Extramedullary plasmacytoma of the oral cavity metastasising to both kidneys in a dog

BACKGROUND: Most extramedullary plasmacytomas (EMPs) aresolitary and located in the head and neck region. They may also occur in the visceral parts of the body. OBJECTIVES: Here, we report a case of oral EMP followed by neoplastic plasma cell metastasis to both kidneys in a neutered male Pomeranian. METHODS: Oral plasmacytoma recurred 11 months aftersurgical removal of an oral mass and partial maxillectomy was performed. Eighteen months after partial maxillectomy, neoplastic masses were detected in both kidneys on computed tomography. The dog died 12 months after detection of bilateral kidney neoplasms. The resected neoplastic masses were routinely processed for histopathological observation and immunohistochemistry against pan‐cytokeratin, desmin, CD3, and MUM‐1. RESULTS: The recurred mass mainly consisted of well‐differentiated plasma cells and contained a small portion of aggressive cells with malignant features. Monoclonal gammopathy was not observed on serumelectrophoresis performed to exclude multiple myeloma. The mass was composed of plasma cells with high nuclear pleomorphism and abundant mitotic figures. The neoplasm stained positive for MUM‐1 with a more aggressive morphology than in oral EMP. CONCLUSION: Based on serum biomarker and pathological observations, a diagnosis of recurrence and metastasis of oral‐to‐renal EMP was established. To the best of our knowledge, metastasis of oral EMP into the bilateral kidneys, as described in the current case, has not been previously reported in dogs.