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.