External radiation, sometimes called external beam radiation, is the most common type of radiation. In this technique, a large machine called a linear accelerator aims a beam of high-energy radiation at the area affected by the cancer. This form of radiation is given on an outpatient basis 5 days a week, over 5 to 7 weeks, depending on the particular situation.
As shown below, radiation to the breast is delivered from 2 different treatment fields. The 2 fields come from opposite directions and face each other:
- One starts from the side of the breast and faces the middle of the chest (where the breastbone is).
- One starts in the middle of the chest and faces the side.
If the adjacent lymph nodes are also being treated, additional treatment fields may be added.
In order to maximize the amount of radiation delivered to the breast area while avoiding or minimizing radiation to other parts of the body, the radiation oncologist can:
- treat the breast area with angled fields that skim across the chest, just catching the breast area
- place the back edges of the 2 fields as close to the breast area as possible
- use special blocks in the opening of the machine to avoid radiation to normal tissue
- place special devices, called wedges, in the path of the beam to bend the dose of radiation away from normal tissues under the breast
External radiation boost
During the final week or so of the 5- to 7-week radiation regimen, you will also receive a supplemental dose of radiation targeted directly to the area around your surgery, where the cancer was. This dose is called the "boost" and is usually delivered in a method similar to your regular radiation. The boost dose will be calculated such that each day you’ll get a slightly higher amount of radiation than you did for the first few weeks of your regimen. A different treatment field is used for the boost dose, so a separate planning and set-up session is usually required before the boost radiation is started.
Most people get their boost dose with a special form of external radiation called electrons. This form is used because the dose can be targeted specifically to a small area near the skin surface, sparing the tissue underneath. You will receive this dose from the same machine that is used for your other therapy, and you'll probably lie in the same position.
One other, less common way to give external radiation is external beam partial-breast radiation.