Assessment of BRAF mutation in pulmonary Langerhans cell histiocytosis in tissue biopsies and bronchoalveolar lavages by droplet digital polymerase chain reaction.

The neoplastic nature of pulmonary Langerhans cell histiocytosis (PLCH) is still debated. As the detection of BRAF and MAP2K1 mutations in patients with PCLH is now considered for such assessment, the aim of our study was to evaluate digital droplet polymerase chain reaction (ddPCR) in PCLH diagnosis. We retrospectively analyzed BRAF detection in a cohort of 42 PCLH tissues and 18 bronchoalveolar lavages (BALs) by ddPCR, immunohistochemistry, high-resolution melting PCR (HRM), and next-generation sequencing (NGS). The presence of BRAF mutation was assessed by at least two concordant techniques to further evaluate specificity and sensitivity of each method. The BRAF mutation prevalence was detected in 18 out of 41 cases by ddPCR, 10 out of 36 cases by HRM PCR, and 16 out of 31 cases by NGS. BRAF immunohistochemistry sensitivity was 94%, and specificity was 79%. HRM PCR sensitivity was only 59%, and specificity was 100%. NGS sensitivity and specificity were 100% for interpretable cases (n = 31), but in 11 cases, this technique was non-contributive. The analysis of BAL samples by ddPCR revealed a BRAF mutation both in tissue and in BAL samples in one patient, a wild-type status both in tissue and in BAL samples in two patients, and a wild-type BRAF status in BAL and a BRAF mutation in tissue samples in four patients. The study supports the usefulness of ddPCR for BRAF status assessment in either tissue or BAL samples to increase the accuracy of PLCH diagnosis.