Multicenter Prospective Trial of Hypofractionated Radiation Treatment, Toceranib, and Prednisone for Measurable Canine Mast Cell Tumors

Background: Mast cell tumors (MCT) are common cutaneous tumors in dogs and when not amenable to surgical excision can present a therapeutic challenge. New treatment protocols for unresectable MCT are needed. Hypothesis: The combination of toceranib, prednisone, and hypofractionated radiation treatment (RT) will be well tolerated and efficacious. Animals: Seventeen client-owned dogs with measurable MCT amenable to RT. Methods: Prospective clinical trial. All dogs received prednisone, omeprazole, diphenhydramine, and toceranib. Toceranib was administered for 1 week before initiating RT, consisting of 24 Gy delivered in 3 or 4 fractions. Results: On an intent-to-treat basis, the overall response rate was 76.4%, with 58.8% of dogs achieving a complete response and 17.6% a partial response. The median time to best response was 32 days, and the median progression-free interval was 316 days. The overall median survival time was not reached with a median follow-up of 374 days. The most common toxicoses were gastrointestinal and hepatic. Conclusions and Clinical Importance: The combination of hypofractionated RT, toceranib, and prednisone was tolerated and efficacious in the majority of dogs. Response rates and durations were higher than those reported for toceranib as a single-agent treatment for MCT. This combination is a viable treatment option for unresectable MCT.