Circumscribed 5 mm mass in Hoffa's fat pad with peripheral signal voids due to susceptibility artifacts caused by hemosiderin deposits. The lesion was surgically excised. Histopathologic diagnosis: pigmented villonodular synovitis (PVNS).

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).