Rosai-Dorfman disease mimicking IgG4-related disease.

Rosai-Dorfman disease (RDD) is a rare histiocytic disorder that may clinically and histologically resemble IgG4-related disease (IgG4-RD), especially in the presence of IgG4-positive plasma cell infiltration. In this case, a 69-year-old woman with generalized lymphadenopathy, constitutional symptoms, and elevated IgG4 levels was initially suspected to have IgG4-RD based on core needle biopsy. However, further evaluation with excisional lymph node biopsy revealed emperipolesis and S100-positive histiocytes, confirming the diagnosis of RDD. This case illustrates the diagnostic challenge of differentiating RDD from IgG4-RD, given the overlap in both clinical presentation and histopathological features, and highlights the importance of excisional biopsy combined with careful clinical evaluation to ensure an accurate diagnosis and appropriate management.