Dermal melanoma with plasmacytoid differentiation in a dog

A 10-y-old spayed female Staffordshire Terrier dog was evaluated because of a cutaneous left ear base mass. Cytology revealed sheets of cells with anisocytosis and anisokaryosis, round-to-oval or plasmacytoid cytoplasm, and round, central, or eccentric nuclei; binucleate cells were present. Cytologic findings were consistent with a round cell tumor (plasmacytoma or agranular mast cell tumor), amelanotic melanoma, or anaplastic carcinoma. Histologically, neoplastic cells were polygonal to elongate, with round-to-oval nuclei and prominent nucleoli and arranged in sheets and nests on a fibrovascular stroma. Neoplastic cells with plasmacytoid morphology (round, glassy, eosinophilic cytoplasm with eccentric nuclei) were present in ~30% of the neoplasm. There were 18 mitoses in 2.37 mm