Prognostic Value of Histologic and Immunohistochemical Features in Feline Cutaneous Mast Cell Tumors

Mast cell tumors (MCTs) are common tumors of the skin in cats. Histologically, feline cutaneous MCTs have been classified as mastocytic (well-differentiated or pleomorphic) and atypical/poorly granulated. The biologic behavior ranges from benign to malignant, but prognostic factors are not well defined. Histotype, mitotic rate, cytoplasmic granules, eosinophils and lymphocytic infiltrates were evaluated retrospectively in 25 feline cutaneous MCTs. Immunohistochemistry was applied to assess KIT (CD117) pattern and KIT immunoreactivity score, telomerase activity (hTERT) and proliferative index (MIB-1/Ki67-index). Follow-up information was obtained via telephone interviews. There were 15 well-differentiated, 7 pleomorphic and 3 atypical/poorly granulated MCTs. Immunohistochemical analyses revealed cytoplasmic expression of CD117 in 13/25 samples (52%), and TERT was expressed in 15/22 samples (68%), with no relation to histotype. Mitotic rate, KIT immunoreactivity score and Ki67-index were significantly higher in pleomorphic MCTs. Follow-up data were available in all cases. Five cats (20%) died of tumor-related causes. Multiplicity of lesions, pleomorphic histotype, KIT immunoreactivity score and high mitotic rates/Ki67-indexes were statistically correlated with an unfavorable outcome. Mitotic rate was the strongest predictive variable. Our findings suggest that histological type, CD117 immunohistochemistry and tumor proliferative activity may be of help to identify those cases of feline cutaneous MCT that should be considered potentially aggressive and therefore require close monitoring. Aberrant KIT protein localization and presence of telomerase activity warrant further exploration as potential therapeutic targets.