Whole-breast irradiation with or without a boost for patients treated with breast-conserving surgery for early breast cancer: 20-year follow-up of a randomised phase 3 trial

{Background Since the introduction of breast-conserving treatment, various radiation doses after lumpectomy have been used. In a phase 3 randomised controlled trial, we investigated the effect of a radiation boost of 16 Gy on overall survival, local control, and fibrosis for patients with stage I and II breast cancer who underwent breast-conserving treatment compared with patients who received no boost. Here, we present the 20-year follow-up results. Methods Patients with microscopically complete excision for invasive disease followed by whole-breast irradiation of 50 Gy in 5 weeks were centrally randomised (1:1) with a minimisation algorithm to receive 16 Gy boost or no boost, with minimisation for age, menopausal status, presence of extensive ductal carcinoma in situ, clinical tumour size, nodal status, and institution. Neither patients nor investigators were masked to treatment allocation. The primary endpoint was overall survival in the intention-to-treat population. The trial is registered with ClinicalTrials.gov, number NCT02295033. Findings Between May 24, 1989, and June 25, 1996, 2657 patients were randomly assigned to receive no radiation boost and 2661 patients randomly assigned to receive a radiation boost. Median follow-up was 17.2 years (IQR 13.0-19.0). 20-year overall survival was 59.7% (99% CI 56.3-63.0) in the boost group versus 61.1% (57.6-64.3) in the no boost group, hazard ratio (HR) 1.05 (99% CI 0.92-1.19