Osteosarcoma usually develops in adolescents and affects males slightly more often than females. Although the lesion may occur throughout the skeleton, in 50% of patients it occurs in the region of the knee. The distal femur is the most common site, and the proximal tibia is the second most common site. Other less common sites are the proximal humerus, proximal femur, and pelvis. Most osteosarcomas originate in long bones in the regions of highest growth, the metaphyses. The initial symptom is pain, and the patients have a tender, bony mass. At the time of diagnosis, most osteosarcomas are stage IIB lesions that have a cortical defect and a soft tissue mass outside the bone. The most common site for metastases is the lungs, and all patients with known osteosarcoma should have chest imaging, preferably with CT.

A small percentage of osteosarcomas in adults occur in association with Paget disease of bone. Severe, unremitting increased pain with or without pathologic fracture is the primary clinical manifestation of sarcomatous transformation in Paget disease. The osteosarcomas may be hard to see within pagetic bone, but on MRI there should be a lack of the normal fatty marrow in Paget disease.

Diagnostic Studies. Radiographs characteristically demonstrate a permeative destructive lesion in which amorphous neoplastic bone can be detected. Typically, the lesion is predominantly dense, or osteoblastic, but may also have a mixed pattern or appear purely osteolytic. Other major characteristics include early cortical destruction, lack of containment by periosteal new bone formation, and a poorly defined margin. On radiographs, the amorphous, neoplastic bone may produce a pathognomonic “sunburst” pattern in which spicules of neoplastic bone arise perpendicular to the long axis of the limb in the periosteum; these spicules have been described as having a “hair on end” appearance. The incompetent periosteal reaction may also take the form of a triangular area of bone at the cortical margins produced by periosteal elevation and reaction (Codman’s triangle). Any of these periosteal reactions are suggestive of an aggressive bone tumor. Tumors treated with adjuvant chemotherapy can become heavily ossified or necrotic. Satellite nodules of tumor in the bone that may rarely be separate from the main mass can be seen in the peripheral margin.