Computational FEM Model, Phantom and Ex Vivo Swine Breast Validation of an Optimized Double-Slot Microcoaxial Antenna Designed for Minimally Invasive Breast Tumor Ablation: Theoretical and Experimental Comparison of Temperature, Size of Lesion, and SWR, Preliminary Data

Microwave ablation modeling with AMICA antenna: Validation by means a numerical analysis

BACKGROUND AND OBJECTIVES

Percutaneous microwave thermal ablation is based on electromagnetic waves that generate dielectric heating, and it is widely recognized as one of the mostly used techniques for tumor treatment. The aim of this work is to validate a predictive model capable of providing physicians with guidelines to be used during thermal ablation procedures avoiding collateral damage.

METHODS

A finite element commercial software, COMSOL Multiphysics, is employed to implement a tuning-parameter approach. Governing equations are written with reference to variable-porosity and Local Thermal Non-Equilibrium (LTNE) equations are employed. The simulations results are compared with available ex-vivo and in-vivo data with the help of regression analysis. For in-vivo data simulations, velocity vector modulus and direction are varied between 0.0007 and 0.0009 m/s and 90-270°, respectively, in order to use this parameter as a tuning one to simulate - and lately optimize with respect to the differences from experimental outcomes - all the possible directions of the blood flow with respect to the antenna, whose insertion angle is not registered in the dataset.

RESULTS

The model is validated using reference data provided by the manufacturer (AMICA), which is obtained from ex-vivo bovine liver. The model accurately predicts the size and shape of the ablated area, resulting in an overestimation lesser than 10 %. Additionally, predictive data are compared to an in-vivo dataset. The ablated volume is accurately predicted with a mean underestimation of 6 %. The sphericity index is calculated as 0.75 and 0.62 for the predictions and in-vivo data, respectively.

CONCLUSION

This study developed a predictive model for microwave ablation of liver tumors that showed good performance in predicting ablation dimensions and sphericity index for ex-vivo bovine liver and for in-vivo human liver data with the tuning technique. The study emphasizes the necessity for additional development and validation to enhance the accuracy and reliability of in-vivo application.

A review of antenna designs for percutaneous microwave ablation

Microwave (MW) antenna is a key element in microwave ablation (MWA) treatments as the means that energy is delivered in a focused manner to the tumor and its surrounding area. The energy delivered results in a rise in temperature to a lethal level, resulting in cell death in the ablation zone. The delivery of energy and hence the success of MWA is closely dependent on the structure of the antennas. Therefore, three design criteria, such as expected ablation zone pattern, efficiency of energy delivery, and minimization of the diameter of the antennas have been the focus along the evolution of the MW antenna. To further improve the performance of MWA in the treatment of various tumors through inventing novel antennas, this article reviews the state-of-the-art and summarizes the development of MW antenna designs regarding the three design criteria.

Double Slot Antenna for Microwave Thermal Ablation to Treat Bone Tumors: Modeling and Experimental Evaluation

According to statistics of the American Cancer Society, the number of young people diagnosed with bone tumors is growing. Surgery and radiotherapy are the common treatments, however they have several side effects which affect the patient’s life. Therefore, a cheaper and less side-effect therapy called thermal ablation has been explored. The goal of this paper is to measure the therapeutic temperatures and the viability of a double slot antenna designed to treat bone tissue by microwave ablation. The antenna (at an operating frequency of 2.45 GHz) was designed, modeled, constructed, and experimentally evaluated. The finite element method was used to predict the antenna performance by means of 2D axisymmetric models. The modeling parameters were used to build the antenna. The experimental evaluation shows that the antenna behavior is repeatable and the standing wave ratio (SWR) was around 1.5–1.8. Temperatures around 60–100 °C were achieved over the bone tissue. The antenna insertion modifies the antenna performance. An insertion lower than 3.5 cm is not recommended because the convection effects modified the tissue temperature. The thermal patterns showed a heat focus near to the slots, which makes it possible for use in the treatment of small tumors.

Computational FEM Model and Phantom Validation of Microwave Ablation for Segmental Microcalcifications in Breasts Using a Coaxial Double-Slot Antenna

Introduction

Cancer is the second leading cause of death worldwide. Breast cancer is the second most common cause of cancer-related mortality, accounting for 11.6% of the total number of deaths. The main treatments for this disease are surgical removal of the tumor, radiotherapy, and chemotherapy. Recently, different minimally invasive technologies have been applied (e.g., emission of electromagnetic waves, thermal and chemical means) to overcome the important side effects of these treatment modalities. The objective of this study was to develop and evaluate a predictive computational model of microwave ablation.

Materials and Methods

The predictive computational model of microwave ablation was constructed by means of a dual-slot coaxial antenna. The model was compared with an experiment performed using a breast phantom, which emulates the dielectric properties of breast tissue with segmental microcalcifications. The standing wave ratio (SWR) was obtained for both methods to make a comparison and determine the feasibility of applying electromagnetic ablation to premalignant lesions in breasts. Specifically, for the analysis of segmental microcalcifications, a breast phantom with segmental microcalcifications was developed and two computational models were performed under the same conditions (except for blood perfusion, which was excluded in one of the models).

Results

The SWR was obtained by triplicate experiments in the phantom, and the measurements had a difference of 0.191 between the minimum and maximum SWR values, implying a change of power reflection of 0.8%. The average of the three measurements was compared with the simulation that did not consider blood perfusion. The comparison yielded a change of 0.104, representing a 0.2% change in power reflection. Discussion. Both experimentation in phantom and simulations demonstrated that ablation therapy can be performed using this antenna. However, an additional optimization procedure is warranted to increase the efficiency of the antenna.

Modeling Heat Transfer in Tumors: A Review of Thermal Therapies

It is quite challenging to describe heat transfer phenomena in living systems because of the involved phenomena complexity. Indeed, thermal conduction and convection in tissues, blood perfusion, heat generation due to metabolism, complex vascular structure, changing of tissue properties depending on various conditions, are some of the features that make hard to obtain an accurate knowledge of heat transfer in living systems for all the clinical situations. This theme has a key role to predict accurately the temperature distribution in tissues, especially during biomedical applications, such as hyperthermia treatment of cancer, in which tumoral cells have to be destroyed and at the same time the surrounding healthy tissue has to be preserved. Moreover, the lack of experimentation in this field, due to ethical reasons, makes bioheat models even more significant. The first simple bioheat model was developed in 1948 by Pennes (J Appl Physiol 1:93-122, 1948) but it has some shortcomings that make the equation not so accurate. For this reason, over the years it has been modified and more complex models have been developed. The purpose of this review is to give a clear overview of how the bioheat models have been modified when applied in various hyperthermia treatments of cancer.

Feasibility of Using a Novel 2.45 GHz Double Short Distance Slot Coaxial Antenna for Minimally Invasive Cancer Breast Microwave Ablation Therapy: Computational Model, Phantom, and In Vivo Swine Experimentation

Microwave ablation (MWA) by using coaxial antennas is a promising alternative for breast cancer treatment. A double short distance slot coaxial antenna as a newly optimized applicator for minimally invasive treatment of breast cancer is proposed. To validate and to analyze the feasibility of using this method in clinical treatment, a computational model, phantom, and breast swine in vivo experimentation were carried out, by using four microwave powers (50 W, 30 W, 20 W, and 10 W). The finite element method (FEM) was used to develop the computational model. Phantom experimentation was carried out in breast phantom. The in vivo experimentation was carried out in a 90 kg swine sow. Tissue damage was estimated by comparing control and treated micrographs of the porcine mammary gland samples. The coaxial slot antenna was inserted in swine breast glands by using image-guided ultrasound. In all cases, modeling, in vivo and phantom experimentation, and ablation temperatures (above 60°C) were reached. The in vivo experiments suggest that this new MWA applicator could be successfully used to eliminate precise and small areas of tissue (around 20–30 mm²). By modulating the power and time applied, it may be possible to increase/decrease the ablation area.