Diagnostic Studies. Periosteal chondromas appear on radiographs as a radiolucent oval or oblong defect visualized as a crater-like deformity of the periphery of the cortex. The lesion is underlined by a thin, distinct cortical reaction. The tumor has little or no calcification. MRI can identify the density of the cartilage as high on T2-weighted sequences.

The radiographic differential diagnosis includes osteocartilaginous exostosis in younger patients (see Plate 6-6), juxtacortical chondrosarcoma (see Plate 6-17), as well as juxtacortical (parosteal and periosteal) osteosarcomas (see Plate 6-16).

On gross examination, the lesion has the consistency of mature cartilage without evidence of ossification or calcification. Histologic examination reveals hypercellular cartilage, and nuclear atypia may be present.

Treatment/Prognosis. Periosteal chondromas are active benign stage 2 tumors that require marginal excision to prevent recurrence. Because of their peripheral location and their cortical rim, this can usually be achieved without bone grafting or extensive reconstruction. There is a risk of recurrence after excision. Sarcomatous transformation has not been documented, and rare reports of such transformation probably reflect the resemblance of periosteal chondroma to juxtacortical chondrosarcoma.