This trial included patients that were diagnosed with breast cancer and had the cancer completely removed by surgery (mastectomy). Patients on the trial were given anti-cancer drugs in the form of chemotherapy or hormonal therapy (or a combination of both). Another therapy was offered to some patients was radiotherapy to treat the site of the mastectomy. Radiotherapy treats breast cancer using high energy x-rays to destroy the cancer cells and the aim of radiotherapy is to reduce the risk of the tumour coming back. In addition, when given in conjunction with anti-cancer drug treatment, it may also improve long-term survival. Postoperative radiotherapy is routinely given to patients at higher risk of recurrence than the patients enrolled in this trial (for example when 4 or more lymph nodes under the armpit are involved or the tumour is large). In patients (such as those in this trial) where there are less than 4 lymph nodes involved by cancer or there are no lymph nodes involved but there are other features of the cancer which increase the risk of the cancer recurring, it is not clear whether postoperative radiotherapy is needed. Currently, there is wide variation in the use of radiotherapy across the UK and internationally, for patients in this risk group. The decision whether to give radiotherapy or not is generally based on local preference rather than established guidelines. This study will help researchers decide whether radiotherapy is helpful for women with this particular type of cancer. Half of the patients in the trial received radiotherapy to the chest wall and half did not. In every other aspect the treatments were the same.