Design and Development of Metamaterial Based Antennas for Hyperthermia Application

Abstract

Hyperthermia treatment for cancer involves raising the temperature of cancerous tissues or tumors in the body. The elevated temperature is typically maintained within a specific range (usually between 410C - 450C) for one hour. This approach utilizes various heating techniques, such as electromagnetic (EM) heating, ultrasound, hyperthermia perfusion, and conductive heating, tailored to the patient's condition, tumor location, and size. Among these, microwave (MW) hyperthermia stands out as a promising non-invasive technique due to its ability to create targeted hot spots in tumors while minimizing damage to healthy tissues. The effectiveness of hyperthermia treatment depends on several factors, including the type and size of the applicator, operating frequency, and the specific absorption rate (SAR). Two critical parameters, penetration depth (PD) and effective field size (EFS), describe the SAR and significantly influence the design of hyperthermia applicators. Applicators are generally categorized into planar and non-planar designs, which operate across specific industrial, scientific, and medical (ISM) frequency bands such as 434 MHz, 915 MHz, and 2450 MHz. The choice of frequency is crucial, as lower frequencies allow deeper tissue penetration but require larger applicators, whereas higher frequencies provide shallower penetration and are more suitable for superficial tumors. To address these challenges, advancements in metamaterial based antennas offer a promising direction. Metamaterials, engineered to manipulate EM waves, enhance PD and EFS, improving hyperthermia efficacy. This work focuses on the development of compact planar applicators compatible with different tissue types, featuring precise focusing abilities. These applicators are designed, optimized, and analyzed using CST Microwave Studio software, based on the finite integration technique. To prevent burns in the skin's upper layers, the applicators are positioned at a specific distance from the body and integrated with a water bolus. Testing is conducted on heterogeneous human phantom models and Gustav voxel models to validate the performance of the applicators. The study involves designing various applicators to improve the efficacy of hyperthermia treatments. The first applicator, a compact Archimedean double spiral antenna (DSA) operating at 2.45 GHz, targets superficial tumors. This antenna achieves a bandwidth of 120 MHz, which increases to 180 MHz when integrated with an artificial magnetic conductor (AMC) acting as a reflector. The AMC also enhances the antenna’s gain from 1.17 dB to 4.28 dB, focusing energy toward the tumor. Simulations with a heterogeneous phantom reveal significant improvements in PD and EFS, and thermal analysis indicates that the applicator can maintain tumor temperatures between 41°C and 45°C at 2.5 W of power. To further enhance PD and EFS, a frequency selective surface (FSS) lens is integrated with the DSA. This lens focuses energy more effectively toward deeper tumors, converting the bidirectional radiation pattern into a unidirectional one. Simulations show a peak tumor temperature of 43°C at 2.5 W of power, demonstrating its suitability for treating larger tumor areas. For even deeper tumors, a compact focused metamaterial based applicator is developed. This design combines the DSA with an AMC and an FSS lens. The AMC directs radiation forward, while the FSS lens enhances energy focusing. This applicator achieves uniform heating patterns and a high PD, maintaining a temperature of 44°C at 2.5 W input power for tumors located 16 mm beneath the skin. To optimize PD further, a novel spiral shaped frequency-selective surface (SFSS) lens is introduced. This spiral lens, integrated with the DSA, ensures uniform heating while protecting superficial tissue layers from hot spots. The design is tested on heterogeneous phantom models and voxel models, both with and without a water bolus. The results show improved PD and EFS compared to previous designs. Thermal analysis confirms a peak tumor temperature of 44.7°C at 1.9 W of power, validating its effectiveness for deep seated tumors. For practical applications, the applicators are enclosed in a protective Teflon layer and integrated with a PVC water bolus layer to ensure safety and environmental durability. The designs are experimentally validated through SAR measurements in tissue mimicking phantoms, showing consistent results with simulated data. Prototypes are fabricated and tested for different configurations, confirming their efficacy in generating controlled heating for hyperthermia. This work demonstrates significant advancements in hyperthermia treatment through the development of compact, efficient, and targeted applicators. The integration of metamaterial structures enhances PD and EFS, addressing the limitations of conventional applicators. The findings highlight the potential of these designs to treat a wide range of tumor types effectively while ensuring patient safety. Future research could focus on further miniaturization, real-time monitoring, and adaptive mechanisms to enhance the precision and versatility of hyperthermia applicators.