Rare Subcutaneous Manifestation of Rosai-Dorfman Disease on FDG PET/CT in an Adolescent.

Rosai-Dorfman disease (RDD) most commonly presents as painless, massive cervical lymphadenopathy. In this case, a 17-year-old girl presented with a progressively enlarging, painful mass on her back. Ultrasound revealed subcutaneous soft tissue lesions in the posterior trunk, and FDG PET/CT demonstrated hypermetabolic activity within these lesions as well as in an enlarged left inguinal lymph node. Histopathologic examination confirmed the diagnosis of Rosai-Dorfman disease. This case underscores the importance of including RDD in the differential diagnosis of atypical subcutaneous masses, particularly in light of its potential for uncommon extranodal involvement.