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Introduction: Axillary lymph node dissection (ALND) has always been part of breast cancer (BC) treatment. However, during the past 25 years, the surgical management of the axilla has shifted towards a more conservative approach. Until now, ALND has remained the standard surgical technique when the sentinel lymph node (SLN) is macrometastatic. However, ALND may now be considered overtreatment for early-stage BC. The SINODAR-ONE trial is a prospective non-inferiority multicenter randomized study aimed at assessing the role of ALND in patients undergoing either breast-conserving surgery (BCS) or mastectomy for T1-2 BC presenting 1-2 macrometastatic SLNs. Objectives: The primary endpoint was to evaluate whether sentinel lymph node biopsy (SLNB) only was associated with clinically relevant worsening of the prognosis compared with ALND in terms of overall survival (OS). The secondary endpoint was to evaluate whether there was increased regional (lymph node recurrence) or distant recurrence in terms of recurrence-free survival (RFS) in patients with macrometastatic SLN who did not undergo ALND. Methods: Patients were randomly assigned (1:1 ratio) to either removal of ≥10 axillary level I/II non-SLNs followed by adjuvant therapy (standard arm) or no further axillary treatment (experimental arm). Results: The trial started in April 2015 and ceased in April 2020, involving 889 patients. The majority of the patients (75.2%) underwent BCS; 328 of 439 patients (74.7%) in the standard treatment arm, and 333 of 440 patients (75.7%) in the experimental treatment arm. 218 patients (24.8%) underwent mastectomy. SLN status at randomization was comparable between the two groups of treatment, with a median number of two SLNs removed and a median number of one positive SLN in both arms. The median number of non-SLNs identified at definitive histopathological evaluation was 16 (interquartile range [IQR] 12–21) in the ALND group. Overall, 193 of 439 patients (44.0%) in the standard treatment arm had additional macrometastases in the removed axillary lymph nodes. However, the median number of positive non SLNs was 0 (IQR 0–1) in the ALND group. Median follow-up was 34.0 months. There were eight deaths (ALND, 4; SNLB only, 4), with a 5-year cumulative mortality of 5.8% and 2.1% in the standard and experimental arm, respectively (p = 0.984). There were 26 recurrences (ALND 11; SNLB only, 15), with a 5-year cumulative incidence of recurrence of 6.9% and 3.3% in the standard and experimental arm, respectively (p = 0.444). Only one axillary lymph node recurrence was observed in each arm. The 5-year OS rates were 98.9% and 98.8%, in the ALND and SNLB only arm, respectively (p = 0.936). Conclusion: The 3-year survival and relapse rates of T1-2 BC patients with 1-2 macrometastatic SLNs treated with SLNB only, and adjuvant therapy, were not inferior to those of patients treated with ALND. These results do not support the use of routine ALND in patients undergoing BCS. However, given the low number of patients treated with mastectomy, there is no certainty that ALND omission can be extended also to this sub-group. In order to collect further evidence regarding the safety of the experimental treatment in patients candidates for mastectomy, the reopening of the enrollment of these patients as part of a single-arm experimental study started in June 2022. Citation Format: Damiano Gentile, Wolfgang Gatzemeier, Andrea Sagona, Erika Barbieri, Alberto Testori, Valentina Errico, Alberto Bottini, Simone Di Maria Grimaldi, Giulia Caraceni, Luca Boni, Paolo Bruzzi, Bethania Fernandes, Davide Franceschini, Ruggero Spoto, Rosalba Torrisi, Marta Scorsetti, Armando Santoro, Giuseppe Canavese, Corrado Tinterri. Preservation of axillary lymph nodes in breast cancer patients undergoing mastectomy with 1-2 metastatic sentinel lymph nodes: The current status and future perspectives of the multicenter randomized clinical trial SINODAR-ONE [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-06-01.