Residual DCIS Versus No Residual Breast Tumor After Neoadjuvant Chemotherapy in ypN0 Breast Cancer: A Retrospective Clinical, MRI, and Survival Comparison

Background/Objectives: The prognostic significance of residual ductal carcinoma in situ (ypTis) compared with no residual tumor in the breast (ypT0) after neoadjuvant chemotherapy (NAC) remains uncertain. We compared clinicopathological, MRI, treatment, and survival outcomes between these groups. Methods: This single-center retrospective study included consecutive patients with stage II–III breast cancer who achieved ypN0 and ypT0/is status after NAC and definitive surgery between 2019 and 2023. Univariable and multivariable logistic regression analyses were performed to identify factors associated with ypTis. DFS and OS were assessed using Kaplan–Meier analysis and log-rank tests. Results: Among 175 patients, 139 (79.4%) had ypT0 and 36 (20.6%) had ypTis. Median follow-up was 51 months. Clinical T-stage distribution differed significantly between groups (p = 0.010), with cT4 disease in 13.9% of ypTis versus 2.2% of ypT0 patients. HER2 positivity and MRI-detected satellite foci were more frequent in the ypTis group. In multivariable analysis, HER2 positivity (OR 2.50, 95% CI 1.05–5.93; p = 0.038), and MRI-detected satellite foci (OR 2.41, 95% CI 1.12–5.16; p = 0.024) were associated with ypTis. Local recurrence occurred in 3/36 (8.3%) ypTis and 1/139 (0.7%) ypT0 patients. DFS did not differ significantly between groups (5-year estimates: 78.7% vs. 78.9%; p = 0.860). OS also showed no significant difference (5-year estimates: 96.8% vs. 87.3%; p = 0.320). Conclusions: Residual DCIS was not associated with statistically significant differences in DFS or OS following NAC. However, these findings do not establish prognostic equivalence and should be interpreted cautiously given the limited number of ypTis patients and outcome events.