Abstract

Background: Erdheim-Chester disease is a rare non-Langerhans histiocytosis characterized by multisystem histiocytic infiltration. Its coexistence with myeloproliferative neoplasm is uncommon and suggests a biological overlap between histiocytic and myeloid neoplasms. Case Presentation: A 68-year-old man was admitted for progressive diplopia with a history of myeloproliferative neoplasm treated with Hydroxyurea. Laboratory tests showed persistent thrombocytosis and a CALR type 1 mutation. Imaging demonstrated bilateral perirenal infiltration (“hairy kidney”), circumferential peri-aortic infiltration (“coated aorta”), and symmetric long-bone osteosclerosis. Perirenal biopsy revealed CD68-positive and CD1a-negative and S100-negative histiocytic cells. Next-generation sequencing identified the BRAF V600E mutation. The patient was diagnosed with Erdheim-Chester disease associated with CALR- mutated myeloproliferative neoplasm. Clinical status remained stable after supportive treatment. Conclusion: This case highlights the value of imaging, histopathology, immunohistochemistry, and molecular testing in diagnosing Erdheim-Chester disease and guiding targeted therapy.