Hypofractionation for early breast cancer: First results of the UK standardization of breast radiotherapy (START) trials

LBA518 Background: The START Trials (ST-A and ST-B) test the hypothesis that breast cancer is as sensitive to fraction (Fr) size as late reacting normal tissues, with an a/β value of about 4Gy. Methods: The phase III randomised START Trials tested hypofractionated post-operative RT in women with completely excised invasive breast cancer (T1–3, N0–1, M0). Centres opted for either ST-A or ST-B. ST-A tested 50Gy in 25Fr (5 wks) vs 41.6Gy vs 39Gy, both in 13Fr (5 wks). ST-B tested 50Gy in 25Fr (5 wks) vs 40Gy in 15Fr (3 wks). Stratification was by centre, surgery and boost. The primary endpoint was local-regional (LR) relapse. Late normal tissue effects (NTE) were assessed by breast photographs, clinical examination and quality of life (QL) questionnaires. Survival analysis methods were used to estimate rates of relapse and NTEs, and hazard ratios (HR) (with 95%CI). Smoothed estimates of absolute differences in relapse rates were obtained from the rates in the 50Gy control arms and the HR. Results: 2236 (ST-A...