Eosinophilic granuloma (EG) is a benign tumor-like condition which is characterized by a clonal proliferation of Langerhans-type histiocytes and defined as a local form of Langerhans cell histiocytosis (LCH). The radiographic appearances of EG are quite different depending on the phase of the disease and the site of involvement. A status of EG in the bone is divided into acute and chronic phases. Radiologically acute phase of EG is difficult to differentiate from a malignant bone tumor such as Ewing's sarcoma or acute osteomyelitis. Chronic phase of EG may mimic a chronic osteomyelitis or a benign bone tumor. We report 3 children's cases of EG in the pelvis which showed quite different radiological features and clinical courses. A 6-year-old boy (Case 1) had an osteolytic lesion with slightly defined margins in the right acetabulum. A 4-year-old boy (Case 2) had a radiologically similar-looking lesion in the left acetabulum. These lesions resembled radiologically chronic osteomyelitis (Brodie's abscess) or a benign bone tumor and healed spontaneously after biopsy. A 2-year-old boy (Case 3) had an osteolytic lesion with ill-defined margin in the ilium. It was difficult to differentiate from a malignant tumor such as Ewing's sarcoma, or acute osteomyelitis. The lesion became enlarged after needle biopsy. In spite of an additional curettage, the osteolytic lesion remained in the left pelvis in 1 year. Treatment for EG is controversial. Curettage of the affected site and bone grafting is usually accomplished. However, some EG heal spontaneously. It is of great importance to understand the natural course of EG and this knowledge will give us the opportunity to avoid unnecessary treatment. EG with poor osteolytic margins may progress further after biopsy. EG with well-defined margins may heal spontaneously after biopsy only.
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