Osteosarcoma: subtle, rare, and misleading plain film features.

Osteosarcoma is a malignant, mesenchymal, osteoid, and bone-forming tumor [1-3]. In most cases, typical radiographic features clearly illustrate the aggressive bone-forming nature of the lesion. These features include long-bone metaphyseal location, mixed areas of lysis and sclerosis, cortical destruction, periosteal new bone, and soft-tissue mass. However, numerous factors contribute to misleading radiographic patterns of osteosarcoma. They include histologic low-grade, lytic, or minimally sclerotic lesions, early detection, confinement to the intramedullary canal, benign-appearing periosteal reaction, rare intraosseous locations (subchondral, diaphyseal, intracortical), and rare skeletal sites (e.g., soft tissues, skull, ribs, tarsal bones). Secondary osteosarcomas, such as those arising from infarcts or fibrous dysplasia, may also produce a confusing radiologic picture. The purpose of this pictorial essay is to illustrate cases of osteosarcoma that proved to be difficult diagnostic dilemmas because of their subtle, rare, or misleading plain film features.