The above-described features are suggestive of the diffuse form of pigmented villonodular synovitis.

Pigmented villonodular synovitis (PVNS) is a rare benign proliferative condition affecting synovial membranes of joints, bursae, or tendons, possibly resulting from neoplastic synovial proliferation with villous and nodular projections and hemosiderin deposition. PVNS is most commonly monoarticular (the knee followed by the hip joints are the most common sites) but occasionally can be oligoarticular. The histology of PVNS can look similar to some aggressive neoplasms (e.g., synovial sarcoma), and imaging, therefore, has a crucial role in guiding the pathologist.

PVNS has two forms:

• diffuse form characterized by joint effusion and extrinsic bony erosions, but the joint space is unaffected.

• localized forms characterized by only a soft-tissue mass

Special thanks to Dr. Amr Gamil, MD.

Editor's note: Per the 2020 WHO Soft Tissue and Bone Tumors Classification (5th ed.), the recommended terminology is tenosynovial giant cell tumor with pigmented villonodular synovitis no longer recommended (although remains in common use).