Answer: Langerhans cell histiocytosis (eosinophilic granuloma)

Histology: Langerhans cell histiocytosis in bone, formerly known as eosinophilic granuloma of bone, occurs predominantly in children and young adults. Characteristically, radiographic lesions are well-defined and punched out. Histologically lesions consist of heterogenous mixture of cells including rounded macrophage-like cells (the Langerhans cells), multinucleated giant cells, and eosinophils. This heterogeneous cells population of inflammatory cells raises the possibility of osteomyelitis. However, the macrophage-like-cells in the Langerhans’ cell histiocytosis are positive with S-100 protein and the antigen CD-1A. Therefore, any case of osteomyelitis in children or young adults should be studied with these antibodies to exclude the possibility of Langerhans cell histiocytosis.

Discussion: The lesion is not osteomyelitis, although histologically osteomyelitis may look like the present case. The reactivity with antibody to CD-1A and S-100 protein in the stromal cell is diagnostic of Langerhans cell histiocytosis.

Radiologically this lesion may be confused with non-ossifying fibroma. It is a well defined lytic lesion in the metaphyseal region. However, the heterogenous population of inflammatory cells is not characteristic of non-ossifying fibroma. Also, the spindle cells and a storiform pattern which is seen in non-ossifying fibroma is not present in this case.