Pires, Yumay

Abstract Erdheim-Chester disease (ECD) and Langerhans cell histiocytosis (LCH) are two rare diseases typically separated by clinical, radiologic, and histologic criteria. However, coexistence of both entities is rare, with fewer than 50 cases reported. Although there are reports of patients with LCH and ECD in the same organ, there have been no reported cases of the two diseases involving the lung. We report two cases of ECD-LCH “overlap syndrome” occurring in the lung. In both cases, the diagnoses were supported by typical immunohistochemical patterns, and in both cases, the BRAF V600E mutation was identified by next-generation sequencing and confirmed by droplet digital polymerase chain reaction. In neither case was either diagnosis suspected. The recognition of overlap histiocytoses is important. Although LCH and ECD may differ in treatment and prognosis, in some cases, such as the two patients reported here, they may have the same activating mutations and may be able to be treated with the same targeted therapy.