+ 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


+ 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

  • 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

+ Prognosis

  • 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%


+ Diagnosis

  • 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

+ Treatment

  • 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

+ Metastasis

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