Langerhans' cell histiocytosis (LCH) is manifested in a variety of ways, the most common being the eosinophilic granuloma, a localized, often solitary bone lesion that occurs predominantly in the pediatric age group. The hallmark of LCH is the proliferation and accumulation of a specific histiocyte: the Langerhans' cell. In bone this may cause pain and adjacent soft-tissue swelling, but some lesions are asymptomatic. LCH can involve any bone, but most lesions occur in the skull (especially the calvarium and temporal bones), the pelvis, spine, mandible, ribs, and tubular bones. Imaging diagnosis of the disease in bone is first based on the plain radiographic appearance, which is usually a central destructive, aggressive-looking lesion. In the skull, the lesions develop in the diploic space, are lytic, and their edges may be beveled, scalloped or confluent (geographic), or show a "button sequestrum." Vertebral body involvement usually causes collapse, resulting in vertebra plana. With significant recent improvements in the quality of gamma cameras, imaging techniques, and in studying children, bone scintigraphy at diagnosis and on follow-up usually reveals the sites of active disease, especially when the involvement is polyostotic. CT and MR imaging are very useful in providing detailed cross-sectional anatomic detail of the involved bone, including the bone marrow and the adjacent soft tissues. CT is better suited for demonstrating bone detail and MR imaging for bone marrow and soft-tissue involvement.

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Dysplasia epiphysealis hemimelica (DEH) is an osteocartilaginous overgrowth involving one or multiple epiphyses or ossification centers, usually in a lower extremity on one side of the body. Characteristically the involvement is hemimelic i.e. either the medial or lateral part of the ossification center is involved. We have studied 24 patients with DEH and are adding 15 new cases to the literature. Because of the variable manifestations of the dysplasia and its different degrees of involvement in the affected children, we have subdivided it into localized, classical and generalized forms. In the generalized form, there is involvement of a whole lower extremity from the pelvis to the foot, and some of these patients show megaepiphyses with enlargement of a whole epiphyseal center, not only its medial or lateral part. We have also described and illustrated other special features of the dysplasia especially the advanced bone age and the metaphyseal and growth plate involvement.

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