X-rays and MRI of the knee are performed.
On x-rays, there is a inhomogeneous soft tissue swelling in the popliteal fossa, as well as a cystic, polygonal lesion with marginal sclerosis in the distal, dorsal segment of the femur. MRI demonstrates a large, oval mass extending in the soft tissues of the popliteal fossa and growing deeply into the femur from the dorsal side. In the T2* gradient-echo sequence, the signal intensity of the lobulated mass appears strongly hypointense. The popliteal artery is displaced medially and supplies pathological vessels to the tumor.
What is your diagnosis?
Diagnosis: Pigmented Villonodular Synovitis (PVNS)
PVNS is a rare, benign but locally aggressive proliferative disorder of the synovium, affecting individuals aged 20 to 50 years, with no significant gender predilection. It can affect any synovial joint, tendon sheath, or bursa, but the knee joint is the most commonly involved site. The localized form of PVNS affects tendon sheaths, the diffuse form commonly large joints such as the knee, hip, or shoulder leading to progressive joint destruction.
The pathology of PVNS is characterized by neoplastic-like proliferation of synovial tissue containing giant cells, foam cells, inflammatory infiltrates and hemosiderin deposition. Hemosiderin is a characteristic by-product of chronic hemorrhage in PVNS creating the diagnostic "blooming artifact" on MRI. In the knee, PVNS typically involves the suprapatellar pouch, the posterior compartments, and the infrapatellar fat pad. In extensive PVNS, several soft-tissue compartments, and the articular bones can be involved, that is critical for planning surgical treatment.
X-ray findings are nonspecific in the early stages. As PVNS progresses, periarticular erosions with a sclerotic rim secondary to bone invasion may be found. Asymmetric joint space narrowing may be observed in advanced cases.
MRI plays a pivotal role in characterizing PVNS. Aside from synovial hypertrophy, joint effusion and bone invasion, the key findings of MRI are “blooming artifacts” in gradient-echo sequences. If PVNS is suspected, at least one GRE sequence should be acquired to hallmark hemosiderin deposition within the synovium as areas of low signal intensity and to differentiate PVNS from other joint pathologies.
