[Status of Langerhans cell histiocytosis in children and adults]

Whether Langerhans cell histiocytosis (LCH) is an inflammatory disorder or a neoplasm is an ongoing debate. On the basis of the identification of the BRAF V600E mutation in 2010, LCH should be defined as an inflammatory myeloid neoplasia. Mutually exclusive BRAF V600E (50-60%) and MAP2K1 (12-25%) mutation renders the ERK activation. BRAF V600E mutations were detected in approximately 50-60% of patients with Erdheim-Chester disease (ECD), and these patients are being treated with vemurafenib, a selective BRAF V600 kinase inhibitor, approved by the US Food and Drug Administration. The Japan Langerhans cell histiocytosis study group-02 protocol study showed five-year overall survival rate of 92% and event-free survival of 46% in risk-organ involvement of positive multi-system LCH. We aimed to improve the quality of life by reducing relapses and development of late complications. It is essential to achieve a close cooperation between hematologists catering to pediatric and adult patients with LCH. Furthermore, novel targeted therapies of MAPK inhibitors are required along with intensified chemotherapy to achieve better outcomes in patients with LCH in Japan.