Review Question - QID 3511

The radiograph, CT scan, bone scan, MRI, and histologic slide are consistent with a diagnosis of a parosteal osteosarcoma. The radiograph and MRI show a dense bone forming lesion which is "stuck-on" the proximal humerus. These radiographs actually show a multi-focal presentation of this lesion with distinct lesions stuck on the bone. Bone scan shows the lesion to be metabolically active. MRI shows the very dark signal characteristic of heavy matrix calcification. The characteristic histology section shows mature appearing bone with surrounding fibroblastic stroma lacking cytologic pleomorphism. Illustration A shows the pathological specimen and demonstrates that this is a surface lesion which appears to be "stuck-on" the cortex of the bone.

While osteochondroma and myositis ossificans can be easily confused with parosteal osteosarcoma, there are key imaging findings to differentiate these entities. Specifically, cross-sectional imaging of osteochondromas will show continuity of the medullary cavity of the bone and lesion while myositis ossificans typically shows a thin egg-shell like calcification and often does not actually touch bone.

Messerschmitt et al review the diagnosis, treatment, and outcomes in osteosarcoma. As described by these authors, parosteal osteosarcoma shows the best 5-year survival compared to other bone tumors and osteosarcoma in general. This is thought to be due to the histologic low grade of this tumor.