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.