Ovarian leiomyoma with an epithelioid differentiation in a dog

A 12-years-3-months-old female Toy Poodle underwent ovariohysterectomy, and an 8-mm ovarian mass was incidentally identified on pathological examination. On the cut surface, the mass consisted of a yellowish-white solid oval area surrounded by a soft white crescent-shaped peripheral area. Histopathologically, the oval area was composed of interlacing fascicles of spindle neoplastic cells with elongated to cigar-shaped nuclei, whereas the crescent-shaped peripheral area contained solitary or clustered polygonal to round neoplastic cells scattered within myxoid stroma. Immunohistochemically, both tumor cell populations were positive for α-smooth muscle actin, desmin, and vimentin. Cytokeratin expression was observed in the polygonal to round neoplastic cells, but not in the spindle neoplastic cells. Collectively, the tumor was diagnosed as an ovarian leiomyoma with an epithelioid differentiation.