Background: After the ACOSOG Z0011 trial, sentinel lymph node (SLN) biopsy became the standard for staging early breast cancer, demonstrating that in select patients with limited lymph node involvement, SLN biopsy alone was sufficient. However, this trial excluded patients with extracapsular extension (ECE), leaving its impact on recurrence rates unclear. This study evaluates the impact of ECE on recurrence rates in SLN-positive breast cancer patients and compares recurrence rates in patients with ECE based on the type of axillary surgery. Methods: We conducted a cohort study of 1,579 SLN-positive breast cancer patients treated at Seoul National University Hospital from 2009 and 2018. Included were patients with invasive breast cancer, T1 or T2 tumor stage, and positive pathology results in both frozen and permanent SLN biopsy. Radiation status and type of surgery (breast-conserving or mastectomy) were not considered. Those who received neoadjuvant chemotherapy were excluded. Patients were divided into two groups based on the presence (N=266) or absence (N=1,313) of ECE. Recurrence rates were analyzed by evaluating disease free survival (DFS) using Kaplan-Meier and Cox regression models. Results: The presence of ECE was associated with significant differences in patient characteristics. ECE-positive patients were older (51.8 vs 50.2 years, p=0.023) and more likely to undergo SLNB+ALND (82.0% vs 65.7%, p<0.001) compared to ECE-negative patients. They also had larger tumors and more advanced nodal disease (both p<0.001).To reduce selection bias, we performed propensity score matching (PSM) with a 1:2 matching ratio. After PSM to account for pathologic T stage, number of positive SLNs, type of axillary surgery, and molecular subtype, the comparison between ECE-positive (N=252) and ECE-negative (N=441) patients showed no significant differences in DFS (log-rank p = 0.96). Using the same PSM data for multivariate Cox regression analysis, adjusted for tumor size, nodal involvement, and molecular subtype, there was no significant difference in DFS between ECE-positive and ECE-negative groups (HR = 0.936, 95% CI: 0.609-1.437, p = 0.762). Among the 266 ECE-positive patients, we compared DFS between those who underwent SLNB alone and those who received SLNB followed by ALND. The addition of ALND did not significantly improve DFS among ECE-positive patients (log-rank p=0.33). Conclusion: Our analysis revealed that the presence of ECE in SLN-positive breast cancer patients did not significantly affect recurrence rates. Furthermore, among ECE-positive patients, additional ALND after SLNB did not significantly improve DFS. These findings suggest that while ECE may indicate more advanced disease characteristics, it does not independently predict recurrence when other factors are adjusted for. Consequently, in certain cases, it may be feasible to omit ALND in ECE-positive patients with positive SLN. These results highlight the need for prospective studies to confirm these results and investigate potential therapeutic strategies. Citation Format: Hyelim Kang, Min Jung Lee, Changhoon Lee, Jinyoung Byeon, Eunhye Kang, Ji-Jung Jung, Hong-Kyu Kim, Han-Byoel Lee, Hyeong-Gon Moon, Wonshik Han. Impact of Extracapsular Extension on Recurrence in Sentinel Lymph Node Positive Breast Cancer Patients: A Retrospective Cohort Study [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P3-09-12.
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