Rosai-Dorfman disease of the breast.

A 56-year-old female was recalled for assessment following screening mammography that demonstrated a new 9-mm indeterminate density in the left breast. Clinical breast examination was normal. Ultrasound confirmed a 9-mm predominantly well-defined hypoechoic breast mass. Core biopsy demonstrated large histiocytes with emperipolesis and positive staining for S100, which is consistent with Rosai-Dorfman disease (RDD). Multidisciplinary team discussion concluded case concordance. The patient was discharged back to the screening programme. RDD is a rare, benign condition that may mimic breast cancer. This case demonstrates that identification of RDD on core needle biopsy may help avoid unnecessary surgery.