Rosai-Dorfman disease of the breast: a clinicoradiologic and pathologic study.

Rosai-Dorfman disease (RDD) is an uncommon histiocytic disorder typically involving lymph nodes and less frequently extranodal tissues. RDD involving the breast is rare and may clinically and radiologically mimic neoplastic and non-neoplastic disorders. We report seven patients with breast RDD, describe their clinicoradiologic and pathologic features, and discuss the differential diagnosis. Patients, ranging from 15 to 74 years of age, presented with unilateral and unifocal (5/7) or bilateral and multifocal (2/7) masses. RDD was either confined to the breast (6/7) or concurrently involved a lymph node (1/7). Masses ranged from 8 to 31 mm, categorized as Breast Imaging-Reporting and Data System (BI-RADS) 4 (6/7) or 5 (1/7). All cases showed similar morphology with many large histiocytes displaying emperipolesis with associated fibrosis and dense lymphoplasmacytic infiltrate. The abnormal histiocytes co-expressed CD68/CD163, S100, OCT2, and Cyclin D1 (7/7), and were negative for CK AE1/AE3 (7/7), CD1a (7/7), and BRAF V600E (6/6). Flow cytometry (n = 3), kappa/lambda in situ hybridization (n = 5), and IgG4/IgG immunohistochemistry (n = 1) did not reveal lymphoma or IgG4-related disease. No mycobacterial or fungal organisms were identified on acid-fast bacillus (AFB) and Grocott methenamine silver (GMS) stains (n = 5). Three patients underwent complete excision and none recurred or progressed to systemic disease during follow-up (88-151 months). In summary, breast RDD should be included in the differential diagnosis of a mass-forming breast lesion. Histopathology with ancillary studies and clinicoradiologic correlation is essential for accurate diagnosis and optimal clinical management. Patients with RDD of the breast have an excellent prognosis after complete excision.