Mast Cell Tumors: Cats vs Dogs

Mast cell tumors (MCTs) account for 2%–15% of tumors in cats. Cytologic diagnosis is easy to obtain, but there is no reliably predictive grading scheme. Feline MCTs are classified into cutaneous and visceral forms, with cutaneous predominating. Cutaneous tumors are histologically divided into atypical and mastocytic forms. The latter are more common and further classified as poorly differentiated or well differentiated. Atypical MCTs usually affect cats <4 years of age and spontaneously regress over 4–24 months. Visceral MCTs most commonly affect the spleen and intestines.

Complete staging should be performed in all cats with visceral MCTs, multiple cutaneous nodules, signs of systemic disease, abnormalities on abdominal palpation, or tumors with abnormal behavior or histology. Buffy coat smear evaluation is recommended; it is specific for MCTs and has been reported in cats with single cutaneous tumors. Multiple cutaneous tumors may represent metastasis from visceral MCT. Effusions can be seen in up to one-third of visceral MCT cases. Histamine blockade is recommended and should be continued until complete surgical excision is confirmed, or for life in cases with visceral disease. Surgical excision is preferred, with good prognosis for most cutaneous tumors. The exceptions are poorly differentiated mastocytic tumors that are more likely to metastasize; wider surgical margins are recommended. Cats with surgical excision of splenic MCTs survive 12–19 months with splenectomy alone. Overall prognosis for intestinal MCT is poor; metastasis is common.

MCTs are more common in dogs than in cats, and although it is tempting to assume that these tumors have similar behaviors in both species, there are some very important differences: First, tumor grade is a key prognostic factors for cutaneous MCT in dogs but not in cats. When dogs have multiple cutaneous MCTs, they usually are independently rising de novo tumors and do not affect prognosis; in cats, multiple cutaneous MCT can represent metastasis from a primary visceral tumor and often are more difficult to treat. Most dogs with visceral mast cell disease have a primary cutaneous MCT that has metastasized to liver and/or spleen. Cats with visceral MCT more commonly have a primary visceral tumor, usually arising in the spleen or intestine. Surgery is not recommended for dogs with stage-IV MCT, but most cats with a primary splenic MCT have metastasis to liver, bone marrow, and/or skin at the time of diagnosis, yet splenectomy is still recommended. This substantially helps improve signs and efficacy of follow-up therapy. There are reports of remaining metastatic disease spontaneously regressing post splenectomy. Feline MCTs release serotonin in addition to histamine. I, therefore, prefer cyproheptadine instead of diphenhydramine, as it blocks both H1 and serotonin receptors. It has the additional benefit of increasing appetite.—Dennis Bailey, DVM, DACVIM (Oncology)

Mast cell tumors in cats: Clinical update and possible new treatment avenues. Henry C, Herrera C. J FELINE MED SURG 15:41-47, 2013.