Radiation therapy is a highly targeted and highly effective way to destroy cancer cells in the breast that may stick around after surgery. Radiation therapy is relatively easy to tolerate and its side effects are limited to the treated area. Most radiation therapy is administered by a radiation oncologist at a radiation center and usually begins three to four weeks after surgery. The radiation is used to destroy undetectable cancer cells and reduce the risk of cancer recurring in the affected breast. Radiotherapy causes side effects because it affects healthy tissue as well as cancer cells. Radiation therapy may be used to treat breast cancer at almost every stage. It's an effective way to reduce your risk of breast cancer recurring after surgery. It can also help control the spread of breast cancer and offer pain relief for advanced breast cancer, the standard type of radiation therapy, delivers radiation in the form of high-powered energy beams, such as X-rays or protons, to your entire breast from a machine outside your body.