A case of Erdheim-Chester disease initially mistaken for retroperitoneal lymphoma.

Erdheim-Chester disease (ECD) is an infrequent, autoimmune disorder that is not Langerhans histiocytosis and is characterized by bilateral sclerosis of the diametaphyseal medullary regions of the long bones and possible multiorgan involvement. A mononuclear foam histiocyte infiltrate determines the condition's histopathologic form and extensive fibrosis. A case of ECD in a 52-year-old man with retroperitoneal infiltration and bone involvement is described here, in whom the diagnosis was established only after a 3-year course of multiple nonspecific symptoms. Establishing a diagnosis of ECD is challenging, especially due to vague and nonspecific presenting symptoms. Nonetheless, it is important to recognize both its unique radiographic findings, particularly, symmetric patchy sclerosis of the long bones, and the pathognomonic infiltration of foamy histiocytes with dense fibrosis, as patients can benefit from interferon-alpha therapy if ECD is established.