Encapsulated radioactive sources emitting ionizing radiation in the form of high energy photons are widely used for treatment of cancer known as brachytherapy. In this treatment radioactive sources are placed directly in the tumor volume (interstitial brachytherapy) or in close proximity to the tumor in body cavities (intracavitary irradiation). Recently there has been considerable interest in the development of brachytherapy sources emitting lower energy photons in the energy range of 10 to 100 keV because they offer advantages over higher energy photon emitters in terms of ease of radiation shielding within the patient, and radiation protection of medical personnel, patient and his family. Following this trend, Samarium-145 sources for brachytherapy were developed by BEST INDUSTRIES in the Phase I grant. These sources emit photons primarily in the energy range of 38 to 45 keV and decay with a half life of 340 days. In addition to the advantages of the ease of radiation shielding, Samarium-145 sources emit photons with energies just above the K-absorption edge of iodine (33.2 keV). This characteristic makes Samarium-145 sources an ideal candidate for radiation enhancement with thymidine analog iodinated deoxyuridine (IUdR) by stimulating cascades of Auger electrons resulting from photoionization of the iodine. Thus, Samarium-145 offers a new possibility for maximizing the effects of IUdR radiosensitization in combination with brachytherapy. This Phase II project will develop prototype Samarium-145 sources, determine their physical dosimetric characteristics and test their efficacy in an animal tumor model.