Rosai-Dorfman disease of the breast is a rare benign inflammatory disorder that can mimic breast cancer clinically and on imaging studies. Both men and women may present with a palpable lump or tenderness and can have either a single lesion or multiple lesions. Strong staining of histiocytes with S100 is characteristic of Rosai-Dorfman. Although nonspecific, emperipolesis is also characteristic and supports the diagnosis when present. Early pathologic diagnosis of Rosai-Dorfman disease of the breast is key because the disease can be treated conservatively

Typical breast imaging findings are classified as suspicious or highly suspicious for malignancy, and breast involvement less commonly mimics fibrocystic disease or fibroadenoma.

• Mammography may show a high-density lobulated mass with a partially circumscribed or ill-defined margin. Nodules can be large and can infiltrate the breast parenchyma. Small diffuse breast nodules have also been reported.

• On breast US, the mass appears hypoechoic with indistinct or angulated margins and shows increased vascularity on color Doppler imaging. Often, it can be associated to diffuse radial hypoechoic projections protruding from the lesion surrounded by an echogenic fat halo suggesting malignancy (it's related to edema or local inflammatory changes). 

Acknowledgments to Dra Cristina Meléndez Muñoz for contributing with the microscopic images.