Case Report: When an unknown fever unmasks Erdheim-Chester disease: a case showing the use of multimodality imaging and the successful interferon alpha therapy.

Background: Erdheim-Chester disease (ECD) is an extremely rare histiocytic neoplasm with a highly variable and often unpredictable clinical course and prognosis. Multiple organs can be affected mimicking other systemic diseases both clinically and radiologically. This makes timely diagnosis more challenging. Case Presentation: We report the case of a 39-year-old male presented with a 15-day history of fever and retrosternal chest pain. Ha has a long history of diabetes insipidus, hypogonadism of unknown aetiology. On exploration, computed tomography revealed retroperitoneal fibrosis and "hairy" kidney appearance. Subsequent multimodality imaging studies showed the extensive multisystemic involvement including the skeletal system, retroperitoneum, pituitary, retroperitoneum, kidneys, heart, central nervous system, myocardium, pleura, lymph nodes and GI tract. Histopathology confirmed CD68+/CD1a- histiocytosis. Molecular analysis for BRAF mutation is currently pending. Pegylated interferon α and corticosteroids induced rapid clinical and biochemical improvement. Therapeutic response using a follow-up 18F-FDG PET scan revealed subsequent improved systemic lesions. Conclusion: A prolonged history of unexplained endocrine abnormalities in the context of fever of unknown origin should evoke the diagnosis of ECD. Multimodal imaging is essential for diagnosis and for monitoring treatment response. This case highlights the importance of the multidisciplinary approach that integrates clinical evaluation, histological analysis, and imaging studies for the optimal the management of this disease. Early initiation of immunomodulatory therapy can lead to better outcomes, even prior to confirmation of molecular findings.