Secretory carcinoma of the canine mammary gland

Secretory carcinoma is an uncommon variant of breast cancer, characterized by the presence of intracellular and extracellular eosinophilic secretion. Here, we report the cytologic, histologic, and immunohistochemical findings of a secretory carcinoma diagnosed in the left inguinal mammary gland of a 3-year-old female German Shepherd Dog. The fine-needle aspiration cytology showed numerous large branching sheets of neoplastic cells and isolated ce lls with cytoplasmic vacuoles. Histologically, the tumor was composed of ce lls with clear cytoplasm and prominent vacuoles that pushed the nuclei to t he periphery, resembling signet ring cells. These cells were arranged in so lid or tubular structures with lumenal spaces filled with eosinophilic secr etion. Immunohistochemical reactions to cytokeratin (CAM 5.2) and alpha-lac talbumin were strongly positive in all neoplastic cells, and staining for v imentin and S100 protein was negative. The cytomorphologic and immunohistoc hemical features of this tumor are similar to those seen in tumors in women , hence enabling the diagnosis of a rare case of primary secretory carcinom a of the canine mammary gland.