Immunohistochemical prognosticators in feline cutaneous and visceral mast cell tumors.

Background. Mastocytomas (MCTs) are common tumors of the skin, intestinal tract, and spleen in cats. Histologically, feline MCTs are classified as mastocytic (well differentiated or poorly differentiated/pleomorphic) and histiocytic/atypical. The biological behavior ranges from benign to malignant, but prognostic factors are not well defined. Methods. Cell differentiation, growth pattern, infiltrating eosinophils, lymphoid aggregates, erythrophagocytosis and collagen degeneration were evaluated retrospectively in 48 feline MCTs. Immunohistochemistry was also applied to assess KIT pattern (CD117), cell proliferation (Ki67/MIB1-index) and telomerase activity (hTERT). Follow-up information was obtained via telephone interviews. Results. Histologically, there were 36 cutaneous MCTs (18 well differentiated, 10 pleomorphic and 8 atypical), 7 splenic (5 well differentiated and 2 atypical) and 5 intestinal (1 pleomorphic and 4 atypical). Immunohistochemistry revealed cytoplasmic expression of CD117, either focally clustered or diffuse, in 91% of cases; proliferative activity and inflammatory infiltrates were more marked in pleomorphic and atypical MCTs. h-TERT was expressed in 52% of samples, regardless of the histotype. Survival data were achieved in 26 cases. Six cats (23%) died because of tumor-related causes (5 pleomorphic and 1 atypical; 4 cutaneous and 2 intestinal). Ki67-index and inflammatory infiltrate were negatively associated to survival. Conclusion. Our findings suggest that cell morphology (i.e. pleomorphic type), accompanying inflammatory infiltrate and Ki67 immunohistochemistry may be of help to decide which cases are to be considered more at risk and therefore require closer monitoring. Aberrant KIT localization and telomerase activity advise further exploration of these pathways as potential therapeutic targets.