Flat DIN 1 (flat epithelial atypia) on core needle biopsy: 63 cases identified retrospectively among 1,751 core biopsies performed over an 8-year period (1992-1999).

Uniform management of flat DIN 1 (flat epithelial atypia) on core needle biopsy (CNB) concerning surgical excision or clinical follow-up are lacking. In a retrospective review of 1,751 CNB over an 8-year period, we found 63 cases with flat DIN 1 as the most advanced lesion; follow-up was available in 55 cases. Of the 63 patients, 24 had a subsequent biopsy for 15 days to 10 years after the initial CNB, an infiltrating carcinoma was found in nine (14.3%) patients, seven (11.1%) in the ipsilateral, and two (3.2%) in the contralateral breast. Five underwent an excisional biopsy of the ipsilateral breast within less than 3 months of the initial CNB; none had either an invasive or intraepithelial carcinoma. Based on our findings, we consider flat DIN 1 a marker of slightly increased risk for subsequent development of invasive breast carcinoma. When flat DIN 1 is found on CNB as the most advanced lesion after mammographic correlation, an excisional biopsy is not mandatory; however, close follow-up is advised with repeat mammograms for early detection of any clinically occult carcinoma in the vicinity of flat DIN 1 that may have been missed by the CNB.