Extranodal Rosai–Dorfman Disease With Increased IgG4-Positive Plasma Cells Involving the Breast: A Case Report With Review of the Literature

Rosai-Dorfman disease (RDD) is a rare histiocytic disorder that most commonly affects lymph nodes but can also present in extranodal sites, including the breast. Immune-related RDD may be associated with increased plasma cells, with a significant proportion of the plasma cells expressing IgG4. These cases typically lack the other features of IgG4-related disease. We report a case of a 20-year-old woman who presented with a painless mass in the left breast. Prior biopsy was diagnosed as RDD with increased plasma cells. Ultrasound scan showed a well-circumscribed hypoechoic hypervascular solid subcutaneous mass with preservation of the surrounding breast parenchyma. Clinical assessment and imaging showed no evidence of regional lymphadenopathy or involvement of other organ systems. A wide local excision was performed. The excision showed a firm, unencapsulated fibrotic subcutaneous nodule. The histopathology revealed an expansion of the subcutaneous tissue, characterized by sheets of large histiocytes with round-to-oval nuclei and abundant pale to eosinophilic cytoplasm, set against a background of lymphoplasmacytic inflammation and stromal fibrosis. Several histiocytes demonstrated emperipolesis. The lesion was completely excised with clear margins. Immunohistochemistry showed that the histiocytes were positive for S100 and CD68, and negative for CD1a and AE1/AE3. The morphology and immunohistochemistry confirmed the diagnosis of extranodal RDD limited to the subcutaneous tissue of the breast. Extranodal RDD involving the breast is exceedingly rare and can mimic carcinoma both clinically and radiologically. Accurate diagnosis requires histologic and immunohistochemical confirmation.