Histopathological and Immunohistochemical Characterization of Sebaceous Adenoma and Epithelioma in Dogs

Sebaceous gland tumors are neoplasms originating from the sebaceous gland and are the third most common type of skin tumor, accounting for 21-35% of all cutaneous neoplasms in dogs. According to their histopathological characteristics, sebaceous gland tumors can be classified into adenoma as a benign tumor and epithelioma as a malignant tumor. Sebaceous epithelioma is distinguished from sebaceous adenoma by containing 90% or more reserve cells. However, this simple numerical criterion is insufficient to histologically distinguish between epitheliomas and adenomas. In addition, sebaceoma in humans, a similar tumor to sebaceous epithelioma, is a term used for tumors with more than 50% of reserve cells, unlike epithelioma. Therefore, we aimed to compare and characterize the histological and immunohistochemical profiles of comprehensive sebaceous adenoma, epithelioma, and borderline tumors that have more than 50% but less than 90% of reserve cells. A total of 14 canine sebaceous tumors were diagnosed as seven adenomas, four borderline tumors, and three epitheliomas. Histologically, the sebaceous adenomas showed nodules consisting of mature sebocytes surrounded by monolayer basaloid cells. In contrast, the portion of the reserve cells was increased, the portion of lipidized cells was decreased, and the majority of lipidized cells were found to be immature in sebaceous epithelioma. In the sebaceous adenomas, necrosis was not observed and mitotic figures were rarely seen. However, necrosis and mitotic figures were highly frequent in both borderline tumor and sebaceous epithelioma. Immunohistochemistry revealed that borderline tumor and sebaceous epithelioma showed significantly higher expression against Ki-67 than sebaceous adenoma. We conclude that it is more accurate to employ the cut-off value of 50% reserve cells in humans rather than the current 90% reserve cells for classifying sebaceous gland tumors in dogs, thereby providing new insight into the characterization of the sebaceous gland tumors.