External radiation covers all types of therapy in which radiation is beamed into the body from an outside source, most often an X-ray machine or linear accelerator. It's usually given in the form of a sequence of treatments, typically five days a week for six to eight weeks, with weekends off to give the body time to recover. In recent years, doctors have developed another radiation schedule for women whose lives won't accommodate a six-week radiation schedule. Known as "accelerated breast irradiation," it usually involves giving higher doses of radiation over a shorter period, such as for three weeks rather than six.

Radiation can be done before a tumor is surgically removed in order to shrink it, or after surgery to catch stray cancer cells, or it can be used as a stand-alone therapy.

Standard radiation therapy, which has been in use in various forms for more than two decades, is a form of external radiation that uses large cross-firing beams from an X-ray machine or linear accelerator to target the tumor and the edge of surrounding tissue, sometimes called the margin. Many hospitals now offer a variety of more advanced radiation techniques, such as 3-D conformal radiation therapy (known as 3D-CRT), in which MRIs and other computer imaging techniques are used to map the tumor three-dimensionally so that the beams of radiation can more closely match the outlines of the tumor and damage less of the surrounding tissue.

In recent years, new computer-assisted imaging techniques, new radiation machines, and other technological advances have vastly increased the number of options available under the umbrella of external radiation. For more specific information about choosing or advocating for the latest advances in radiation therapy for breast cancer, see Guide to Intensity-Modulated Radiation Therapy and the Latest Technology.