In The Lancet, Vratislav Strnad and colleagues present 5-year results of a large, international, randomised trial testing standard whole-breast radiotherapy against accelerated partial breast irradiation (APBI) after breast-conserving surgery, in a selected low-risk population of women. The APBI technique entailed a 4–5-day postoperative course of radiotherapy delivered via radioactive sources inserted into breast tissue surrounding the operation site, the so-called tumour bed. The study design tested for non-inferiority with a primary endpoint of local recurrence in 1184 patients recruited from 16 centres, and 5-year local recurrence was less than 2% in both arms. A predefined 3% non-inferiority margin was upheld by a difference in local relapse rates of 0·52% (95% CI −0·72 to 1·75) in favour of whole-breast irradiation. There were no statistical differences in disease-free or overall survival, and adverse effects were similarly mild in both groups.