Biphasic mammary tumor in a cat: Case report

Feline mammary tumors (FMT) are the third most common neoplasm in older domestic cats. Among these, biphasic tumors (BT) are particularly rare. BT feature two distinct neoplastic cell populations, a carcinomatous component and a non-epithelial one. This paper describes the histological and immunohistochemical features of a biphasic mammary tumor in a 7-year-old Domestic Shorthair female cat. Histopathology identified two malignant cell populations: a tubular epithelial proliferation and a highly cellular and pleomorphic mesenchymal population. Immunohistochemistry (IHC) for pan-cytokeratin, cytokeratin 18, cytokeratin 14, and cytokeratins 5/6 labeled the epithelial component of the tumor. The mesenchymal population was diffusely positive for vimentin and α-SMA, occasionally positive for calponin, and negative for p63. Ki-67 index was higher in the epithelial cells (88.8 %) compared to the interstitial cells (22.9 %). Based on histological and immunohistochemical findings, a diagnosis of carcinosarcoma was made. This study highlights the importance of including mammary BT in the differentials of feline mammary neoplasms, despite their rarity.