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Background
Erdheim Chester disease (ECD) is a rare non-Langerhans cell histiocytosis that can have a broad range of clinical and radiological presentations. It is characterized by excessive production and accumulation of foamy histiocytes and Touton giant cells in various tissues and organs. ECD most frequently affects adults in fifth decade, with a slight male predominance. The diagnosis is difficult because of the protean manifestations of this disease, which can involve virtually any organ system. Sclerotic lesions in the long bones of the lower extremities are the most common finding, but the cardiovascular system, retroperitoneum, endocrine system, central nervous system, lungs, skin, and orbits are also frequently involved. There are no uniform diagnostic criteria, and the diagnosis and treatment require integration of clinical information, imaging studies, and pathology. Despite being a non-malignant entity, ECD can be fatal due to severe organ dysfunction. The prognosis for untreated ECD is exceptionally poor and therapy is recommended at diagnosis in all patients except for those with minimally symptomatic disease.

Educational Goals / Teaching Points
Describe the histology/pathology of ECD. Discuss common clinical presentations and outcomes. Illustrate the wide range of imaging appearance of ECD affecting multiple organ systems. Discuss and illustrate differential diagnostic possibilities for various imaging findings.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The most common features include skeletal involvement with bilateral osteosclerotic lesions of long bones of the lower limbs, diabetes insipidus, cardiovascular involvement with circumferential thickening of the aorta ("coated aorta"), and retroperitoneal fibrosis ("hairy kidney"). Renal and vascular structures are the most frequently affected abdominal organs. Cardiovascular and CNS involvement are associated with a worse prognosis. ECD has a predilection for the posterior fossa and spinal cord; however, pituitary, dural, facial sinus, and orbital involvement are also relatively frequent. Cardiac disease most often involves the pericardium, in about 80% of patients. Myocardial infiltration is detected in more than one-third of patients, with the most characteristic findings of right atrial pseudotumor and infiltration of the atrioventricular sulcus seen in 20 – 40% of patients. Early recognition of cardiovascular involvement of ECD is important because of the associated high morbidity and mortality.

Conclusion
ECD can involve virtually any organ system and can have a broad range of clinical and radiological presentations. Imaging plays a key role in the diagnosis, management, and follow-up of ECD. Although the diagnosis can be suggested on imaging, histopathology and immunohistochemistry are required for confirmation.