Go, Ronald S

Abstract Malignant histiocytic neoplasms (MHNs) are rare tumors derived from the mononuclear phagocyte system (MPS), encompassing histiocytic sarcoma, Langerhans cell sarcoma, interdigitating dendritic cell sarcoma, and other high-grade MPS lineage tumors. Despite advances in understanding histiocytic neoplasms, MHNs remain diagnostically challenging and lack standardized treatment algorithms. Current classification systems differ in lineage framing and fail to address mixed or ambiguous phenotypes, contributing to diagnostic uncertainty and inconsistent care. To address these gaps, the Histiocyte Society convened an international working group of pathologists and oncologists, including World Health Organization and International Consensus Classification contributors, to harmonize nomenclature, define minimum diagnostic criteria, and develop pragmatic treatment recommendations. Using a modified Delphi process and case-based review, the group formulated >40 consensus statements spanning classification, pathology, molecular testing, clinical evaluation, and therapeutic strategies. Key recommendations include the adoption of a unified MHN designation, use of a minimum immunophenotypic panel, integration of broad molecular profiling, and the documentation of previous hematopoietic malignancy. Treatment algorithms emphasize surgical resection for unifocal disease and targeted therapy or immune checkpoint inhibition for multifocal disease when actionable mutations or programmed death ligand 1 expression are present. These consensus recommendations aim to reduce diagnostic ambiguity, standardize reporting, and improve outcomes for both pediatric and adult patients. Future priorities include international registries to refine risk stratification and biomarker-driven trials exploring targeted therapy, immunotherapy, and combination approaches.

Erdheim–Chester disease (ECD) is a rare non-Langerhans cell histiocytosis characterised by multiorgan pathological histiocytic infiltration. Cardiac involvement occurs in 40–75% of patients and increases mortality. A 55-year-old woman with chest pain and dyspnoea was found to have an interatrial septal mass and aortitis suspicious for ECD without other organ involvement. After two inadequate transcatheter interatrial biopsies were obtained using ultrasound guidance alone, diagnostic samples were successfully obtained using intraprocedural frozen section examination. The patient started genotype-targeted treatment with good response. Biopsy confirmation of ECD is required to guide treatment; however, cardiac biopsies are uncommonly performed when other organs are affected. Our unique case of isolated cardiovascular involvement highlights how frozen sectioning with multimodal intraprocedural imaging guidance can improve diagnostic yield of endomyocardial biopsies. It also highlights how diagnosis of rare conditions requires careful multidisciplinary evaluation of affected organs, procedural risk and diagnostic yield. The use of frozen sectioning can improve diagnostic yield of endomyocardial biopsies.

PURPOSE BRAF and MEK inhibitors are standard treatments in histiocytic disorders, such as Erdheim-Chester disease (ECD). Some patients lack MAPK-pathway alterations, making these treatments less effective. METHODS We describe three patients with histiocytic disorders who have novel non-MAPK pathway alterations. These alterations were studied through genomic and in silico analyses when applicable, then treated with off-label medications rationally selected on the basis of genomic alterations. RESULTS Patient 1 had rapidly progressive ECD involving the CNS. A CSF1R in-frame deletion (p.S560_P566del) was identified, and in silico modeling predicted a gain-of-function mutation. This alteration was targeted with pexidartinib, which led to a clinical complete response (CR) within 2 months, and a partial response (PR) on imaging after 3 months. After 15 months, the disease became resistant to pexidartinib and transformed to histiocytic sarcoma. Patient 2 has skin-only involvement of a xanthogranuloma disorder. A KIF5B-FGFR1 fusion was identified on RNA sequencing and targeted with pemigatinib. At 24 months of follow-up, she remains in a clinical PR. Patient 3 has ECD involving the bone marrow, gastrointestinal tract, and subcutaneous tissues. A MEF2C-FLT3 fusion was identified and targeted with sorafenib. He achieved a clinical CR and radiographic PR within 3 months, which has continued for 30 months. CONCLUSION We report three patients with histiocytic disorders harboring novel alterations who had sustained responses to off-label kinase inhibitors specific to their histiocytic disorder. Pathogenic variants outside of the MAPK pathway, including variants of unknown significant, may be targeted with readily available small molecules.

Lin et al. report the long‐term follow‐up of a phase II trial involving 95 adult patients with Langerhans cell histiocytosis (LCH), investigating the combination of methotrexate and cytarabine (MA). After a median follow‐up of 6.5 years, the study showed high response rates, with 90% overall response; 55% of patients free from an event such as disease progression, poor response or death; and 93% of patients were alive, though nearly half experienced febrile neutropenia. This prospective study helps fill gaps in understanding adult LCH treatment, indicating that fixed‐duration chemotherapy can yield durable responses despite its associated risks. It emphasizes the importance of personalized treatment decisions, considering both fixed‐duration chemotherapy and continuous targeted agents based on patient and disease‐specific factors.Commentary on: Lin et al. Long term follow‐up of methotrexate and cytarabine in adult patients with Langerhans cell histiocytosis. Br J Haematol 2025; 206:576‐584.

Abstract In this retrospective study, BRAF mutation status did not correlate with disease extent or (event-free) survival in 156 adults with Langerhans cell histiocytosis. BRAFV600E was associated with an increased incidence of second malignancies, often comprising hematological cancers, which may be clonally related.