Preclinical Evaluation of Submillimeter Diameter Microwave Interstitial Hyperthermia Applicators

Ultra miniature coaxial cable has been used, with microscopic techniques, to fabricate interstitial hyperthermia applicators having diameters of 0.20 mm, 0.33 mm, and 0.58 mm; commercial applicators have a diameter of 1.1 mm. Animal studies with the 0.33 mm diameter applicators have shown that they produce less local tissue trauma than the larger-diameter devices. All of these applicators operate at 915 MHz and have similar heating patterns because they use the conventional monopole design and the catheters have been approximately scaled to the dimensions of each size applicator. We have measured the heating (SAR) patterns of these applicators in tissue-simulating phantoms, both singly and in arrays, using a miniature electric field probe. As an intermediate step to patient trials, we have examined the ability of these applicators to provide effective heating of perfused tissue, using pig thigh and liver as models. Test results suggest that the durability and power handling capabilities of our submillimeter applicators are adequate for use in patients. These new applicators should be useful in the percutaneous treatment of deep-seated tumors and in intraoperative treatments. The applicators also permit intraluminal or intravascular access to tumors.