Langerhans cell histiocytosis (LCH) is a myeloid neoplastic disorder characterized by a clonal proliferation of modified dendritic cells with the Langerhans cell phenotype (CD1a+, CD207/Langerin+) [[1], [2], [3]]. These cells can accumulate in one or more tissues or organs, typically accompanied by an eosinophil-rich inflammatory background. Recurrent activating mutations in genes of the MAPK pathway are observed in >85 % of LCH cases, playing a key role in the pathogenesis of the disease. Notably, BRAFV600E mutations have been identified in 50–60 % of LCH cases. Alternatively other genes within the MAPK pathway, particularly MAP2K1, are involved (20–30 % of cases), and these mutations exhibit mutual exclusivity [1,[4], [5], [6]].

LCH has a broad clinical spectrum with highly variable organ and system involvement. It can be a single-system or multi-system disease, with unifocal or multifocal involvement within a single system. Bone is the most commonly affected site, which is often accompanied with soft tissue extension. Other commonly affected systems include skin, lung, liver, spleen, gastrointestinal tract, and bone marrow [1]. Multi-system LCH can be classified into low-risk and high-risk categories based on the extent of organ involvement and the presence of disease in “risk organs” such as the liver, spleen, or bone marrow [1,2,7,8].

Diagnosis of LCH requires lesional tissue examination, with histopathologic examination demonstrating proliferation of characteristic neoplastic Langerhans cells with pale eosinophilic cytoplasm and distinctive coffee bean-shaped or cleaved nuclei, admixed with inflammatory cells including eosinophils, macrophages, lymphocytes, neutrophils, and giant cells. The diagnosis is confirmed by immunohistochemical evidence of neoplastic cells expressing CD1a, Langerin, and S100 [9].

LCH typically affects children but can also occur in older patients [10]. The incidence rate of LCH is estimated to be 2.6 to 8.9 cases per million in children under 15 years old, compared to only 1 to 2 cases per million in adults [11]. While LCH has been extensively studied in children, less is known about its natural history in adults. To better understand the expected clinical course of adult patients who are diagnosed with LCH on bone biopsies, we retrospectively analyzed a cohort of osseous LCH cases diagnosed in our Musculoskeletal Service.