INTRODUCTION: Shortening the duration of postoperative radiotherapy (RT) for breast cancer while maintaining efficacy and safety has become a significant trend. The 3-week regimen of 40-42.5 Gy in 15-16 fractions is now a preferred option in clinical practice. Following the publication of the 5-year outcomes from the Fast-Forward trial, interest in 1-week regimens has surged, prompting the initiation of multiple studies. However, trials exploring the 1-week regimen for regional nodal irradiation (RNI), especially involving internal mammary nodes (IMN), remain scarce. Additionally, the optimal fractionation scheme for tumour bed boost in the era of ultra-hypofractionated regimens is still debated. To address these gaps, we initiated the adjuvant regional nodal radiation therapy for one week in breast cancer (ARROW) trial to evaluate the feasibility of a 1-week regimen for RNI of 26 Gy in five fractions, with optional sequential tumour bed boost of 10.4 Gy in two fractions. The findings from our trial are expected to extend the application of ultra-hypofractionated regimens to include sequential tumour bed boosts and RNI, pioneering its use in IMN irradiation. METHODS AND ANALYSIS: The ARROW trial is an open-label, single-arm, multicentre phase II trial, encompassing four teaching hospitals in China. Enrolled patients will receive a total of 26 Gy in five fractions to ipsilateral whole breast/chest wall and regional regions, including supraclavicular/infraclavicular nodes, IMN and any portion of the undissected axilla deemed at risk. A sequential tumour bed boost of 10.4 Gy in two fractions is delivered in patients at high risk for recurrence, which is at the discretion of the radiation oncologist. The sample size for the ARROW trial was 197 patients. Both intensity-modulated radiation therapy and proton therapy are permitted. The primary endpoint is acute radiation-induced toxicity, graded according to Radiation Therapy Oncology Group (RTOG) criteria and CTCAE V.3.0. Secondary endpoints include cosmetic outcomes for breast-conserving surgery, late radiation-induced toxicity, local regional recurrence, distant metastasis, invasive tumour-free survival, overall survival and quality-of-life assessment. ETHICS AND DISSEMINATION: The trial has been approved by the Ethical Committee of Ruijin Hospital affiliated to Shanghai Jiao Tong University School of Medicine, as well as approvals from the ethical committees of each participating centre have also been obtained. Research findings will be submitted for publication in peer-reviewed journals. TRIAL REGISTRATION NUMBER: ARROW trial: NCT04509648.