Results of staging studies reflect the tumor’s aggressive nature. Bone scans show intense radioisotope uptake and reveal metastatic lesions to other bones. MRI provides more detailed information about intraosseous involvement, skip lesions, and soft tissue extension. Chest CT scans are also needed to detect pulmonary metastases not always seen on chest radiographs. Staging studies are essential to monitor the response to chemotherapy and plan the surgical approach.

Osteosarcomas vary widely in appearance, and it may be difficult to distinguish non-neoplastic, reactive osteoid from trauma or other tumors compared with the immature neoplastic osteoid in osteosarcoma, especially if the tissue sample is small.

Treatment/Prognosis. Forty years ago amputations were routinely performed for patients with osteosarcoma and most patients still eventually died of metastatic disease. However, preoperative (neoadjuvant) chemotherapy has substantially improved the life expectancy from less than 20% to 70% in patients who present without metastases. In addition, most patients now undergo limb salvage procedures compared with the amputations previously performed.

PAROSTEAL OSTEOSARCOMA

Parosteal (juxtacortical) osteosarcoma arises between the cortex and muscle as a low-grade stage IA surface tumor, most commonly in adolescents and young adults (see Plate 6-16). It usually presents as a fixed, painless mass on the posterior aspect of the distal femur (over 50% of cases); other sites include the proximal humerus and proximal tibia. Parosteal osteosarcoma is distinguished from classic osteosarcoma by its much slower, less aggressive clinical course and broad attachment to the adjacent cortex. Pain is less common with parosteal osteosarcoma than conventional osteosarcoma.

Diagnostic Studies. Radiographs show a dense, heavily ossified, broad-based fusiform mass that appears to encircle the metaphysis. Invasion into the overlying displaced soft tissues is rare. Late in the disease, the tumor extends through the underlying cortex to invade the medullary canal as well, converting to a stage IB tumor; about 10% of these tumors exhibit areas of dedifferentiation into high-grade sarcoma and when dedifferentiated these parosteal osteosarcomas are considered stage IIB lesions.