Rosai-Dorfman disease in the breast.

Rosai-Dorfman disease (also known as sinus histiocytosis with massive lymphadenopathy) is a rare benign proliferative disorder of histiocytes that typically involves the lymph nodes and can also involve extranodal sites. Rosai-Dorfman disease confined to the breast is extremely rare, but important to recognize as it can mimic malignancy. We present the case of a 63-year-old woman who presented with a palpable breast lump that was highly suspicious for malignancy based on mammogram and ultrasound appearance. Biopsy revealed inflammatory tissue with lymphoplasmacytic and histiocytic predominance. The diagnosis of Rosai-Dorfman was made based on characteristic staining of histiocytes with S-100 and the presence of emperipolesis. Early recognition of this benign disease entity spared the patient further investigation and surgical intervention.