Both therapies combined can halve the 15-year risk of death from the disease. More than a million women worldwide each year are diagnosed with breast cancer.
Surgery will remove the cancer and radiotherapy is given to kill any residual cells in the breast.
Chemotherapy and tamoxifen, a hormone therapy for oestrogen-sensitive cancers, are often recommended as extra treatment if appropriate, to stop the disease coming back.
"The standard chemotherapy and hormonal therapy that have long been used to prevent breast cancer recurrence not only have an effect on 5-year survival but they have a long-lasting effect on 15-year survival," said Professor Sarah Darby, of the Radcliffe Infirmary, in Oxford, England.
"What we have got now is very good. It is better than people ever realized."
