Histologic features consist of mature trabeculae with a peculiar pattern of cement lines similar to that seen in Paget disease of bone. Enmeshed in a low-grade stroma, the trabeculae often contain varying degrees of cartilage that is not obviously malignant. Because of these bland overall features, this tumor is frequently underdiagnosed as benign, leading to inadequate intracapsular or marginal excision and recurrence.

Treatment/Prognosis. Treatment of the parosteal osteosarcoma is wide excision that can usually be accomplished with a limb-salvaging procedure. Unless dedifferentiation (favored by repeated recurrence or prolonged neglect) is present, the prognosis is good and chemotherapy is not indicated because the risks outweigh the benefits in non-dedifferentiated parosteal osteosarcoma.

PERIOSTEAL OSTEOSARCOMA

Periosteal osteosarcoma also is an uncommon variant of osteosarcoma. It primarily affects young adults, usually presenting as an enlarging, often painless mass that grows on the external surface of the bone.

Diagnostic Studies. Radiographs show a largely external, poorly mineralized mass in a crater-like area of cortical erosion with an irregular margin and periosteal reaction. The incidence of pulmonary metastases is higher than in parosteal osteosarcoma, and the prognosis is worse. The radiographic differential diagnosis includes classic osteosarcoma (see Plates 6-14 and 6-15), periosteal chondroma (see Plate 6-5), and juxtacortical chondrosarcoma (see Plate 6-17).

Bone scans show a disproportionate increase in radioisotope uptake throughout the tumor, considering its predominantly radiolucent appearance. CT depicts a tumor sitting in a shallow cortical defect with only moderate calcification.

On gross examination, the tumor appears to be composed primarily of cartilage; microscopic examination, however, reveals areas of malignant mesenchymal stroma containing neoplastic osteoid scattered in and about the lobules of low-grade mature cartilage.

Treatment/Prognosis. Because of its intermediate aggressiveness and accessible location, the lesion is almost always amenable to excision with a wide margin. Adjuvant systemic chemotherapy is indicated only for higher-grade tumors. Prognosis for patients with periosteal osteosarcoma is fair.