BACKGROUND: Radiation therapy is commonly used as an adjunct to incomplete surgical excision in dogs with mast cell tumors (MCT), but the optimal dose and fractionation regimen have yet to be determined. HYPOTHESIS: We assessed outcomes (time to local recurrence, patient survival and toxicity) of a large population of dogs with MCT that received adjunctive radiation therapy. ANIMALS: Three hundred dogs with 302 MCT treated using adjunctive radiation therapy. METHODS: Retrospective observational study. Clinical records of 4 veterinary radiation centers were reviewed. RESULTS: Local recurrence rates were similar regardless of radiation protocol with 6.6% of patients developing recurrent cutaneous MCT at a median of 526 days. Local recurrence rate was similar between high and low-risk MCT. Mast cell tumor related death was reported in 19% of all dogs, with 13% of dogs with low-risk MCT dying of their disease compared to 29% of dogs with high-risk MCT. No SC MCT (SCMCT) recurred after radiation therapy and only 7% of dogs with SCMCT were reported to have died of their disease. Mild late toxicity was common in both protocols and severe late toxicity occurred in 1.9% of dogs many years after treatment. CONCLUSIONS AND CLINICAL IMPORTANCE: Our study supports the use of adjunctive radiation for the long-term control of incompletely or narrowly excised cutaneous and SCMCT in dogs. More moderate dose and fractionation protocols may be appropriate in the adjunctive treatment of low-risk MCT in dogs. Large multicenter prospective studies are required to establish the optimal dose and fractionation for MCT of different risk categories.