Pigmented Villonodular Synovitis, Knee

35-year-old male with chronic left knee pain. The images demonstrate a proton density isointense, STIR mildly hypointense lesion with scattered regions of signal loss. The lesion heterogeneously enhances and is intimately associated with the posterior cruciate ligament. These imaging findings are characteristic of pigmented villonodular synovitis or PVNS for short. PVNS is a benign proliferative condition affecting the synovial membranes of joints, bursa, or tendons. It results from neoplastic synovial proliferation with villous and nodular projections with scattered hemosiderin deposition. It is most common monoarticular. Occasionally, PVNS can look like aggressive neoplasms such as rhabdomyosarcoma, synovial sarcoma, or epithelial sarcoma.