Childhood Langerhans cell histiocytosis hematological involvement: severity associated with BRAF^V600E loads

• LCH-HI is life threatening, defined as hemoglobin ≤7 g/dL and/or platelets ≤20 × 10⁹/L.

• LCH-HI severity, associated with BRAF^V600E variant and macrophage activation, may require early MAPK inhibitors.

Hematological involvement (HI) is one of the life-threatening risk organs (ROs) in Langerhans cell histiocytosis (LCH). Lahey criteria have defined HI since 1975 as hemoglobin <10 g/dL, platelets <100 × 10⁹/L, leukopenia (white blood cell count <4 × 10⁹/L), and/or neutrophils <1.5 × 10⁹/L. Among the 2313 patients aged <18 years enrolled in the French National Histiocytosis Registry (1983-2023), 331 developed HI (median age at diagnosis, 1 year); median follow-up lasted 8.1 years. Bone marrow aspirate smears and biopsies may show reactive histiocytes, hemophagocytosis, or myelofibrosis but never confirm the diagnosis. Fifty-eight patients (17%) developed macrophage-activation syndrome, sometimes related to acute Epstein-Barr virus or cytomegalovirus infection, sometimes months before typical LCH manifestations appeared. Hemoglobin and platelet thresholds for initiating transfusion(s) appear to accurately distinguish 2 groups: mild HI (MHI; >7 g/dL and >20 × 10⁹/L, respectively) and severe HI (SHI; ≤7 g/dL and/or ≤20 × 10⁹/L). Each entity has different organ involvements, laboratory parameters, mutational status, blood BRAF^V600E loads, drug sensitivities, and outcomes (MHI and SHI 10-year survival rates, 98% and 73%, respectively). Since 1998, mortality first declined with combination cladribine-cytarabine therapy and then with MAPK inhibitors since 2014. Forty-one patients (12%) developed neurodegenerative complications that have emerged as a risk for long-term survivors. These results suggest limiting the HI-RO definition to SHI, because it encompasses almost all medical complications of LCH. Future clinical trials might demonstrate that targeted therapy approaches would be better adapted for these patients, whereas MHI can be managed with classic therapies.