A Rare Case of Malignant Plasmacytoma at a Previous Injection Site in a Cat.

A 6-year-old castrated male Bengal cat developed a subcutaneous mass at a previous vaccination site, which was initially diagnosed as cutaneous T-cell lymphoma following surgical excision at a local hospital. The cat was referred for metastatic evaluation and further treatment. Computed tomography revealed a residual tumor at the excision site, and an additional thoracic subcutaneous mass was identified. Both lesions were surgically excised and submitted for histopathologic and immunohistochemical examination. Despite the initial diagnosis of lymphoma, immunohistochemistry demonstrated that the neoplastic cells were negative for multiple lineage-specific markers, including CD3, Pax5, CD20, and cytokeratin, but showed strong MUM-1 positivity in mitotically active cells. These findings supported a diagnosis of plasmacytoma. This case emphasizes a diagnostic challenge in distinguishing round cell tumors in cats and the critical role of immunohistochemistry in achieving an accurate diagnosis. The tumor also showed aggressive clinical behavior, including suspected distant metastasis, and may have arisen in association with chronic inflammation at a prior injection site.