some limitations because tion zone are still very challenging. The volume of air it used a simple lung model for simulation. in the lung changes with respiratory movement, so tis- Constructing a porous lung structure composed of sue parameters of the lung also change. Yang et al. many alveoli or air sacs and taking into account the [26] performed three-dimensional reconstruction of rich capillary network around the lung would be the lung models at different respiratory stages and studied follow-up work. the change trend of lung tissue parameters during res- Liu et al. [28] explored the feasibility of constructing piration and the effect of these tissue parameters on 3D lung models of specific patients in the lung tumor MWA coagulation zone. During the ablation process, MWA simulation. Lung tumor models were attached they divided the tissue parameters into dynamic and to or not attached to the trachea, around the main static to simulate respectively. The sinusoidal function bronchus or around the bronchioles located in the was used to represent periodic physical parameters of deep lung, respectively. The results showed that ther- lung tissue, and the fixed parameter value was used mocoagulation effects were different for lung tumors to represent a certain stage of the respiratory process, under different locations. thus running through the whole respiratory process Tian et al. [51] studied the influences of bronchi on including end expiratory, end inspiratory and inter- ablation results. When building the simulation model, mediate state. By setting different angular frequency they placed a bronchiole near the opening at the tip values in sinusoidal functions, the effect of respiratory of coaxial antenna and set the airflow velocity to frequency on the temperature distribution in the abla- 3 L/min that matched the velocity of human body. tion zone was also studied. The results showed that They found that the airflow would take away part of there was no significant difference between different the heat, thus forming a nonuniform temperature respiratory frequencies on the damage zone. At the distribution. end of exhalation, the coagulation zone formed by Anai et al. [72] conducted in vivo experiments to the antenna was more concentrated in the tip and verify the effects of bronchi on ablation results. They slot. The backward heating effect was weaker, and the compared ablation results between the experimental damage to healthy tissue around the antenna was group (occlusion of unilateral main bronchi) and the less. Using the dynamic physical parameters, it was control group (without any treatment) and found that 14 J. LIU ET AL. the diameter and volume of coagulation zone in the diameter and coagulation zone in the experimental experimental group were significantly larger than group were larger than those in the control group. those of the control group. The limitation was that the experiment caused serious complications. 6.2.2. The influence of blood vessels on ablation results 7. Discussion Wang et al. [73] studied the effects of blood flow MWA is a promising therapeutic technique in the parameters (e.g. vessel diameter and vessel-antenna treatment of malignant tumors. However, some thera- spacing) on the temperature distribution and coagula- peutic parameters cannot be accurately set before tion volume of MWA. It was found that the coagula- tion area on the side of vessels was smaller than that clinical ablation surgery. As a result, it will be useful to on the other side, and the distance between the develop numerical tools that can correctly predict the MWA temperature distribution. In this paper, the simu- antenna and vessel had a more significant effect on lation techniques of MWA for lung tumors in recent the ablation result than the diameter of vessel. However, if the blood vessel was located outside the years were reviewed in order to provide some help for predicted coagulation zone, the ablation results would the future research. Previous studies have shown that tissue properties, hardly be affected by the blood vessel. The blood frequency, power and tip temperature significantly temperature also affected the volume of coagulation affect hyperthermia, especially MWA [75–77]. The tis- zone, which was obvious near the blood vessel. The sue thermophysical and electrical parameters change closer the blood temperature was to the body tem- perature (37 C), the closer the volume of coagulation with the increase of temperature, which has a signifi- cant impact on the ablation effects [23]. With the zone was to the actual measured value. increase of blood perfusion rate, thermal conductivity Vaidya et al. [74] comprehensively considered vas- and permittivity, the coagulation zone will decrease. cular morphological parameters and the construction of ablation antennas and studied their effects on the The increase of electrical conductivity has a positive thermal damage zone. In order to evaluate the effects effect on the ablation results. When other parameters are kept constant, the coagulation zone will expand of geometric parameters on thermal damage, they with the increase of frequency and power. However, introduced three indicators: the average lesion bound- ary displacement D ð%Þ, the maximum Nusselt num- in order to avoid damage to normal tissue, the use of ber Nu and the relative ablation volume V : The low power for a long time is more reasonable [16]. max rel Compared with other parameters, the applied power, results showed that the most important contribution frequency and blood perfusion rate have relatively sig- to the directional effect measurement D was the par- ameter d (the vertical distance between the center of nificant effects on the coagulation zone volume [34]. It antenna slot and the vessel-antenna connection). The is worth noting that the coagulation zone under the most important factor affecting Nu was the param- dynamic tissue parameters is larger, but the backward max heating effect is also more significant [26]. Although eter r (blood vessel radius) and V decreased with the rel decrease of the distance between antenna and blood increasing power and frequency may improve ablation vessel. The directional effect of thermal damage efficiency, this approach may not be appropriate for occurred between 0.4 mm and 0.5 mm radius of blood all types of tumors. For tumors with small radius (r < 1 cm), increasing power has no significant effect vessels, and blood vessels were classified according to on the ablation results. For larger tumors (r 1.5 cm), the condition under which the directional effect occurred. However, this study only considers the influ- higher power will destroy more tumor cells. For slen- ence of a single blood vessel, does not consider der tumors, the use of lower frequency and double- slot antennas will obtain better results, however, for parameters except the blood vessel radius and lacks spherical and oblate tumors, the use of higher fre- the experimental verification of simulation conclusions. quency will produce better results [43]. Therefore, the Anai et al. [72] studied the effects of blood flow on optimal ablation parameters such as frequency, power, the thermal ablation coagulation zone by occluding time and antenna setting should be selected accord- the ipsilateral pulmonary artery of the tumor and set ing to the shape and size of the tumor. Some up the experimental group (occluded pulmonary researchers have also studied the relationship between artery) and the control group (without any treatment) ablation volumes and tip temperatures. The results for in vivo experiments. The results showed that the showed that the higher tip temperature could COMPUTER ASSISTED SURGERY 15 improve MWA results [78]. Additionally, in the process vessels and tumors still need to be further explored. of simulation, the modeling based on the real anatom- Pennes biological heat transfer equation is the most ical structure of the human body [28] will also make widely used, but the ablation results obtained by the the prediction results more accurate. porous media heat transfer model are closer to the Although many scholars have established various real situation. Although the relationship between bio- logical tissue parameters and temperature has been simulation models of lung tumors MWA and made studied, the biological tissue parameters in most certain achievements, there are still challenges in the microwave simulation models still choose fixed values. simulation of temperature field: (1) Most of the models The construction of internal structures of lung tissue are based on the results of ex vivo porcine lung such as blood vessels and tumors for modeling and experiments or tissue phantom experiments and do simulation analysis is still a problem worthy of in- not consider the individual differences. (2) The effects depth study. The solution of these problems will pro- of the large blood vessels, the heart and other organs vide some guidance for improving the prediction around lung tissue are ignored. In the reconstruction accuracy of MWA temperature distribution and coagu- of the lung tissue model, these organs need to be lation zone of lung tumors. reconstructed together to get a more realistic model [28]. (3) Although some scholars have studied the rela- tionship between biological tissue characteristic Acknowledgements parameters and temperature, most microwave simula- The authors would like to thank the anonymous reviewers tion models still use fixed values for biological tissue for their constructive comments and suggestions. parameters. (4) The main problem of lung tumor MWA is local recurrence [79], and pneumothorax is the most Author contributions common serious complication [80,81] after percutan- eous ablation. The main cause of pneumothorax Conceptualization, S.W. and H.G.; investigation, Y.H. and X.L.; seems to be associated with the insertion of the resources, Z.C.; writing—original draft preparation, J.L. and J.W.; writing—review and editing, S.W. and H.G.; funding antenna [82]. Therefore, some flexible MWA applica- acquisition, S.W. All authors have read and agreed to the tors are under development [83,84] and can offer the published version of the manuscript. possibility of reducing the risk of pneumothorax. The limitations of this review include the lack of in- Disclosure statement depth understanding of treatment parameters for clin- ical application. Surgical path planning, another key No potential conflict of interest was reported by the technique for lung tumor MWA, has not been author(s). reviewed. In addition, the studies related to Hyperbolic equation and Weinbaum-Jiji equation are Funding not involved in the comparison of heat transfer mod- This work was supported by the National Natural Science els. Because the achieved MWA results may be linked Foundation of China [Grant No. 61871005 and No. to the specific antenna, the summarized data are not 82171941]. necessarily general. References 8. Conclusions [1] Sung H, Ferlay J, Siegel RL, et al. Global cancer statis- MWA has been proved to be a safe and feasible alter- tics 2020: GLOBOCAN estimates of incidence and native to surgery with acceptable morbidity and mor- mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2021;71:209–249. tality in medically inoperable patients with lung [2] Siegel RL, Miller KD, Fuchs HE, et al. Cancer statistics, tumor. The model simulation techniques of lung 2022. CA Cancer J Clin. 2022;72(1):7–33. tumor MWA in recent years were mainly reviewed in [3] Alexander ES, Dupuy DE. 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Journal
Computer Assisted Surgery
– Taylor & Francis
Published: Dec 31, 2023
Keywords: Lung tumor; microwave ablation; temperature distribution; simulation modeling
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References
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