Pulmonary Langerhans cell histiocytosis masquerading as adenocarcinoma of the lung.

Langerhans cell histiocytosis (LCH), also designated as histiocytosis-X or eosinophilic granuloma, is an uncommon dendritic cell disorder characterized by the infiltration of abnormally proliferating Langerhans cells into 1 or more organs.1 The clinical presentation of LCH varies depending on the site(s) of involvement and aggressiveness of disease. In children, it may present as LettererSiwe disease, an acute disseminated form of LCH that is invariably fatal if left untreated. Conversely, in patients with a unifocal lesion, it may remain undiagnosed for prolonged periods. Among adults, it usually occurs in association with smoking, either as a unifocal eosinophilic granuloma (approximately one-third of patients), or by affecting multiple organ systems; lungs and bones are the most commonly involved sites.1