CASE
A 22-year-old woman presented to her family physician with complaints of knee pain. When a thorough examination raised suspicions of a meniscal injury, the physician ordered radiography and MRI of the knee. The radiograph showed a large, aggressive lytic lesion involving the metaphysis and epiphysis of the distal femur, with expansion of the bone (Figure 1). No periosteal reaction was apparent. A narrow transition zone separated the lesion from normal bone. On coronal T1-weighted MRI, the lesion had low signal intensity and did not extend intra-articularly (Figure 2). A fast spin echo, proton density weighted, axial MRI demonstrated high intensity cystic areas representing fluid-fluid levels indicative of bleeding (Figure 3). The lesion extended beyond the cortex of the bone. What do these images show?
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