A 400 MHz Hyperthermia System using Rotating Spiral Antennas for Uniform Treatment of Large Superficial and Sub-Surface Tumors

Numerous studies have shown that hyperthermia can significantly increase the tumor-killing effects of radiation therapy and/or chemotherapy. Superficial tumors are usually treated with waveguide-based systems operating at 915 MHz which do not heat evenly or deeply enough for typical disease that spreads below and across the skin surface. A 400 MHz system that has been shown capable of treating somewhat larger and deeper volumes in previous animal and human clinical trials is investigated in this effort with the intent to further improve the technology. Real-time temperature, oxygen, and power reflection coefficient measurements are added for accurate control and deposition of the prescribed thermal dose while also assessing physiological response of the treated area. The heating performance of three spiral antenna applicators is evaluated in two ways. First the power deposition pattern is simulated with Ansoft HFSS for a single stationary microstrip spiral applicator radiating into muscle tissue. Next the power deposition averaged over one cycle of rotation is simulated with Ansoft HFSS for a single asymmetrically rotated spiral, and for larger heating patterns an asymmetrically rotated two antenna spiral array. To verify the simulations, the power deposition pattern of the largest dual spiral scanning applicator was evaluated in a muscle equivalent tissue phantom using infrared imaging of a 'split-phantom' load and the results compared to the HFSS simulated patterns. The data show that these spiral antennas heat uniformly above 50% of the maximum heating rate just beyond the perimeter of the scanning or fixed spiral structure, producing for the largest dual spiral a 17 cm diameter region at a depth of 3 cm. The results of a previous human clinical trial using these spiral applicators is presented along with our plans to optimize the applicator in terms of real time control and extent of heating based on new HFSS modeling and thermal analysis tools.