Novel labial adnexal sebaceous basaloid proliferation associated with sebaceous epitheliomas and adenocarcinomas in Labrador retrievers.

A distinctive peritumoral epithelial proliferation was identified exclusively in the labial region of Labrador retrievers in association with sebaceous neoplasms. To characterize this lesion, 515 sebaceous neoplasms diagnosed in dogs of different breeds between 2017 and 2025 were retrospectively reviewed. Among these, 2 of 255 epitheliomas (0.7%) and 22 of 41 adenocarcinomas (54%) arising in the lips exhibited a distinctive peripheral basaloid epithelial proliferation. The lesion was observed only in the lower lip of Labrador retrievers, occurring in association with either solitary or multifocal neoplastic nodules. No similar proliferations were identified after examining 103 labial lesions of other histotypes diagnosed in the same timeframe. Histologically, the proliferations consisted of 1- to 2-cell-thick epithelial cords surrounding pre-existing adnexal units. The cells showed minimal atypia and low mitotic activity. Immunohistochemically, they exhibited a cytokeratin (CK)5+/p63+/alpha-smooth muscle actin-/CK7- phenotype, supporting derivation from basal reserve cells of cutaneous glands and excluding differentiation toward mature apocrine epithelial or myoepithelial cells. The exclusive occurrence in Labrador retrievers suggests a possible breed-related predisposition. Although the biological behavior of this novel lesion is unknown, its recognition may be important in diagnostic pathology to avoid misinterpretation with an invasive adenocarcinoma.