Thymic Rosai-Dorfman Disease: A Rare Presentation With Unusual Histopathologic Features.

Rosai-Dorfman disease (RDD) represents a clinicopathological entity within the spectrum of histiocytic neoplastic disorders, characterized by histiocytic proliferation with distinctive histopathological features and heterogeneous clinical presentations. We report an unusual presentation of thymic RDD in a 38-year-old female patient, with an anterior mediastinal soft tissue mass identified on computed tomography. Histologically, sections of the thymus demonstrated characteristic RDD features accompanied by a complex microenvironment containing both necrotizing and nonnecrotizing granulomatous inflammation. Immunohistochemical stains revealed diffuse positivity for S-100, CD68, and OCT2 in the lesional histiocytes, with focal cyclin D1 expression and absence of CD1a immunoreactivity. CD20, CD5, and CD3 highlighted surrounding reactive B-cell and T-cell lymphocytes. The patient's postoperative course was unremarkable. Subsequent 3-month surveillance demonstrated no evidence of disease recurrence.