For patients with stage I breast cancer, careful resection of tumour margins with a sector surgical technique is vital for a good outcome. Now, a study has investigated the long-term outcomes of the addition of postoperative radiotherapy for improving tumour control in 381 women with pT1N0 breast cancer. In this analysis, outcomes were assessed at 20 years of follow up. Postoperative radiotherapy was shown to benefit tumour control outcomes within the first 5 years of treatment. However, the cumulative incidence of first breast cancer was similar for both arms of the trial. Therefore, radiotherapy protects against disease recurrence in the first 5 years of follow up, but the similar recurrence rates beyond this point indicate that late recurrences are new tumours.