The following is an excellent summary from Breastcancer.org:
Radiation therapy — also called radiotherapy — is a highly targeted, highly effective way to destroy cancer cells in the breast that may stick around after surgery. Radiation can reduce the risk of breast cancer recurrence by about 70%. Despite what many people fear, radiation therapy is relatively easy to tolerate and its side effects are limited to the treated area.
Your radiation treatments will be overseen by a radiation oncologist, a cancer doctor who specializes in radiation therapy.
In this section you can learn more about radiation therapy, including: