Distinct [18F]FDG-PET imaging features of a newly recognized and yet uncharacterized RDD-ECD overlap disease entity

A newly recognized histiocytosis entity, encompassing clinical and histopathologic features of Rosai-Dorfman disease (RDD) and Erdheim-Chester disease (ECD), is driven by MAP2K1 mutations [1, 2].[ 18 F]fluorodeoxyglucose ([ 18 F] FDG) positron emission tomography (PET) features have not yet been reported.This 46 year-old man presented with a two-year history of clinical hallmarks resembling RDD rather than ECD, including lymphadenopathy and painless testicle enlargement [3], being also visible on [ 18 F]FDG-PET (A).Testicular RDD-ECD involvement was also reported in 6/13 patients by Razanamahery et al. [2].Diffuse omental proliferations,