Periosteal Osteosarcoma, Tibia

21-year-old male with pain in the proximal left leg. There is an infiltrative T1 hypointense lesion circumferentially extending along the medial cortical margin of the proximal left tibia. The hypointense signal extends into the medullary cavity of the proximal tibia. The lesion is STIR hyperintense and avidly enhances. Imaging findings are compatible with a periosteal osteosarcoma. Periosteal osteosarcoma is the second most common type of surface osteosarcoma after the parosteal subtype. Periosteal osteosarcoma arise from the inner germinative layer of periosteum. Lesions tend to be diaphyseal with the femur and tibia most commonly affected. The lesion arises from the cortical margin with intramedullary extension being rare.