Osteosarcoma in the dog is a highly aggressive cancer that is often fatal. Management of this disease involves controlling the local tumor as well as its regional and distant metastasis. Despite aggressive therapy with radiation therapy, surgery and chemotherapy, median survival is about one year. The AKT/mTOR pathway is constitutively active in many forms of human cancer. We have found that this pathway is also activated in canine osteosarcoma. We have also shown that this pathway can be blocked using the drug rapamycin. Several studies in human cancer cell lines have shown that rapamycin can increase the radiosensitivity of in vitro. The goals of this study were to test the effect that the combinations of rapamycin and radiation have on cellular proliferation, clonogenicity and cell cycle kinetics in canine osteosarcoma. Results suggest that treatment intensification by a 2-week interradiation treatment interval coupled with interradiation chemotherapy is an effective treatment for dogs.