Multiple cutaneous mast cell tumors displaying epitheliotropism in a male cat.

A 16-year-old castrated male domestic medium-hair cat was referred for examination of multiple cutaneous mast cell tumors displaying epitheliotropism. Staging tests were negative for distant metastasis. Toceranib phosphate therapy was initiated. The cat improved clinically and had a strong partial response. Tumor progression was noted 13 wk later, at which time the cat was treated with lomustine. The cat was euthanized 11 d later. Epitheliotropism is an uncommon finding in mast cell tumors and may be associated with more aggressive behavior. Key clinical message: Feline cutaneous mast cell tumors displaying epitheliotropism may follow a more aggressive clinical course. Toceranib should be considered as an initial treatment for these tumors. Further research is needed to determine the prognosis for cats with epitheliotropic mast cell tumors.