Small cell osteosarcoma of the soft tissue.

The lesion we described is an osteoid producing mitotically active spindle cell soft tissue neoplasm, which has light and ultrastructural features of an osteosarcoma. By immunohistochemistry it marks with p30/32MIC2-directed antibodies but fails to react with antibodies associated with other soft tissue neoplasms. The precise histogenesis of the lesion is unknown, but failure to elicit a periosteal reaction makes the periosteum less likely to be the primary tissue of origin. The failure to demonstrate any radiologic or histologic evidence of involvement of the underlying bone leaves the soft tissue as the most reasonable site of origin, although the parosteal region cannot be completely excluded.