A case of pigmented villonodular synovitis of the knee successfully diagnosed by fine-needle aspiration of the joint fluid

We report a case of pigmented villonodular synovitis (PVNS) of the knee diagnosed in a 69-year-old woman. The fine-needle aspiration smear of the joint fluid showed loose aggregates of mononuclear cells relatively rich in cytoplasm with bland nuclei containing a single nucleolus. Hemosiderin-containing cells and multinucleated cells were also observed. Histopathological study of the resected synovial tumor confirmed the diagnosis of PVNS.The characteristic cytological features of PVNS are as follows : i) aggregates of bland mononuclear cells, and ii) presence of hemosiderin-containing mononuclear cells and multinucleated cells. Recognition of these features can facilitate the cytodiagnosis of PVNS.