Hyperthermia has been shown to substantially increase the effectiveness of radiation and drugs in the clinical management of cancer, but the response of tumors to combined therapy is highly dependent upon the uniformity and localization of heating within the target volume. Recent studies have shown that applying the heat, and radiation concurrently will significantly improve tumor control, and is most dramatic for long duration interstitial heating with concurrent brachytherapy. The few interstitial heating technologies which are compatible for combined therapy exhibit an inability to effectively control the temperature distribution along the length of the implant, which often translates into poor heating uniformity and a low minimum thermal dose. This proposal describes a research plan to develop an interstitial heating system which will be more effective than former technologies in delivering simultaneous interstitial thermoradiotherapy to previously difficult to treat sites such as brain. This includes: l) the development of directly implantable multielement ultrasound applicators that feature dynamic control of the longitudinal power deposition and allow for the simultaneous insertion of radiation sources; 2) the development of treatment optimization and control software to preplan the stereotactic placement of applicators with respect to thermal and radiation dosimetry considerations, and regulate power levels to attain uniform temperature distributions of maximum thermal dose. The impact of this approach will be most significant for targets in brain where sparse implants and non-parallel alignment of sources are often necessary. In the future, this system may be readily adapted for use in other sites such as prostate, where advantages such as the ability to adjust the power deposition along the length of each applicator and precisely control the localization of heating are essential. The primary objectives are to produce interstitial applicators and associated planning & control programs which will provide more uniform and bigger overall thermal dose distributions than currently available interstitial heating technologies, and to facilitate the delivery of simultaneous thermoradiotherapy. The development of this heating technique will provide an improved heating modality ready for Phase I/II trials of long duration simultaneous thermoradiotherapy.