BEAM SHAPING FOR MICROWAVE WAVE-GUIDE HYPERTHERMIA APPLICATORS

Purpose: Hyperthermia treatments commonly use single element microwave waveguide applicators. The microwave beam patterns produced by these applicators are often non-uniform. As a result, hot spots are formed i n the heated tissue and therapeutic temperatures are reached in only s mall areas of the treatment field. We have constructed new coupling bo luses that improve the heating patterns of external microwave applicat ors. Methods: The microwave beam transmitted through the bolus is modi fied by microwave absorbing saline/gelatin pads. The pads can be desig ned to result in a uniform heating pattern over a large area or altern atively, complex heating patterns can be generated for specific clinic al applications. An analysis of the effect of bolus design parameters on microwave absorption patterns is presented. The heating patterns of the MA-100 and MA-120 microwave waveguide applicators have been measu red in muscle and fat phantom materials with both the manufacturer's b oluses and the new boluses. Results: In the case of the MA-100, the ar ea above the 70% heating level measured in a muscle phantom was increa sed by a factor of 2.3 by an absorbing pad bolus. Similarly, the heati ng area of the MA-120 was increased by a factor of 2.6 by an absorbing pad bolus. The boluses were tested in a clinical setting by measuring tissue temperature profiles in patients under different bolus arrange ments. The area over which therapeutic temperature was achieved was in creased considerably when the absorbing bolus was used. A second bolus was designed for the MA-120 to produce a ring heating pattern for the treatment of a breast cancer patient who had developed recurrences at the periphery of a skin graft. The heating pattern produced in a musc le phantom is compared with tissue temperature profiles measured durin g the hyperthermia treatment of this patient. Conclusions: Microwave a bsorbing filters using saline pads significantly improve the heating p atterns of microwave wave guide hyperthermia applicators. This improve ment was confirmed in clinical application where much greater areas of homogeneous heating were observed. The technology was extended to pro duce complex heating patterns for special clinical applications.