Pigmented villonodular synovitis and plasmacytoid lymphoma in a dog.

A 4-year-old Labrador Retriever was examined because of progressive left hind limb lameness involving the stifle. A villous synovial mass was evacuated by synovectomy. Initially, the macroscopic and histopathologic features suggested a malignant fibrosarcomatous process; however, further histologic studies revealed lesions consistent with pigmented villonodular synovitis. Nine months later, the dog developed a large retroperitoneal tumor, with metastasis to the lungs and liver. The dog was then euthanatized. By histologic and electron microscopic examinations, the tumor was found to be a primitive plasmacytoid lymphoma.