•2nd most common skin tumor (8% in Great Britain and 20% in USA)

•feline mast cells also contain surface-bound immunoglobulins capable of secreting histamine, heparin, and other vasoactive compounds

•feline mast cells also have phagocytic capability and can endocytose erythrocytes

•degranulation can cause coagulation disorders, GI ulceration, and anaphylactoid reactions


General Considerations

•2 types: mastocytic (similar to dog and more common) and histiocytic (less common)

•edema, eosinophilia, and collagen denaturation associated with canine MCT are not seen in feline MCT

•50% of cats with cutaneous MCT have tumors develop in other sites within weeks to months after surgery

Mastocytic Mast Cell Tumor

General Considerations

•mean age: 10 years

•no sex or breed predilection, but Siamese cats are over-represented in some reports

•cutaneous MCT are often malignant (44%-59%) and associated with multiple cutaneous or visceral involvement

•feline MCT are usually solitary (< 1 cm diameter), firm, and raised with superficial ulceration in 25%

•20% multiple

•sites: 45% of cutaneous MCT found on head and 21% on limbs

•2 clinical forms: flat pruritic plaque-like lesion and discrete subcutaneous nodule

•feline cutaneous MCT is subdivided into compact and diffuse forms which may have prognostic significance

•histologic grading system used in dogs does not have prognostic value in cats

Compact Mastocytic Mast Cell Tumors

•compact MCT accounts for 50%-90% cases and most are benign

•compact MCT: homogenous cords and nests of slightly atypical mast cells with basophilic round nuclei, ample eosinophilic cytoplasm, and distinct cell borders, with 50% eosinophils

Diffuse Mastocytic Mast Cell Tumors

•diffuse MCT are more anaplastic and many are malignant with presenting signs of generalized, pruritic, and miliary lesions combined with alopecia, erythema, excoriations, and diffuse lichenification

•diffuse MCT: less discrete and infiltrates into the subcutaneous tissue, larger nuclei > 50% of cell size, 2-3 mitotic figures per HPF, marked anisocytosis with mononuclear and multinucleated giant cells, and eosinophils are common

Histiocytic Mast Cell Tumor

•mean age: 2.4 years

•sex predisposition: ± male

•breed predilection: ± Siamese

•spontaneously regressing multicentric histiocytic form of MCT in Siamese < 4 years

•spontaneous regression occurs over 4-24 months

•diagnosis: histology with histiocyte-like cells with equivocal cytoplasmic granularity, randomly scattered lymphoid aggregates, and eosinophils

•DDx: granulomatous nodular panniculitis or deep dermatitis


•metastatic rate for cutaneous MST variable: 0%-24% with recurrence ± metastasis more likely for diffuse form

•local tumor recurrence usually observed < 6 months

•spontaneous regression is possible for histiocytic form and observation is appropriate

•histopathologic grade has no prognostic significance

•incomplete excision is not associated with a higher rate of local tumor recurrence

•cutaneous MCT is not associated with metastatic disease [2.5% (4/160)] or death

•12-month DFI 84%, 24-month DFI 65%, and 36-month DFI 52%



•FNA of cutaneous mass: granules stain blue with Giemsa and purple with toluidine blue and appear more eosinophilic with hematoxylin and eosin stains

•tissue biopsy and histology required for diagnosis of histiocytic MCT

•disseminated disease: hematology, serum biochemistry, buffy coat smear, bone marrow aspirate, and coagulation profile

•cats with systemic mastocytosis will have eosinophilia, basophilia and peripheral mastocytosis (50%)

•mast cells can account for up to 25% of white blood cells in cats

•coagulation abnormalities reported in 90% of cats with splenic MCT, but rarely clinically significant

•methylated metabolites of histamine in urine may be a valuable diagnostic technique for mastocytosis


•surgery: resection for solitary cutaneous MCT, splenectomy for splenic MCT, and resection (5-10 cm margins recommended) with end-to-end anastomosis for intestinal MCT

•wide surgical margins are not required for cutaneous MCT, unless diffuse form, due to their benign behaviour

•effectiveness of adjunctive therapy unknown

•combination chemotherapy protocols using prednisone, vincristine, cyclophosphamide, and methotrexate have not offered a survival advantage over surgery alone


•2 forms of metastasis in feline MCT:

•dermal and subcutis MCT metastasize to lymph node and viscera (i.e., similar to dog)

•MCT originates in hematopoietic cell precursors and disseminates throughout viscera







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