Hyperthermia has been shown to increase the effectiveness of radiation therapy in the treatment of superficial tumors. However, the technical aspects of heating deep seated tumors have not yet been thoroughly solved, although interstitial techniques, focussed ultrasound and possibly phased array microwave systems have the potential to heat these tumors. An alternative nonsurgical approach to the above methods is to improve the temperature distributions by utilizing intracavitary heating devices is the small amount of control over the induced temperature distribution and the shallow penetration depth of the applied energy. In this study we propose to use our experience with ultrasound intracavitary transducer arrays to overcome the above mentioned limitations. First, ultrasound penetrates well in tissue. Second, the transducer elements can be made small and thus, arrays of transducers can be used. By driving each of these transducers with separate signals, the temperature distributions can be controlled. Third, it is possible to further enhance the penetration depth and temperature control by utilizing electrical focussing of the ultrasonic field. (Our studies indicate that therapeutic temperatures can be achieved in tissues up to 4-5 cm from the cavity wall by using electrical focussing). We have shown in our preliminary experiments that an ultrasonic transducer array of practical size can be constructed and that enough acoustical power can be produced to induce hyperthermia in dog's prostate in vivo. In addition, the physical characteristics of these arrays have been optimized using simulations and experiments. During the past few years the commercial transrectal diagnostic ultrasound arrays have been developing rapidly and in many cases an endocavity transducer offers the best images of the tumors located close to cavities. In this proposal we plan to use our experience to construct clinical intracavitary ultrasonic arrays, combine them with a commercial transrectal ultrasound imaging array, develop the driving hardware and software, evaluate the system in animal experiments and then test the feasibility and toxicity of intracavitary ultrasonic hyperthermia combined with sequential radiation therapy in a Phase I patient trial. Once the feasibility of heating the tumors has been established a dose escalation Phase I trial with hyperthermia combined with simultaneous radiotherapy will be executed. Our goal is to develop a hyperthermia system that is easy to use and which produces good temperature distributions in selected, clinically significant tumors.