Mast cell tumours in dogs less than 12 months of age: a multi-institutional retrospective study

Objectives: To describe the clinicopathological and genetic characteristics of mast cell tumours in dogs less than 12 months old. Materials and methods: Retrospective review of dogs aged less than 12 months when diagnosed with mast cell tumours at three referral hospitals in the UK. Results: Sixteen pure-bred dogs were included, of which 11 were female. The median age at first presentation and diagnosis were 7.6 and 9 months, respectively. In 13 dogs the mast cell tumours were cutaneous and in three they were subcutaneous. Four cutaneous mast cell tumours were described as high-grade (Patnaik or Kiupel) and nine were Patnaik grade II; three had mitotic index of >5 in 10 high-power fields. Of the three subcutaneous tumours, two had an infiltrative growth pattern and one had mitotic index of 10 per 10 high-power fields. Of 10 tested dogs, seven had c-kit mutations in exon 11 and Ki-67 score was above the cut-off value in nine. Four of 12 cases showed evidence of metastasis in the regional lymph nodes. After varying treatment protocols, all patients were alive and disease free at a median of 1115 days after diagnosis. Clinical significance: The prognosis of mast cell tumours in dogs less than a year old appears better than the adult counterparts, even without extensive treatment.