Flat epithelial atypia (DIN 1a, atypical columnar change): an underdiagnosed entity very frequently coexisting with lobular neoplasia

Aims: Flat epithelial atypia of the breast [FEA; synonyms: ductal intraepithelial neoplasia (DIN) 1a, atypical columnar change] is increasingly recognized by pathologists and shows distinct genetic alterations. The aim of this study was to determine its biological significance as an incidental finding in breast biopsy specimens. Methods and results: On the assumption that both FEA and lobular neoplasia (LN) derive from progenitor cells in the terminal ductal‐lobular unit, we investigated the association between FEA and LN semiquantitatively in 111 excisional breast biopsy specimens which contained LN, but did not contain ductal carcinoma in situ (DCIS) or invasive carcinoma. Ninety‐six cases (86.5%) revealed coexistence of LN and FEA (P