Innovative sequential therapy in a rare case of refractory Erdheim-Chester disease with bilateral pulmonary involvement.

Background: Erdheim-Chester Disease (ECD) is a rare, multisystemic histiocytosis with complex diagnosis and management. Case Presentation: We report a case of a 44-year-old male with ECD,confirmed by lung biopsy, presenting with persistent fatigue, intermittent low-grade fever, and bilateral interstitial lung disease. The patient initially received vincristine and prednisone, followed by maintenance therapy with 6-mercaptopurine, which resulted in both symptomatic and radiographic improvement. However, disease progression was noted after six months. Second-line therapy using a cytarabine-based regimen adapted from the Japan LCH Study Group-02 protocol achieved sustained remission for over two years. Conclusion: This case highlights the diagnostic complexity and therapeutic challenges of pulmonary-predominant ECD. It further underscores the potential utility of sequential cytotoxic chemotherapy as an alternative strategy in settings where targeted therapies, such as BRAF inhibitors, are not accessible. Future studies are needed to validate the role of such regimens in broader ECD populations.