Treatment of canine mast cell tumours with prednisolone and radiotherapy.

This retrospective study describes 35 dogs with non-resectable, grade I-III mast cell tumours on the head or limb treated with prednisolone (40 mg m(-2) daily) for 10-14 days prior to radiotherapy (4 x 800 cGy fractions at 7-day intervals) from a 4 MV linear accelerator. Prednisolone was continued at a reduced dose rate (20 mg m(-2)) during radiotherapy and for 2 months or longer afterwards. Eighteen of 24 tumours (75%) decreased in size in response to prednisolone treatment. By 6-8 weeks following radiotherapy, 12 dogs had achieved a complete remission and 19 a partial response. Two tumours remained static and two progressed during the course of treatment. The overall response rate was 88.5%. With long-term follow-up, 11 dogs experienced local recurrence (n = 4), metastasis (n = 5) or both (n = 2). The median progression-free interval was 1031 days (95% CI 277.44-1784.56, Kaplan-Meier), with 1- and 2-year progression-free rates of 60 and 52%, respectively. Tumour grade did not predict the prognosis for this group of dogs, but tumour location did affect the outcome. Dogs with tumours located on the limb survived longer than those with tumours on the head. The combination of prednisolone with radiotherapy appears to have a useful role in the management of measurable mast cell tumours sited on the head and distal extremities.