BACKGROUND: The incidence of flat epithelial atypia of the breast has been increasing owing to the increasing use of ultrasonography (US) and US-guided core needle biopsy. However, reports describing the sonographic features of flat epithelial atypia are fewer than those on its mammographic features.

CASE REPORT: A 47-year-old female had a regular 6 month-interval follow-up US for multiple bilateral breast masses detected on US. A new focal non-mass-like lesion measuring 2 cm in maximum diameter was noted in the 10 o’clock direction of the right breast on the follow-up US. Mammography was not performed during the follow-up period. The patient had no breast symptoms and no family history of breast cancer or other pertinent medical history. US-guided 14-gauge core needle biopsy yielded a pathological diagnosis of flat epithelial atypia without upgrade to malignancy, and this was further confirmed after complete excision of the non-mass-like lesion.

CONCLUSIONS: Flat epithelial atypia can present as a focal non-mass-like lesion on US, which has not been reported previously.

Keywords: Breast, Breast Diseases, Ultrasonography, Mammary, Mammography, Ultrasonography