LAUSR

Abstract Purpose The UK-FAST-Forward study showed that ultra-hypofractionated whole-breast irradiation (ultra-HF-WBI) involving five fractions of 26 Gy radiation over 1 week was not inferior to HF-WBI. However, it is not used in Japan due to safety concerns. In April 2022, we commenced a multi-institutional, single-arm, phase II trial. Our aim is to confirm the safety of ultra-HF-WBI after breast-conserving surgery (BCS) for breast cancer in Japanese women. Method We plan to enroll 98 patients from 13 institutions. The primary endpoint is the proportion of late adverse events of grades ≥2 within 3 years. Discussion We believe that this highly promising clinical study can positively impact the Japanese guidelines for breast cancer treatment. The results will help us decide whether or not ultra-HF-WBI can be used as a more convenient alternative to WBI. Registration number and date This trial was registered in the UMIN Clinical Trials Registry (UMIN000047080) on March 4, 2022.