Answer: Enchondroma

Histology: It is extremely difficult to differentiate enchondromas from low grade chondrosarcomas. The presumptive diagnosis should be secure even before a histologic study is undertaken. An important factor supporting a diagnosis of enchondroma is that the patient is asymptomatic. Also, radiographically, the lesion should not show evidence of growth. That is, the lesion should be well defined and show not cortical erosion. Enchondromas are non-growing lesions in the adult, while chondrosarcomas are growing lesions. Therefore, evidence of growth indicates malignant behavior. Histologically, enchondromas show bland cartilage with occasional binucleated chondrocytes. An important histologic feature of enchondromas is cartilage lobule encasement by bone. This indicates that the nodules of cartilage are non-growing and that they have had time to mineralize at the edges. Another feature of enchondromas is the presence of bone marrow between lobules of cartilage.

Discussion: This lesion is not a chondrosarcoma because it was assymptomatic and the radiographic features are that of a benign cartilage lesion (even though it is very large). The lesion lacks the permeative growth pattern of chondrosarcomas. There is no penetration of the marrow space and the surrounding of native bone trabeculae.

The lesion is not a chondromyxoid fibroma. Although chondromyxoid fibromas have distinct chondroid-like areas, mature hyaline cartilage is not present in chondromyxoid fibroma. Also, chondromyxoid fibromas do not show heavy mineralization as is present in the radiographs on this case.