Answer: Chondroblastoma

Histology: The lesion is composed of round stromal cells with a mixture of multinucleated giant cells. There is a chondroid matrix which may vary in quantity. A chondroid matrix often calcifies to give a characteristic “chicken wire” pattern of calcification. Stromal cells are round with a rounded nucleus which is often grooved.

Discussion: Chondroblastomas are lesions of immature cartilage that favor the epiphyseal ends of long bones. They usually occur in patients who are skeletally immature. However, older patients may also develop chondroblastoma. The presence of cartilage in a giant cell containing lesion is almost always indicative of chondroblastoma.

Giant cell tumors occur in the ends of long bones and also have mixtures of giant cells and stromal cells. However, giant cell tumors never contain chondroid material. The presence of chondroid in this case removes the possibility of conventional giant cell tumor.

Enchondromas, by contrast, occur in the metaphyseal or diaphyseal portions of the bone. They are frequently heavily calcified by plain radiography. They consist histologically of mature hyaline cartilage without a mixture of stromal cells and giant cells.

Chondrosarcomas are growing lesions that produce an aggressive x-ray picture with endosteal erosion or cortical expansion. There is permeation of the marrow cavity. The present lesion is sharply well defined and has a sclerotic rim, features indicative of a benign process.