Thermal Evaluation of a Micro-Coaxial Antenna Set to Treat Bone Tumors: Design, Parametric FEM Modeling and Evaluation in Multilayer Phantom and Ex Vivo Porcine Tissue

Abstract

Bone cancer is rare in adults, the most affected persons by this disease are young people and children. The common treatments for bone cancer are surgery, chemotherapy, and targeted therapies; however, all of them have side-effects that decrease the patient’s quality of life. Thermotherapy is one of the most promising treatments for bone cancer; its main goal is to increase the tumor temperature to kill cancerous cells. Although some micro-coaxial antennas have been used to treat bone tumors, most of them are designed to treat soft tissue. Therefore, the purpose of this work is to analyze the thermal behavior of four micro-coaxial antennas specifically designed to generate thermal ablation in bone tissue to treat bone tumors, at 2.45 GHz. The proposed antennas were the metal-tip monopole (MTM), the choked metal-tip monopole (CMTM), the double slot (DS) and the choked double slot (CDS). The design and optimization of the antennas by using the Finite Element Method (FEM) allow to predict the optimal antenna dimensions and their performance when they are in contact with the affected biological tissues (bone, muscle, and fat). In the FEM model, a maximum power transmission was selected as the main parameter to choose the optimum antenna design, i.e., a Standing Wave Ratio (SWR) value around 1.2–1.5. The four optimized antennas were constructed and experimentally evaluated. The evaluation was carried out in multilayer phantoms (fat, muscle, cortical, and cancellous bone) and ex vivo porcine tissue at different insertion depths of the antennas. To fully evaluate the antennas performance, the standing wave ratio (SWR), power loss, temperature profiles, and thermal distributions were analyzed. In the experimentation, the four antennas were able to reach ablation temperatures (>60 °C) and the highest reached SWR was 1.7; the MTM (power loss around 16%) and the CDS (power loss around 6.4%) antennas presented the lowest SWR values depending on the antenna insertion depth, either in multilayer tissue phantom or in ex vivo tissue. These proposed antennas allow to obtain ablation temperatures with an input power of 5 W after 5 min of treatment; these values are lower than the ones reported in the literature.

Thermal Evaluation of Multi-Antenna Systems Proposed to Treat Bone Tumors: Finite Element Analysis

“…The evaluation was based on SWR analysis, tissue volume at ablation temperatures, maximum reached temperatures, and isothermal contours. The double slot (DS) and the monopole (MTM) antennas were chosen to form the arrays, as they were previously designed and tested [ 17 ], i.e., their correct performance has been evaluated by experimentation in phantom and porcine ex vivo tissue. Previous experimental tests showed that both antennas could generate ablation temperatures at bone using 10 W of input power applied for durations between 5 and 10 min.…”

“…Moreover, as this research was focus on bone tumors, the monopole antenna was also proposed to evaluate the possibility to cover a larger region of bone tissue. The double slot and the monopole antennas were previously modeled, optimized, and tested in multi-tissue phantoms and ex vivo porcine tissue by this research group [ 17 ]. The computational models showed SWR values of 1.26 and 1.55 for the double slot (DS) and the monopole (MTM) antennas, respectively.…”

“…A parametric study was performed to obtain the maximum distance between antennas ( ad ) that was needed to enhance the volume of tissue that was affected by the thermal ablation. In order to set the distance between antennas ( ad ), the next specifications, according to previous studies that were done by this research group, were considered [ 15 , 17 ]. The designed antennas reach temperatures higher than 60 °C at approximately 10 mm from the antenna axis, and similar temperatures were reached at approximately 6 mm further to the antenna tip.…”

“…Lately, researchers at the National Institute of Rehabilitation Luis Guillermo Ibarra Ibarra (INR-LGII) started developing micro-coaxial antennas that were designed to specifically treat bone tissue by MWA [ 12 , 13 , 14 , 15 , 16 , 17 ]. Trujillo-Romero et al firstly designed a single slot antenna to treat bone tumors [ 15 ].…”

“…This effect could reduce the antenna lifetime and affect its performance after just a few uses. Therefore, a new set of four different micro-coaxial antennas with a thicker diameter (2.19 mm) was proposed [ 17 ]. The antennas were modeled, built, and tested by using phantoms and ex vivo porcine tissue.…”