Answer: Chondroblastoma

Histology: The most important radiographic criteria in diagnosing a chondroblastoma is a well-marginated lytic lesion in the epiphysis. In the long bones, chondroblastomas are almost exclusively confined to the epiphysis whereas giant cell tumors involve both the epiphysis and metaphysis. Histologically, a chondroblastoma may also resemble a giant cell tumor of bone. However, there are two important differences. First, chondroblastoma often has islands of immature chondroid material, or the extracellular space between the stromal cells has a faint blue chondroid appearance. By contrast, giant cell tumors never contain cartilage. Second, the stromal cells of chondroblastoma are sharply marginated with distinct cytoplasmic membranes. They resemble fried eggs. Giant cell stromal cells tend to form a synctium. Chondroblastomas also have many giant cells like giant cell tumor, but a differentiating feature lies in the stromal cells.

Discussion: Radiologically, non-ossifying fibroma is a well-defined lytic lesion that is always confined to the metaphysis of long bones. Clear cell chondrosarcomas like chondroblastomas, involve the epiphysis. However, the pattern of bone destruction is poorly marginated and suggests an aggressive neoplasm. In addition, clear cell chondrosarcomas occasionally have stippled calcifications.

Histologically, clear cell chondrosarcoma may have areas resembling chondroblastoma. In fact, some feel that a clear cell chondrosarcoma is a malignant version of a chondroblastoma. However, clear cell chondrosarcoma has islands of atypical cartilage indistinguishable from chondrosarcomas in other settings. Histologically, a non-ossifying fibroma is a benign fibrous histiocytoma which may have many multinucleated giant cells. However, the stromal cells tend to be spindle shaped and are arranged in a storiform pattern.