Flat Epithelial Atypia of the Breast

Definition

Breast epithelial proliferation of 1-5 cell layers composed of cells with low-grade nuclear atypia but with architectural features not meeting the criteria for ductal carcinoma in situ

Columnar cell alteration with prominent apical snouts and secretions (CAPSS) with atypia
Columnar cell change with atypia
Columnar cell hyperplasia with atypia

Flat growth pattern
- Proliferative epithelium 1-5 cell layers thick
- Architectural complexity falls short of low grade DCIS and ADH (atypical ductal hyperplasia)
  - No partial or complete filling of ducts
  - Arcades and micropapillary formations absent or very rare
  - Tufts and mounds may be seen
- Involves distended terminal duct lobular units
Low grade nuclear atypia
- Nuclei 2-3x size of a RBC (10-15 microns)
- Minimal pleomorphism
- Short of high grade DCIS
Lined by cuboidal to round cells
- Frequently with eosinophilic cytoplasm
- Frequently with prominent apical snouts
Columnar cell change and hyperplasia (lesions without nuclear atypia) are considered separately