A case of Erdheim-Chester disease-a mimicker of IgG4-related disease and large vessel vasculitis.

The patient was a 57-year-old man who developed bilateral thigh pain and chest tightness 1 year ago. Chest computed tomography (CT) scan showed reticular shadows, thickened interlobular septa in both lung fields, and pericardial effusion. Three months ago, his symptoms worsened. A contrast CT scan revealed increased pericardial effusion, multiple masses in the right atrium, soft tissue shadows suggestive of retroperitoneal fibrosis, and soft tissue shadows around the thoracic and abdominal aorta. He visited University Hospital of University of Occupational and Environmental Health, Japan suspecting IgG4-related disease (IgG4-RD) or large vessel vasculitis (LVV). Based on the involvement of various organs and bilateral thigh pain, Erdheim-Chester disease (ECD) was suspected, and an 18F-fluorodeoxyglucose Positron Emission Tomography (FDG-PET) scan was performed. In addition to increased accumulation around the right ventricle, right coronary artery, and aorta, increased accumulation was confirmed in the distal femurs and proximal tibias on both sides, strongly suggesting ECD. A bone biopsy confirmed the diagnosis of ECD, showing bone fibrosis with CD68-positive and CD1a-negative foam cell infiltration, which is a characteristic of ECD. ECD is an extremely rare form of non-Langerhans cell histiocytosis. ECD affects a wide variety of organs, and its imaging findings can sometimes resemble those of IgG4-related disease or LVV. However, bone lesions are characteristic of ECD and are a key finding for its diagnosis. When systemic organ lesions, including bone lesions, are present, ECD should be included in the differential diagnosis, and PET-CT should be considered.