Canine mast cell tumors are the most common cutaneous malignant tumors in dogs. Histologic grades, I, II, and III, provide very useful information because they are indicative of a patient’s prognosis. Treatment options depend on negative prognostic factors, but in general surgery and radiation therapy are very effective. Chemotherapy could be applied if the patient has either systemic disease or a grade III mast cell tumor. The most commonly used chemotherapy drugs for mast cell tumors are CCNU and vinblastine. A new therapeutic agent, Palladia, targets a receptor on mast cell tumors called c-kit (CD117). Dr. Shiomitsu will investigate if Palladia can enhance the radiosensitivity of canine mast cell tumor cells in vitro and determine the mechanism of radiosensitization when it occurs. Radiosensitization by Palladia may be able to improve local tumor control, and hopefully prolong survival time for radiation oncology patients.