Penile Extramedullary Plasmacytoma in a Dog

Penile tumours are rare in dogs. Reported herein is a case of a penile extramedullary plasmacytoma (EMP) in a 5‐year‐old male cocker spaniel that was brought to a local hospital for an evaluation of a penile mass. The mass was approximately 1.3 cm in diameter at the time of presentation. In fine needle aspiration and histopathological examinations, the neoplastic cells showed eccentric round nuclei, a moderate amount of basophilic cytoplasm, and a peri‐nuclear clear zone, consistent with plasma cell morphology. There was nuclear pleomorphism with mononuclear giant cells and occasional bi‐nucleation. Round cells on the periphery of the mass demonstrated plasmacytic differentiation. Immunohistochemically, the neoplastic cells stained positive for MUM1 and light lambda chain. Based on the cytological and pathological observations, a diagnosis of penile EMP was established.