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

Objectives Mast cell tumours (MCTs) are the most common skin neoplasia of middle-aged to older dogs, but little information exists about their biological behaviour in dogs younger than 12 months. The aim of this retrospective study was to describe the clinicopathological and genetic characteristics of MCTs in dogs less than one year-old.ResultsSixteen pure-bred dogs were included. Sixty nine percent of patients were female. The median age at first presentation and diagnosis was 7.6 months and 9 months, respectively. Thirteen had cutaneous MCTs and three subcutaneous MCTs. Four cutaneous MCTs were described as high-grade (Patnaik or Kiupel) and nine were Patnaik grade II; three had mitotic index (MI) >5 in 10 high power fields (HPFs). Of the three subcutaneous MCTs, two had an infiltrative growth pattern and one had MI of 10/10 HPFs. Presence of c-kit mutation and Ki-67 score were assessed in 10 MCTs: seven had mutations in exon 11 and Ki-67 was above the cut-off value in nine.The regional lymph nodes were assessed in 12 MCTs and four cases were considered metastatic. The patients underwent variable treatment protocols, including active monitoring in seven cases (43.8%). All patients were alive and disease free at a median of 1115 days post-diagnosis. StatementThe prognosis of MCTs in dogs less than a year appears better than the adult counterparts, even without extensive treatment. Based on this study MCTs in this cohort of dogs might show better clinical behaviour despite the presence of negative prognostic markers compared to adult-older dogs.