Histiocytic disorders: insights into novel biology and implications for therapy of Langerhans cell histiocytosis and Erdheim-Chester disease.

Langerhans cell histiocytosis (LCH) and Erdheim-Chester disease (ECD) are caused by mutations of the MAPK pathway, most often BRAFV600E, in myeloid dendritic cells that lead to some overlapping and other unique presentations of the two diseases. LCH occurs in both children and adults, but ECD is primarily found in the latter. The challenges in diagnosing these conditions relates to the rarity of the conditions and that they mimic diseases that are more widely understood, such as certain rashes; bone, lung, and renal diseases; and other malignancies. The histopathology of LCH is definitive, but not so for ECD. Treatment with BRAF and MEK inhibitors has become one of the important advances in the care of these patients.