Redefining criteria for omitting axillary surgery following neoadjuvant systemic therapy in patients with breast cancer.

e12579 Background: Historically considered an indispensable part of the breast cancer treatment, axillary surgery is now increasingly omitted when it does not negatively impact patient outcomes. The SOUND and INSEMA trials have demonstrated the safety of omitting axillary surgery in selected early breast cancer patients, while three single-arm trials (ASLAN, ASICS, EUBREAST-01) are investigating the feasibility of omitting axillary staging surgery in cN0-1 patients who achieve breast pathologic complete response (pCR) after neoadjuvant systemic therapy (NST). However, all studies mandate achieving pCR in the breast, and evidence remains limited for patients who achieve ypN0 irrespective of breast pCR. Leveraging these insights, this study evaluates criteria for omitting axillary surgery regardless of breast pCR, proposing a rationale for future randomized trials. Methods: Stringent criteria were applied to select candidates likely to achieve ypN0. Patients with cT1-T3, cN0 breast cancer were included regardless of subtype. Additionally, a subset of cN1 patients was selected if they met criteria indicative of a favorable axillary response: HER2-positive or TNBC subtype, and a tumor size reduction > 50% on MRI after NST. Patients with suspicious lymph nodes on post-NST ultrasound were excluded. This retrospective study reviewed all patients treated at Seoul National University Hospital between 2017 and 2022 to evaluate clinicopathologic outcomes in eligible cases. Results: A total of 395 patients (median age 51 years, IQR 44–57) met the inclusion criteria. Most patients had cT2 tumors (86.8%), with 49.6% cN0 and 50.4% cN1. Subtypes included were HR+HER2- (10.4%), HR+HER2+ (29.6%), HR-HER2+ (20.0%), and TNBC (40.7%). Histologic grade was predominantly grade II (53.3%) and III (44.7%), with tumor size reduction > 50% observed in 77.5% of patients. After surgery, breast pCR was achieved in 46.1% and axillary pCR in 95.2%. Of the 19 (4.8%) patients with residual axillary disease, 13 had one metastatic node, 4 had two, and 2 had four. During a median follow-up of 4.1 (2.7–5.4) years, 11 patients experienced recurrence: 1 ipsilateral breast, 1 axillary node, 1 supraclavicular node, and 8 distant metastases. The 5- and 10-year DFS rates were 95.8% and 92.6%, respectively, while OS rates were 97.2% and 94.1%. Conclusions: These findings demonstrate a low nodal positivity rate after NST in patients selected with stringent criteria, supporting the potential to refine axillary surgical approaches in this group. A future randomized controlled trial is planned to evaluate the oncologic safety and clinical outcomes of omitting axillary surgery in carefully selected patients. ypN0 (N=376) ypN+ (N=19) p ypT stage <0.001 pCR 28 (7.4%) 1 (5.3%) ypTis 181 (48.1%) 1 (5.3%) ypT1 145 (38.6%) 11 (57.9%) ypT2 19 (5.1%) 6 (31.6%) ypT3 3 (0.8%) 0 (0.0%)