Grant: Project Grant

Most women with node negative breast cancer are treated with breast conserving surgery (BCS) followed by radiotherapy (RT) to reduce the risk of local recurrence. Maintaining the cosmetic appearance of the breast is key to this local therapy. Usually RT is given as a daily treatment to the whole breast for 3-5 weeks called whole breast irradiation (WBI). Recently based on the results of a single trial from the UK and the COVID-19 pandemic, radiation oncologists including those in Canada have switched to larger daily treatments of WBI for only 5 days. However, in the past 4-5 treatments of WBI has been associated with increased breast toxicity and worse long-term cosmetic results. Poor breast cosmesis can have a devastating impact on a woman's quality of life (QOL). Short course RT is very attractive to patients and physicians. There is an alternative for WBI called partial breast irradiation (PBI) where RT is delivered just to the tumour bed area and not the whole breast. The recent experience with PBI is that it does not result in poor cosmesis. The objective of the study is to determine if PBI compared to WBI both given once-a-day over 1 week following BCS results in non-inferior rates of local recurrence and improved cosmesis. The primary outcomes are local recurrence and patient assessed cosmetic appearance of the breast at 3 years. Secondary outcomes are overall survival, toxicity and QOL. A multicentered randomized trial will be performed. The planned sample size is 910 patients (455 per arm) to determine if local recurrence with PBI is not inferior to that with WBI, and if cosmesis is improved with PBI compared to WBI. Short course WBI is increasingly used in women with early breast cancer, but concerns remain about the potential toxicity of this approach. If we are able to show PBI is not inferior to WBI with respect to local recurrence and results in improved cosmesis, women will have a convenient therapy with limited toxicity, which will be widely adopted.