Localized Langerhans cell histiocytosis of the thymus with BRAF V600E mutation: a case report with immunohistochemical and genetic analyses

SummaryWe report a case of localized Langerhans cell histiocytosis characterized by clonal aggregation of Langerhans cells in the thymus and identified with molecular genetic study. A 43-year-old Japanese woman was found to have an anterior mediastinal mass by radiologic studies. Laparoscopy-assisted biopsy was subsequently performed. Histologically, we found subtle nodules scattered in the thymus consisting of aggregated Langerhans cells, which caused destruction of Hassall corpuscles. These Langerhans cells were immunohistochemically positive for S-100, CD1a, and CD207/langerin. Using allele-specific polymerase chain reaction and immunohistochemistry with mutation-specific antibody VE1, the BRAF V600E mutation was identified in aggregated Langerhans cells. At the medical follow-up, the thymic tumor had spontaneously regressed; however, identification of oncogenic BRAF mutation supports the neoplastic nature of the current case.