CSF1R and sinus histiocytosis with massive lymphadenopathy: While RDD was previously considered a reactive disorder due to immune abnormalities and associations with autoimmune conditions, recent discoveries have identified somatic mutations in the MAPK pathway (KRAS, NRAS, MAP2K1, and ARAF) and activating mutations in CSF1R in some cases, suggesting a neoplastic origin in subsets of RDD [18, 35].