10 year old with insidious onset of right medial thigh pain. [Musculoskeletal] [XR] [MR]

[Top]: X-ray shows a lucent, bubbly, lesion of the distal femur at the metaphysis. On the frontal view [top right], there is breakage through the medial femoral cortex into the adjacent soft tissues, not a good sign.

[Bottom]: MRI shows a multicystic lesion filling the distal femur containing multiple locules, many with fluid-fluid, fluid-debris, and fluid-hemorrhage levels. The most common lesions with this striking appearance are aneurysmal bone cyst, giant cell tumor, or telangiectatic osteosarcoma. Unfortunately, there is clearly extension of the bone tumor beyond the bone (yellow arrows), which favors a more aggressive neoplasm from that differential diagnosis - this turned out to be telangiectatic osteosarcoma.