Primary pulmonary Langerhans cell histiocytosis: comprehensive clinicopathologic and molecular genetic analysis of 13 cases.

Primary pulmonary Langerhans cell histiocytosis (PLCH) is a rare entity. To characterize its clinicopathological and molecular features, 13 cases (15 specimens) were retrospectively analyzed, incorporating clinical information, imaging findings, histopathological features, immunohistochemical results, BRAFV600E mutation status, and DNA-based next-generation sequencing data. The patients ranged in age from 17 to 59 years (mean, 30 years), with a male-to-female ratio of 12:1; all male patients were smokers. Computed tomography demonstrated multiple pulmonary cysts in 12 cases, including pneumothorax in 3 cases, and a solitary nodular mass in 1 case. Histologically, 11 cases exhibited classic morphological features, whereas two small biopsy specimens showed subtle nuclear grooves and sparse background eosinophils, leading to an initial misdiagnosis of reactive lesions. Immunohistochemical analysis confirmed expression of S-100 (15/15), CD1a (15/15), Langerin (15/15), and cyclin D1 (3/3) in all cases. BRAFV600E mutation analysis was positive in one of four tested cases. Next-generation sequencing performed in six cases (seven lesions) revealed a non-frameshift BRAF deletion in one case, concurrent MAP2K1 and DUSP4 missense mutations in one case, an isolated DUSP4 missense mutation in one case, and RRAS mutations in two separate specimens from one patient. Four cases exhibited abnormalities in genes associated with DNA damage repair. Additional alterations involved multiple other signaling pathways, as well as abnormalities in hematopoietic regulation, epigenetic modulation, and related molecules. At a median follow-up of 23 months, all twelve patients were alive, although one patient developed extrapulmonary lesions. In conclusion, PLCH predominantly affects adult smokers, shows a marked male predominance, and generally carries a favorable prognosis. In cases with atypical morphology, immunohistochemical and molecular analyses should be integrated with characteristic imaging findings to avoid misdiagnosis. Furthermore, this study provides additional insights into the molecular pathogenesis of PLCH.