Real-world outcomes in adult histiocytosis: the uncharted territory of orphan diseases

Adult histiocytic disorders are rare, and limited data exists on spectrum, clinical presentation and real-world management strategies. Here, we present Canadian single-center experience spanning over 23 years. Ninety-six patients were identified, with Langerhans Cell Histiocytosis (LCH; <i>n</i> = 60), Erdheim-Chester disease (ECD; <i>n</i> = 11); Rosai-Dorfman disease (RDD; <i>n</i> = 19) constituting the majority. At a median follow-up of 38 months, median OS of our cohort was not reached with encouraging 5-year OS of 96.3%, 79.5% and 86% amongst LCH, ECD and RDD patients respectively. Although, 5-year PFS rates were higher for LCH and RDD compared to ECD patients (61.7% vs 47% vs 24.2%; <i>p</i> = 0.320), these were not statistically significant. LCH patients with RO involvement had inferior 2-year PFS compared to those without (33.3% vs 75.8%; <i>p</i> = 0.042). A high incidence of concomitant hematological malignancies, (<i>n</i> = 3/11 in ECD and <i>n</i> = 3/60 in LCH) in our cohort warrants the need for further translational studies to explore possible association.