A large lesion is seen in the left popliteal fossa formed by conglomeration of two masses. Superiorly it is at the level of gastrocnemius attachment, inferiorly it extends to proximal fibular shaft level. There is intra-articular extension into the posterior knee joint space and proximal tibiofibular joint.
Below the knee it causes large paraarticular bony erosions/oedema in the proximal tibia and fibula, occupies the deep flexor compartment, extends through the interosseous membrane into the deep extensor compartment. It is related to soleus muscle. The lesion is hypointense on T1, with thin strands of enhancement seen throughout and surrounding it.