Diagnostic Radiology/Musculoskeletal Imaging/Tumors Basic/Parosteal Osteosarcoma

Parosteal osteosarcoma is the 2nd most common type of osteosarcoma. 80% occur between the ages of 20 and 50. They are commonly found at the distal femoral metaphysis, posteriorly, less commonly in the proximal tibia and humerus. Parosteal osteosarcoma arises from the surface of the bone and is seen as a soft tissue mass often growing around the bone of origin. The osteoid matrix is typically more mature and densely sclerotic, especially centrally. MRI appearance is variable depending on the amount of osteoid formation, cellularity and presence of cartilage, usually appearing quite heterogeneous. If there is little cellularity and cartilage the lesion will be lower signal intensity. They are low grade tumors and grow slowly, but with recurrence can become more aggressive. Metastases are less common than with conventional osteosarcoma but can metastasize to the lung. Differential includes myositis ossificans, cortical desmoid tumor and osteochondroma. Myositis ossificans is typically more dense peripherally rather than centrally, helping in differentiation. Osteochondroma has more normal appearing marrow and cortical bone.



Fig. 1 Parosteal sarcoma arising from the posterior margin of the femur with a large soft tissue mass and dense osteoid matrix.