This is pigmented villonodular synovitis involving a joint, usually the knee. This neoplastic process can develop in the synovium of joints, tendon sheaths, and bursae. Note the tangled mat of red-brown folds and finger-like projections. A related tumor, known as a giant cell tumor of tendon sheath, is similar histologically (multinucleated giant cells, macrophages, hemosiderin) but is a localized mass. [Image courtesy of John Nicholls MD, Hong Kong University]

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