Canine mast cell tumors (MCT) of high grade, nonresectable, with low margins or recurrent after surgery require adjuvant therapies. Aim of this study was to evaluate the toxicity and the efficacy of radiotherapy (RT) with concomitant and subsequent toceranib, chlorambucil and prednisone.

Fourteen dogs were treated with curative RT for MCT (three G2SI of lips, one G3SIII of hard palate, one G2SIII of digit), with adjuvant RT for MCT recurrent after surgery (two G3SIII of shoulder muscles) or resected with low margins (three G2 of interscapular muscles, two G2 of stifle skin, two G2 of penis skin).

The dose to the tumor bed and local lymph nodes was 4 x 800 cGy fractions at 7-day intervals. During the treatment, toceranib (2.5 mg/kg/48 h), chlorambucil (0.1 mg/kg/24 h) and prednisone (0.5 mg/kg/24 h) were administered, continuing toceranib for 6 months, chlorambucil and prednisone for 2 months. Toxicity, response, and survival were evaluated along 2.5 years.

All the dogs achieved a complete remission. All three dogs with MCT G3 experienced local recurrence and metastasis, with progression-free interval (PFI) of 8 months and median survival time (MST) of 13 months. Median PFI and MST were not reached in dogs with MCT G2. Six of 14 dogs have requested temporary suspension of chemotherapy. Five and three dogs experienced, respectively, grade II acute and grade I late radiation morbidity. The protocol is useful in the treatment of canine MCT with slight toxicity; however, an extension of the study is advisable.