Eosinophilic granuloma usually occurs as a solitary, symptomatic lesion in children younger than 20 years of age but can occur as multiple tumors at any age and at any site. Low-grade fever, elevated erythrocyte sedimentation rate, and mild peripheral eosinophilia are occasional associated findings. The skull, supra-acetabular region of the pelvis, and diaphysis of the femur are the usual sites of involvement, although any bone can be affected.

Diagnostic Studies. On radiographs, a small lesion may resemble the punched-out radiolucent lesion of multiple myeloma. Involvement of a vertebral body produces the pathognomonic radiographic appearance of vertebra plana (“coin-on-edge” flattened vertebra) after vertebral collapse. A lesion that occurs in the diaphysis or metaphysis of a limb appears as an oval radiolucent area. Because of its variable radiographic characteristics, eosinophilic granuloma has been called the great imitator.

Bone scans are used to document the presence of multiple lesions (<10% of cases); an intense radioisotope uptake indicates an active lesion. MRI is helpful in delineating the lesion, especially in the spine and hip.

Gross examination of an active lesion reveals soft, vascular, granulomatous tissue covered with a mature bony capsule that may be runny and liquefied. Histologic characteristics are a mixture of Langerhans histiocytes, eosinophils, and occasional giant cells with little background stroma. Birbeck granules can also be seen. The same histologic pattern is seen in each of the forms of histiocytosis X, but the multisystem Hand-Schüller-Christian and Letterer-Siwe diseases have significant morbidity and mortality and are sometimes treated with chemotherapy.

Treatment/Prognosis. The curettage performed to obtain tissue for diagnosis is usually adequate treatment. When the defect is large, bone grafts may be needed to prevent fracture (see Plate 6-30). In a small percentage of patients with a solitary lesion, the visceral manifestations of Hand-Schüller-Christian disease can eventually develop and follow-up or a bone scan should be performed to rule out multifocal disease.