Langerhans Cell Histiocytosis in Adults: A Canadian Multicenter Case Series.

Langerhans cell histiocytosis (LCH) is a rare clonal myeloid neoplasm. Canadian data on clinical characteristics, molecular profile, and treatment outcomes is limited. This study aims to report the initial experience of a Canadian rare diseases program, reflecting "real-world" diagnostic pathways, referral patterns, and treatment heterogeneity across multiple provinces. We conducted a retrospective review of patients managed in an adult-care cohort with histologically confirmed LCH diagnosed between 2000 and 2025 across multiple Canadian centers. Patients diagnosed as children and subsequently transferred to adult care were included. Clinical features, radiologic findings, histopathology, molecular testing, treatment approaches, and outcomes were collected and analyzed. Thirty-one patients were identified, with a median age at diagnosis of 42 years (range: 2-84) and a male predominance (65%). Bone (74%), lung (29%), and skin (16%) were the most commonly involved sites. Concomitant or subsequent malignancies were present in 19% of patients. Molecular testing found BRAFV600E mutations in 44% of tested patients. The most common first-line systemic therapy was cytarabine (n = 10), followed by other drugs such as hydroxyurea, vemurafenib, and cladribine. This series represents the initial experience of a Canadian rare disease referral program and captures the clinical heterogeneity and longitudinal adult care of patients with LCH across multiple provinces. Variability in treatment approaches highlights the need for collaborative prospective natural history studies and coordinated clinical trials.