Lipid-rich carcinomas of the mammary gland in seven dogs: clinicopathologic and immunohistochemical features.

Lipid-rich carcinomas occurred in seven female dogs. Affected dogs were purebred (all but one), intact (all but one), and between 4 and 13 years of age. Five of them had a history of parity, one had pseudopregnancy, and none had received contraceptive steroids. The tumors were single (five cases) or multiple (two cases) well-circumscribed masses of different sizes (varying from 1 to 6 cm in diameter), composed of solid nests and cords of tumor cells separated by a moderate amount of stroma. The tumor cells contained either multiple and small or large and solitary vacuoles that pushed the nucleus to the periphery of the cell (signet-ring cell). A glandular epithelial immunophenotype (cytokeratins 5 and 8 and 8 and 18) was observed in the majority of tumor cells. All tumors lacked both estrogen and progesterone receptors, and five out of seven tumors gave rise to local recurrence and proximal or distant metastases or both.