Rectal plasmacytoma with intracellular hemosiderin in a dog.

An 11-year-old female American Cocker Spaniel was presented with a rectal mass that protruded through the anus. Cytologically, the mass was composed of neoplastic round cells consistent with an extramedullary plasmacytoma. Histiocytic sarcoma, carcinoid tumor, and melanoma were also considered because of the presence of golden-brown cytoplasmic granules. Histopathologic findings and immunohistochemical reactivity for lambda light chains confirmed the cytologic diagnosis of plasmacytoma. Prussian blue staining identified the cytoplasmic granules as hemosiderin. The granules were negative by Fontana Masson and Grimelius histochemistry. To our knowledge, this is the first report of an extramedullary plasmacytoma containing hemosiderin in a dog.