Mast cell tumor in cats typically presents as a skin mass composed of slightly pleomorphic mast cells with 0-1 mitotic figures/10 HPF (400x) and vague granulation.  There can be variably dense eosinophilic infiltrates.  Unlike canine mast cell tumors which have not just one but 2 prognostically significant grading systems (the Patnaik and the Kiupel/MSU systems), there is not a widely recognized grading system for feline mast cell tumors. Molecular parameters such as Kit receptor analysis are also not applicable at this time.  A recent study however, reported by Sabattini and Bettini  in Veterinary Pathology 56(1), 43-49, 2019 finds that tumors with >5 mitotic figures/ 10 HPF and 2 of the 3 criteria such as tumor diameter over 1.5 cm, irregular nuclear shape, and nucleolar prominence, exhibited high grade behavior with reduced survival time (median 349 days). 


This tumor should always be considered to have potential for local invasion and distant metastasis. Tumors may arise in the skin or in the internal organs, usually the spleen. Upon diagnosing this tumor in the cat, check the local lymph nodes, watch adjacent skin for other masses, palpate (gently) the liver and spleen for organomegaly that might indicate visceral mast cell tumor, and check the CBC for circulating mast cells. Aggressive palpation may result in mast cell degranulation, release of vasoactive amines, and hypotension, so care is warranted. It is important to determine if the margins are clean with histologic evaluation as complete excision of this type of tumor is often curative.