Radiation therapy uses high levels of radiation to kill or slow the growth of breast cancer cells. Usually, this therapy follows a lumpectomy or mastectomy. Radiation is given as a small daily dose over several days or weeks. Side effects may include swelling, hair loss, enlarged pores, fatigue, and more. Follow the links below to find WebMD's comprehensive coverage about how radiation thearpy is given, what to expect, side effects, and much more.