Clinico-pathological features and treatment options for canine mast cell tumor with bone marrow involvement in the imatinib era: disappointment or challenge?

Background: Canine mast cell tumor (MCT) with bone marrow (BM) involvement is scarcely documented and is associated with a poor prognosis. Successful treatment strategies have not been described. Hypothesis: Clinico-pathologic findings of affected dogs are not specific. Administration of lomustine or imatinib mesylate, a tyrosine kinase inhibitor, is beneficial. Animals: 13 dogs affected by MCT with BM involvement. Methods: Clinical and laboratory evaluations were performed in each dog on admission and during follow-up. Eight dogs received lomustine, 2 imatinib and 3 were treated symptomatically. Imatinib was administered if the tumor-associated tyrosine kinase KIT was aberrant. Results: On admission 10 dogs had a single cutaneous nodule while 3 had multiple nodules. Involvement of regional lymph nodes, liver or spleen was observed in each case. BM infiltration with mast cells was observed in all dogs. On blood work, nonregenerative anemia, leukopenia or thrombocytopenia were common. Four dogs had circulating mast cells. Increased alkaline phosphatase and alanine transferase were observed in 11 and 10 dogs, respectively. Treatment with lomustine induced partial remission in one out of 8 dogs. Median survival time was 43 days (range, 14-57). Both dogs on imatinib experienced complete remission. Survival time was 117 and 159 days, respectively. Dogs treated symptomatically did not improve and were euthanized after 1, 14 and 32 days.