Cutaneous invasive micropapillary carcinoma of probable apocrine sweat gland origin in a cat

An invasive micropapillary carcinoma (IMC) occurred in the buccal skin of an 18-year-old female cat. Histologically, the tumor had a honeycomb pattern characterized by clusters of neoplastic epithelial cells that were surrounded by empty clear spaces and lined with fibrocollagenous stroma. On immunohistochemistry, the neoplastic cells were positive for cytokeratin (clone CAM5.2; pancytokeratin, clone AE1/AE3) and carcinoembryonic antigen (CEA) but negative for cytokeratin 14, vimentin, S100, smooth muscle actin, and p63. The CEA-positive staining reaction was present along the outermost rim of the neoplastic cell clusters consistent with an "inside-out" immunoreactivity pattern. Examination of the tumor cells by electron microscopy revealed microvilli on the outermost rim of neoplastic cells that were directed toward the surrounding vacant space. Based on histomorphological characteristics, the neoplasm was defined as an IMC of "pure-type." The location site and immunohistochemical features suggest the tumor was most likely derived from the apocrine sweat glands in the buccal skin.