Desmoplastic tricholemmoma in a dog

A 10-year-old mixed breed dog was presented with a 0.8 cm diameter mass below the left eye region. The mass was surgically removed and processed for histopathological examination. Microscopically, tumor cells proliferated in small lobules, nests and cords, and the tumor parenchyma was separated by desmoplastic stroma. Majority of the tumor cells were periodic acid-Schiff (PAS)-positive, and the desmoplastic stroma was densely collagenous and mucinous. Immunohistochemical results showed that the tumor cells were diffusely positive for cytokeratin 15, cytokeratin 19 and CD 34, while cytokeratin 8 reactivity was limited to the tumor cells proliferating in cords. Few tumor cells were positive for nestin. Based on the histopathological findings, the tumor was diagnosed as desmoplastic tricholemmoma.