Abstract: Background and purpose: Breast ductal carcinoma in situ with microinvasion (DCIS-MI) is considered to be the interim stage in the progression from breast ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC). We attempted to study the differences of clinicopathological features and immunohistochemistrybased subtypes of DCIS and DCIS-MI. Methods: In this retrospective study, 317 consecutive DCIS patients were recruited, including 227 (71.6%) cases with pure-DCIS and 90 (28.4%) with DCIS-MI. They were categorized into four groups: luminal-A ［estrogen receptor (ER)+ and/or progesterone receptor (PR)+, human epidermal growth factor receptor 2 (HER-2)-］, luminal-B (ER+ and/or PR+, HER-2+), ERBB2+ (ER-, PR-, HER-2+), and basal-like (ER-, PR-, HER-2-). Results: DCIS-MI patients tended to have larger tumor with higher nuclear grade (P=0.059 for size; P=0.002 for nuclear grade). The proportion of luminal-A tumors decreased while the proportion of basal-like tumors increased in DCIS-MI compared with pure-DCIS (P=0.001). Conclusion: Different distribution of subtypes among DCIS and DCISMI and their distinctive characteristics indicate they are distinct entities. DCIS-MI is a novel stage in the progression of DCIS with distinctive evolutions. Further studies with larger sample size are needed to replicate our observations.

Key words: Breast cancer, Ductal carcinoma in situ, Ductal carcinoma in situ with microinvasion