Abstract PS6-05: First report of clinicopathologic characteristics and surgical outcomes of patients in the Avoid axillary Sentinel Lymph node biopsy After Neoadjuvant chemotherapy (ASLAN) trial (KBCSG-28)

Abstract Background: With advances in neoadjuvant systemic therapy (NST), response to treatment and pathologic complete response (pCR) rates have increased considerably, producing de-escalation of surgery strategies. Multiple trials are ongoing to demonstrate the oncologic safety of omitting breast or axillary surgery. The ASLAN trial (NCT04993625) is a prospective, multicenter, single-arm non-inferiority trial with a target accrual of 178 patients that aims to demonstrate the oncologic safety of omitting axillary surgery in patients with a pathologic complete response in the breast after NST for cT1-3, cN0-1 triple-negative, human epidermal growth factor 2 (HER2)-positive, or low-estrogen receptor (ER) breast cancer. Patients with excellent response to NST on physical examination and radiologic imaging (≤2cm mass on mammogram, breast ultrasound, and MRI or ≤4cm non-mass enhancement on MRI) who are planning to undergo breast-conserving surgery (BCS) were screened for eligibility. After BCS without axillary surgery, patients with a pCR defined as no residual invasive cancer were enrolled in the trial and avoided axillary surgery. Those with residual invasive cancer received additional sentinel lymph node biopsy (SLNB) and/or axillary lymph node dissection (ALND) and were registered into a prospective registry. We aimed to investigate the clinicopathologic characteristics and surgical outcomes of the patients in the ASLAN trial. Methods: The ASLAN trial screened 254 patients who met the inclusion criteria from September 2021 to December 2023. A total of 245 patients who received BCS were included for analysis. Clinicopathologic variables, including the pCR status of the breast and axillary lymph nodes (LNs), were analyzed. Results: Most patients had cT2 (217/245, 88.6%) and cN0 (189/245, 77.1%) disease before NST. Among 56 (22.9%) cN1 patients, a fine-needle aspiration was performed on the suspicious LNs in 37 (66.1%) and 24 (42.9%) had LN metastasis. 130 (53.1%) were TNBC, 113 (46.1%) were HER2-positive, and 2 (0.8%) were low-ER. After BCS, a breast pCR was confirmed in 182 (74.3%) patients, two of whom were dropped from enrollment due to refusal of radiation therapy or lost to follow-up resulting in 180 patients with ongoing follow-up. Among 63 (25.7%) patients with a breast non-pCR, SLNB only was performed in 60, SLNB followed by ALND in one, and no axillary surgery in two (patients refused). 95.1% (58/61) of patients who received axillary surgery had no LN metastasis, and two had micrometastasis in one LN. Patients with a non-pCR had more ER-positive disease (p=0.002), lower Ki67 (p=0.025), larger post-NST size on ultrasound (p=0.006), and no difference in the proportion of cN1 patients (p=0.824). Conclusion: The ASLAN trial completed screening of 245 patients by performing BCS of the breast and enrolled 182 patients who were confirmed to have a pCR on BCS and were omitted axillary surgery. Axillary surgery on cN0-1 patients with excellent radiologic response to NST had LN metastasis in less than 5%. This trial will be the first prospective trial to determine the oncologic safety of avoiding axillary surgery in exceptional responders to NST. Data lock is expected in December 2028. Citation Format: Han-Byoel Lee, Jai Min Ryu, Ji-Jung Jung, Wonshik Han, Sung Gwe Ahn, Hee Jeong Kim, Hyung Seok Park, Ji Soo Choi, Haeyoung Kim, Won Kyung Cho, Jeong Eon Lee. First report of clinicopathologic characteristics and surgical outcomes of patients in the Avoid axillary Sentinel Lymph node biopsy After Neoadjuvant chemotherapy (ASLAN) trial (KBCSG-28) [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 PS6-05.