Rosai-Dorfman disease of the breast: a mimic of breast malignancy.

Rosai-Dorfman disease (RDD) or sinus histiocytosis with massive lymphadenopathy (SHML) is primarily a nodal-based, idiopathic, benign proliferative disorder of histiocytes with 43% of these cases also involving extranodal sites. The breast is an unusual site of occurrence of RDD. We report two cases of this exceptional event. The first represents an intramammary nodal Rosai-Dorfman disease, while the second is an extranodal disease with sole involvement of the breast. In both, the possibility of malignancy was raised. Histological examination of the two breast lesions revealed sheets of characteristic large histiocytes displaying emperipolesis, a microscopic hallmark of this disease. Immunohistochemical and ultrastructural studies were also performed; the former showed cytoplasmic staining of histiocytes for S-100 protein, while histiocytes that engulfed lymphocytes and plasma cells were identified on electron microscopy.