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Background
Erdheim-Chester disease (ECD) is a rare, multisystemic, inflammatory, non-Langerhans cell histiocytic neoplasm. The discovery of recurrent, somatic mutations in mitogen-activated protein kinase (MAPK) signaling pathway, most commonly BRAFV600E has led to a reclassification of ECD from an inflammatory disorder to a neoplastic process. It is now included in revised 2016 WHO classification of hematopoietic tumors and in ‘Langerhans’ (L) group of revised 2016 Histiocytosis Classification of the Histiocyte Society. When symptomatic, ECD most commonly presents with bone pain and fatigue. Neurological manifestations, central diabetes insipidus, exophthalmos and periorbital xanthelasma-like lesions (XLL) are frequently encountered. Interestingly, pathological findings may vary from site of biopsy and may display a spectrum of features. Thus, due to diverse clinical presentation, and variable histological findings, imaging can often be the first sign of the disease. Radiological findings are, however, interpreted in conjunction with clinical and histological findings to establish the diagnosis of ECD.

Educational Goals / Teaching Points
1. Discuss recent updates in classification and pathogenesis of ECD. 2. Describe role of imaging and clinical and radiologic findings of ECD. 3. Discuss abdominal imaging findings of ECD. 3. Review new treatment options and therapeutic response evaluation.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
1. Introduction. 2. Epidemiology. 3. Pathology. 4. Pathogenesis. 5. Newer classification systems (2016 WHO classification of lymphoid neoplasms and 2016 Working group of Histiocytic Society Classification). 6. Mixed histiocystosis: an overlap syndrome. 7. Diagnosis. 8. Role of imaging. 9. Manifestations of ECD. 10. Retroperitoneal disease. 11. Endocrine involvement. 12. Hepatobiliary disease. 13. Testicular disease. 14. Newer imaging findings. 15. Association with nonhistiocytic malignancies and autoimmune diseases. 16. Markers of prognosis. 17. Molecular targets for treatment. 18. Treatment approach. 19. Therapeutic response evaluation and modified PERCIST criteria.

Conclusion
ECD is a multiorgan, inflammatory histiocytic neoplasm that most frequently affects the skeletal system producing the virtually pathognomonic finding of osteosclerosis around the knees. Other typical imaging findings comprise perinephric infiltrates (the hairy kidney), periaortic soft tissue (coated aorta), and right atrial pseudotumor. Radiological findings are, however, interpreted together with clinical and histological features to establish the diagnosis. Besides aiding in diagnosis, imaging helps to determine extent of disease, target sites for tissue sampling, assess treatment response, and predict prognosis.