Pediatric Pathology: Emerging Entities, Current Trends, and Comprehensive Updates, "Histiocytosis".

Histiocytic lesions, now classified as a myeloid-derived neoplasia, consist of mutated monocyte-derived dendritic and macrophage cells infiltrating tissues. They are primarily driven by kinase-activating alterations in mitogen-activated protein kinase (MAPK), Phosphatidylinositol 3-kinase (PI3K)/AKT, or other receptor tyrosine kinase (RTK) signalling pathways, leading to extracellular signal-regulated kinase (ERK) overexpression and growth survival. In children, these include Langerhans cell histiocytosis (LCH), juvenile xanthogranuloma (JXG), Rosai-Dorfman-Destombes disease (RDD), and ALK-positive histiocytosis. Additional diagnostic challenges are discussed including mixed histiocytosis and malignant histiocytic neoplasms (MHN), including Histiocytic Sarcoma and Langerhans cell sarcoma, described in terms of primary or secondary disease, if after a prior hematologic malignancy.