+ General Considerations

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

CUTANEOUS MAST CELL TUMOR

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

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 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 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

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