14-year-old male with posterior thigh mass in the pain. There is a large mass centered within the popliteal fossa of the knee, surrounding the popliteal artery and vein as well as the tibial nerve. The mass extends approximately 10 cm above the knee joint superiorly, abutting the posterior cortex of the distal femur, the medial aspect of the biceps femoris, and the posterolateral aspect of the vastus medialis. There appears to be a thin fat plane between the mass and the distal semimembranosus. At the level of the knee joint, the bulk of the mass is situated between the heads of the gastrocnemius with a small "tail" extending along the anterior margin of the lateral head of the gastrocnemius and additional tissue entering the posterior aspect of the intercondylar notch. The mass does appear to focally invade the medial head of the gastrocnemius, with streaky elements also invading the lateral head of the gastrocnemius along its length distally. Overall the craniocaudal dimension is at least 30 cm with a cross-sectional diameter of up to 6 cm. The mass also abuts the soleus and popliteus muscles. The mass has a lobulated appearance with some focal fatty septa within the mass, but it is predominantly isointense to muscle on T1-weighted images and is predominantly hyperintense on T2-weighted images, with near-fluid signal intensity. The mass enhances in a predominantly peripheral and septal distribution with a more nodular component of enhancement along the posterior aspect at the level of the distal femur. Poorly defined infiltrative enhancement is noted as the mass extends into the lateral head of the gastrocnemius.The menisci are intact and the articular cartilage appears normal. The anterior and posterior cruciate ligaments are intact as are the medial collateral ligament and lateral collateral ligament complex. The extensor mechanism is intact. The bone marrow signal is within normal limits.
Large lobulated mass centered in the popliteal fossa of the knee as described above. Based on its morphology and signal characteristics, the mass may represent a lymphatic malformation. Given its intimate association with the popliteal vessels, extremely extensive cystic adventitial disease is considered a possibility, albeit less likely. The high signal intensity of the mass on T2WI with what appear to be fatty components raise the possibility of a myxoliposarcoma, although the fatty components may simply represent entrapped fat within the lobules of the tumor. Synovial sarcoma may also have a predominantly cystic appearance, although the mass surrounds the neurovascular bundles rather than displacing them, which would be atypical for most malignancies. Follow-up with orthopedic oncology is recommended.