We at PGIMER have been practicing hypofractionated radiotherapy in breast cancer patients for the last 4 decades. Our standard doses have been 35Gy/15#/3wks to the chest wall after mastectomy and 40Gy/16#/3wks after breast conserving surgery (BCS).It is also a routine practice in the UK and in a few centers in Canada. Hypofractionation reduces treatment time to half while maintaining cosmesis and gives control rates equal to conventional fractionation. As breast cancer is a leading cancer in females and radiation therapy is an important part of its local management, hypofractionation helps radiation centers worldwide to meet the growing need for radiation treatment in breast cancer, particularly in developing countries where resources are limited. It also reduces the financial burden on the patient and family. In this study we want to evaluate the impact of reducing the treatment duration from 3 weeks to 1 week. Eligible patients with breast cancer after mastectomy or BCS will be treated with a radiotherapy dose of 26Gy in 5 fractions over 1 week in the study arm and 40Gy in 15 fractions over 2 weeks in the control arm. The primary endpoint of this noninferiority study will be locoregional tumour control. Secondary endpoints will be early and late radiation toxicities, quality of life, contralateral primary tumours, regional and distant metastases, survival and second cancers. A total of 1018 patients will be randomised (1:1) to receive 1 week or 2 weeks of radiotherapy. An event-driven analysis will be performed after at least 94 patients have documented locoregional recurrences.