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

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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, manifesting as faintly [^18^F]FDG-avid omental thickening resembling a fishing net (SUV_max_ 5.5; A, B, C), and symmetric large-joint synovitis were reported as specific features of RDD-ECD [1, 2], Notably, none of these features are characteristic of RDD or hitherto known ECD subtypes. Other RDD and/or ECD features were absent [4–7].

Open biopsy targeted peritoneal lesions (D) localized on the diaphragm (d), peritoneum (p) and greater omentum (go). Histopathology revealed nodular fibrosis, foamy cell infiltrates, pigment deposits and chronic perivascular inflammatory infiltrates (E). Molecular genetic analyses confirmed presence of a characteristic MAP2K1 mutation (p.Q56P).

Diamond et al. effectively treated a patient harboring the identical mutation with MEK inhibitors [8]. FAPI-PET focusing on fibrosis aspects of histiocytosis might help determining disease extent and assessing treatment response [9, 10].

In summary, the newly recognized RDD-ECD overlap histiocytosis demonstrates distinct [^18^F]FDG-PET features setting it apart from RDD and ECD. The concurrent presence of omental proliferations, symmetric large-joint synovitis, and high testicular uptake should raise suspicion for this yet uncharacterized disease.