Acute adverse events of ultra-hypofractionated whole-breast irradiation after breast-conserving surgery for early breast cancer in Japan: an interim analysis of the multi-institutional phase II UPBEAT study

The applicability of ultra-hypofractionated (ultra-HF) whole-breast irradiation (WBI) remains unknown in Japanese women. This study aimed to evaluate the safety and efficacy of this approach among Japanese women and report the results of an interim analysis performed to assess acute adverse events (AEs) and determine whether it was safe to continue this study. We enrolled Japanese women with invasive breast cancer or ductal carcinoma in situ who had undergone breast-conserving surgery, were aged ≥ 40 years, had pathological stages of Tis–T3 N0–N1, and had negative surgical margins. Ultra-HF-WBI was delivered at 26 Gy in five fractions over one week. When the number of enrolled patients reached 28, patient registration was paused for three months. The endpoint of the interim analysis was the proportion of acute AEs of grade ≥ 2 (Common Terminology Criteria for Adverse Events v5.0) within three months. Of the 28 patients enrolled from seven institutes, 26 received ultra-HF-WBI, and 2 were excluded due to postoperative infections. No AEs of grade ≥ 3 occurred. One patient (4