Dermal melanocytoma-acanthoma in an adult mixed breed dog.

A cutaneous melanocytoma-acanthoma was diagnosed in a 7-year-old intact female mixed breed dog. Grossly, this tumor was a solitary and darkly pigmented nodule located in the face. Histologically, the lesions consisted of melanocytic and epithelial tumor cells. The melanocytic component consisted predominantly of large round-to-polygonal and heavily pigmented melanocytic cells arranged in nests and clusters. These melanocytes were positive for S-100 and vimentin. The epithelial component was composed of multiple small horn cysts with concentric keratin within the cyst lumina and was positive for cytokeratin. Atypism was not observed in both components. Since this tumor has previously been reported in only two dogs, this report adds to the data that will help determing predilections of age, breed, sex and site.