A class of new microwave therapeutic applicators

A class of new direct-contact microwave applicators has been developed for diathermy and hyperthermia applications. They consist of three concentric, circular, cylindrical tubes. The inner cylinder may serve as a port for circulating coolant to reduce surface temperature or as a port for introducing ionizing radiation in combination therapy for cancer. The coaxial waveguides formed by the inner and middle cylinders and by the middle and outer cylinders permit the propagation of TE11 and TE21 modes, respectively, for dual-beam, single-frequency use. The combinations give relatively uniform, circularly symmetric heating distributions in the tissue. Heating pattern and leakage level measurements obtained for a 10-cm-diameter dual-beam 2450±50-MHz applicator show that the maximum leakage level at a distance of 5 cm from the surface of the applicator in contact with planar fat-muscle phantoms is less than 4 mW/cm2 for a therapeutic specific absorption rate (SAR) of 235 W/kg. This value corresponds to a net forward power of 26 W. The measured VSWR is about 1.25. The width of the heating pattern is about 4 cm at a depth of 2.2 cm into the simulated muscle, where the SAR is half the maximum, and decreases slowly as the depth into the muscle increases, thus giving it a fairly uniform distribution.