Answer: Pigmented villonodular synovitis

Histology: The lesion consists of multiple nodules composed of an admixture of hemosiderin-laden macrophages, foamy macrophages, and rounded synovial-like cells. Scattered nuclear giant cells are present. The overlying synovium appears hyperplastic.

Discussion: Both localized giant cell tumor of tendon sheath and diffuse giant cell tumor of tendon sheath show identical cytological features. A synonym for diffuse giant cell tumor of tendon sheath is pigmented villonodular synovitis (PVNS). Whereas localized giant cell tumors of tendon sheath have a lobular growth pattern, often surrounded by a collagenous capsule, diffuse giant cell tumor grows in sheets and are multinodular corresponding to a more aggressive appearance both radiographically and at surgery. In contrast to hemosiderodic synovitis, in pigmented villonodular synovitis there is a proliferation of synovial cells within the lesion whereas hemosiderodic synovitis merely contains a reactive synovial lining with underlying histiocytes containing hemosiderin. Synovial sarcoma typically does not involve the joint space and has a much more cellular, monotonous appearance, lacking hemosiderin-laden macrophages, xanthomatous macrophages, and multinuclear giant cells. PVNS are locally aggressive but non-metastasizing. Treatment is aimed at removing as much tumor as possible without causing severe disability for the patient.