Abstract GS2-11: Omission of sentinel lymph node biopsy in clinically T1-2 node-negative breast cancer patients treated with breast-conserving therapy: results of the Dutch BOOG 2013-08 randomized controlled trial after a median follow-up of 5 years

Abstract Background: In early-stage breast cancer, surgical de-escalation from axillary lymph node dissection (ALND) to sentinel lymph node biopsy (SLNB) did not compromise oncologic safety. This has led to the realization that SLNB predominantly provides prognostic information and may itself not influence recurrence or survival. The BOOG 2013-08 trial was therefore designed to evaluate whether omission of SLNB in clinically T1-2N0 patients treated with breast-conserving surgery and whole-breast irradiation (summarized together as BCT) is safe with respect to regional recurrence rates (RR) and distant disease-free survival (DDFS). Methods: The BOOG 2013-08 (NCT02271828) is a multicenter, non-inferiority, randomized phase-3 trial including women with unilateral cT1-2N0 breast cancer treated with BCT. Clinically node-negative was based on negative findings on physical examination and preoperative axillary ultrasound (including negative cyto- or histopathology in case of a suspicious findings). The trial was conducted in the Netherlands between 2015 and 2022 across 25 centers. Patients were randomized (1:1) to SLNB or omission of SLNB. Randomization was stratified by clinical tumor size (3 cm vs ≥3 cm), histological grade (grade I-II vs III), hormone receptor status (HR), HER2 status, age (≤50, 50 ≤ 75, 75 years), primary systemic therapy (PST) and participating center. The primary endpoint was 5-year RR; secondary endpoints as presented here were 3-year RR and DDFS. Cumulative probabilities of RR and DDFS were estimated using the Kaplan-Meier method. Non-inferiority was defined as an absolute difference in RR ≤5% in favor of the control group at 5 years. Analyses were performed per-protocol. Results: A total of 1,733 patients were enrolled across 25 centers. After excluding 159 patients (n=106 due to radiation therapy protocol violations), 1574 patients were included (SLNB: n=749; no-SLNB: n=825). At the time of analysis, 100% of patients had reached 3 years and 73% 5-year follow-up. Median follow-up was 5.0 years (range 0.78-6.86, IQR 1.94). Mean age was 61.5 years, 83% had cT1 tumors and 17% cT2 tumors. Tumor grade was I in 28.7%, II in 54.1% and III in 17.2%. Breast cancer subtype was HR+HER2- in 86.8%, triple negative in 5.8% and HER2+ in 7.4% of patients. In the SLNB arm, micro- and macrometastatic disease was detected in 6.0% and 7.7%, respectively. Preliminary data at a median 5-year follow-up showed that the estimated probability of RR was 0.5% in the SLNB arm (95% CI 0.0-1.1) and 1.2% in the no-SLNB arm (95% CI 0.4-2.0), corresponding to an absolute difference of 0.7% (95% CI -0.3 to 1.7). The confidence interval ranges from -0.3 to 0.7, indicating that the 5% non-inferiority margin is not exceeded. Five-year DDFS was 98.3% (95% CI 97.3-99.3) in the SLNB arm versus 96.1% (95% CI 94.5-97.7) in the no-SLNB arm (log-rank p=0.071).Chemo- or endocrine therapy was administered in 11.3% of patients as PST (12.3% SLNB arm vs. 10.3 no-SLNB arm, p=0.214). In total, 50.0% of patients received systemic therapy (51.2% SLNB vs. 49.0% no-SLNB, p 0.202). Of these 50.0%, 39.4% received chemotherapy with or without targeted therapy. Among ER-positive patients, adjuvant endocrine therapy was administered in 48.6% (SLNB) and 46.6% (no-SLNB), p=0.732, while chemotherapy was given in 9.8% and 8.2%, respectively (p=0.532). Conclusions: In cT1-2N0 breast cancer patients treated with BCT, omission of SLNB resulted in a non-inferior median 5-year RR and DDFS compared with SLNB. These premature findings support the omission of SLNB, at least in case of cT1N0, ER+, grade 1-2 breast cancer. This study further demonstrates that adjuvant hormonal therapy should not be a prerequisite for omission of the SLNB. Citation Format: R. J. Schipper, L. M. van Roozendaal, V. M. Wintraecken, J. M. Simons, M. Moossdorff, E. J. T. Luiten, F. van Duijnhoven, L. de Munck, L. F. C. Dols, R. Koelemij, R. van Eekeren, A. B. Francken, T. van Dalen, K. Keymeulen, J. Volders, J. A. van der Hage, H. Torrenga, P. A. Neijenhuis, P. Nijhuis, D. van Uden, E. Heuts, M. A. Bessems, C. D. Dirksen, M. H. Martens, M. F. Bouwmeester, K. Schenk, A. V. R. J. Bell, S. Maaskant-Braat, L. J. A. Strobbe, S. C. Linn, P. M. P. Poortmans, M. L. G. Vane, V. C. G. Tjan-Heijnen, K. K. B. T. van der Vijver, M. B. I. Lobbes, J. de Vries, L. Emelianova-Hiddink, J. Kandt, L. J. Boersma, J. H. W. de Wilt, S. M. J. van Kuijk, M. L. Smidt, BOOG 2013-08 Study Group. Omission of sentinel lymph node biopsy in clinically T1-2 node-negative breast cancer patients treated with breast-conserving therapy: results of the Dutch BOOG 2013-08 randomized controlled trial after a median follow-up of 5 years abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr GS2-11.