Mammary carcinoma with sebaceous differentiation in a dog

This report describes an invasive mammary carcinoma with a rare distinctive feature characterized by sebaceous differentiation of tumor cells. This tumor occurred in a 10-year-old female mixed breed dog. The patient had two masses in the left fifth mammary gland. Grossly, the masses were firm, whitish to light brown, and superficially ulcerated. On cut surface, they were multilobulated with foci of necrosis. Microscopically, the tumors were composed of two distinctive neoplastic components, intraductal papillary adenocarcinoma and sebaceous carcinoma. The regions of sebaceous tumor were clumped separately, contained well-developed sebaceous cells and keratinized epithelial cells, and were surrounded by few to several layers of basaloid cells. The cells with abundant foamy cytoplasm that resembled sebaceous cells were also found within the intraductal papillary-like nests of mammary carcinoma, providing evidence of sebaceous metaplasia. Sebaceous differentiation in a mammary gland tumor is possible, because skin appendages and ductal apparatus of the mammary gland share a common anlagen. This tumor had an aggressive behavior with lymphatic metastasis. Consequentially, the dog had a poor prognosis.