Tailored axillary surgery with or without axillary lymph node dissection followed by radiotherapy in patients with clinically node-positive breast cancer (OPBC-03/SAKK 23/16/IBCSG 57-18/ABCSG-53/GBG-101-TAXIS).

TPS611 Background: Axillary lymph node dissection (ALND) is standard of care for most patients with clinically node-positive breast cancer (cN+BC), either in the adjuvant setting or in case of residual nodal disease after neoadjuvant chemotherapy (NACT). At least one quarter of patients with breast cancer experience long term morbidity after ALND. The TAXIS trial was designed to investigate if axillary radiotherapy (ART) can replace ALND in patients with cN+ BC who underwent tailored axillary surgery (TAS), which is recognized as one of the most important uncertainties and controversies in axillary management of patients with breast cancer. Methods: To test the remaining indications for ALND in clinical practice, we designed the international, multicenter, phase-III, non-inferiority, randomized controlled TAXIS trial to be pragmatic (average PRECIS-2 score of 4.44). Accordingly, most study domains follow usual care conditions, including eligibility (following routine indications for ALND in clinical practice), surgical skills and techniques, and follow up. The radiation oncology protocol included detailed recommendations and an extensive quality assurance program, which was developed to address the limitations of prior landmark trials and increase acceptability among recruiters. The main objective of the trial is to demonstrate that treatment with TAS and ART is non-inferior to TAS and ALND in terms of disease-free survival (DFS) of patients with cN+ BC detected by imaging or palpation in the upfront surgery setting or in case of residual disease after NACT. Randomization at a 1:1 ratio is mainly performed intraoperatively to either use TAS followed by ALND and regional nodal irradiation excluding the dissected axilla as a target volume (arm A) or to use TAS followed by regional nodal irradiation including the full axilla (arm B). The sample size is based on the primary endpoint DFS. With a type I error of 5% and a power of 80%, 385 events will be needed to show non-inferiority of TAS and axillary RT in comparison to ALND with a non-inferiority hazard ratio of 1.289 (corresponding to a DFS at 5 years of 80% in the ALND arm and 75% in the TAS and axillary RT arm). The sample size was calculated to be 1500 patients (750 per arm). Enrolment started in July 2018 and will stop after reaching 1500 patients, which is expected to be completed by Q4/2025. With currently 51 active sites from Europe, Canada and Korea within the network of the Oncoplastic Breast Consortium (OPBC), >950 patients were randomized by January 2024. Another 14 sites in Europe, USA, and South America will join the trial in 2024. The interim analysis (after 20% of the required events) is expected for Q4/2024 and will be reviewed by the IDMC. The primary endpoint analysis is planned in Q4/2030. Clinical trial information: NCT03513614 .