Adult Langerhans cell histiocytosis (LCH) remains poorly defined. We retrospectively studied 266 newly diagnosed LCH patients to understand the clinical presentation, treatment, and prognosis of adult LCH. The median age at diagnosis was 32 years (range, 18-79 years). At the time of diagnosis, 40 patients had single lesions within a single system, 18 patients had single pulmonary LCH, 26 patients had multiple lesions within a single system (SS-m), and 182 patients had multisystem disease (MS). The most common organ involved in MS patients was the bone (69.8%), followed by the pituitary (61.5%) and lung (61.0%). BRAFV600E , BRAF deletion, and MAP2K1 mutation were detected in 38.8%, 25.4%, and 19.4% patients, respectively. BRAF deletion was found more common in patients with MS LCH compared to single-system LCH (38.5% vs 7.1%, p = .004), also in patients with liver involvement (69.2% vs 14.3%, p < .001). The estimated 3-year overall survival (OS) and event-free survival (EFS) rates were 94.4% and 54.7%, respectively, in SS-m and MS LCH. Multivariate Cox regression showed that involvement of the liver or spleen at baseline predicted poor EFS and receiving cytarabine-based therapy as a first-line treatment and age older than 30 years at diagnosis predicted favorable EFS. The involvement of risk organs and age older than 50 years predicted poor OS, and receiving cytarabine-based therapy predicted favorable OS. Therefore, BRAF deletion was correlated with MS LCH, particularly those with liver involvement. Liver or spleen involvement at baseline indicates a poor prognosis, and a cytarabine-based regimen could be considered as first-line treatment for adult LCH patients.
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