Answer: Chondroblastoma

Histology: This lesion is a cellular neoplasm with rounded stromal cells admixed with multinucleated giant cells. There is a faint bluish background in the extracellular matrix that is an indicator of very early chondroid matrix secretion. The stromal cells are rounded and discohesive, features of chondroblasts. In addition, there is focal calcification of the extracelluar matrix in a “chicken wire” pattern. This indicates chondroblastoma. The radiographic appearance of a well-defined lytic lesion in the epiphyseal end of the long bone is the pattern of chondroblastoma.

Discussion: The lesion is not a giant cell tumor because of the nature of the stromal cells and chondroid matrix. Giant cells tumors do not contain cartilage. In addition, the giant cell tumors, while they involve the epiphyseal end of long bones, usually include the metaphysis. The present lesion is almost entirely confined to the epiphysis.

The lesion is not a chondromyxoid fibroma. The chondroid background would be found in a chondromyxoid fibroma. However, this current lesion lacks the spindle cell and myxoid component of a typical chondromyxoid fibroma. In addition, chondromyxoid fibromas involve the metaphyseal portion of the long bone while this lesion involves the epiphysis.