Cytologic aspect of keratoacanthoma with granulomatous inflammation in a dog

A 5-year-old Pekinese dog was presented for evaluation of a back mass of 1 year duration. Fine needle aspiration cytology of the mass revealed numerous keratinocytes, and a lesser number of multinucleated giant cells along with a low number of spindle cells, which were suggestive of epidermal cyst or hair follicle tumor with secondary granulomatous inflammation. The mass was surgically removed and examined histologically. Microscopically there was partially encapsulated well-demarcated nodule in deep dermis and subcutis. The nodule consisted of central cyst and secondary cysts that were lined by a single or multiple layers of basaloid cells and squamous epitheliums with central laminated keratin. Multifocal aggregation of numerous macrophages and multinucleated giant cells were present. The final diagnosis was keratoacanthoma with granulomatous inflammation. The mass did not recur 5 months after surgery.