For women undergoing radiotherapy following surgery for early-stage breast cancer, breast-related quality of life (BrQoL) is an important consideration. Treating only the part of the breast by radiation where the cancer has been surgically removed (partial breast irradiation or "PBI") rather than the whole breast (whole breast irradiation) can reduce the toxic effects of radiotherapy. This trial aims to evaluate whether there is a difference in patient-reported BrQoL between two total doses of radiation given in five treatments using PBI. If BrQOL for the higher dose of PBI is no worse than the lower dose, using the higher dose would be advised as best practice, given that it is more likely to be more effective in reducing the chance of cancer coming back in the breast than the lower dose.