A Red Lobulated Facial Mass: A Clinicopathological Challenge.

A 32-year-old female presented with a 1-year history of a progressive, lobulated facial nodule. Histopathological examination revealed a dense infiltrate of S100-positive histiocytes exhibiting emperipolesis, the hallmark feature of Rosai-Dorfman disease (RDD). The condition was refractory to initial methotrexate and isotretinoin therapy. Successful management was achieved with systemic dapsone (100 mg daily), resulting in significant regression after 1.5 years, followed by serial excision of the residual lesion for complete clearance. Adult-onset xanthogranuloma (AOXG), sarcoidosis, and anaplastic large cell lymphoma (ALCL) are key differential diagnoses for RDD. They are distinguished by their unique histopathological features, such as AOXG having Touton giant cells, sarcoidosis showing noncaseating granulomas, and ALCL featuring anaplastic CD30+ cells. RDD is identified by S100+ histiocytes exhibiting emperipolesis. We report a rare and clinically challenging case of RDD diagnosed with the aid of histopathology and immunohistochemistry. Additionally, we emphasize the role of a combined approach of serial surgical excisions and systemic dapsone therapy.