Flat Epithelial Atypia of the Breast.

Lesions of the breast characterized by enlarged terminal duct lobular units lined by columnar epithelial cells are being encountered increasingly in breast biopsy specimens. Some of these lesions feature cuboidal to columnar epithelial cells in which the lining cells exhibit cytologic atypia. The role of these lesions (recently designated "flat epithelial atypia" [FEA]) in breast tumor progression is still emerging. FEA commonly coexists with well-developed examples of atypical ductal hyperplasia, low-grade ductal carcinoma in situ, lobular neoplasia, and tubular carcinoma. These findings and those of recent genetic studies suggest that FEA is a neoplastic lesion that may represent a precursor to or the earliest morphologic manifestation of ductal carcinoma in situ. Additional studies are needed to better understand the biologic nature and clinical significance of these lesions.