External whole-breast radiation, sometimes called external beam whole-breast 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 breast affected by the cancer.
For many years, the most common way to deliver this form of radiation was on an outpatient basis 5 days a week, over 5 to 7 weeks, depending on the particular situation. A Gray is the way radiation oncologists measure the dose of radiation therapy; if you were on a 5-week treatment schedule, 50 Gray was the usual total amount given over 5 weeks (2 Gray at each of 25 treatments).
Radiation treatment schedules were developed based on research results. But a 5- to 7-week nearly daily commitment was difficult for some women, especially if they lived far away from a treatment center. So doctors developed and studied a different radiation therapy schedule that involves fewer treatments with higher doses of radiation at each treatment, but the same total radiation dose. This accelerated, or hypofractionated, radiation schedule puts the same radiation dose into a 3- to 5-week schedule.
In 2018, the American Society for Radiation Oncology updated its guideline on whole-breast radiation therapy. The current guideline says that most women diagnosed with breast cancer who will have radiation therapy after lumpectomy should be treated with accelerated whole-breast radiation therapy as the standard of care. The preferred hypofractionated dose schedule is 40 Gray in 15 doses or 42.5 Gray in 16 doses.
As shown below, radiation to the breast is delivered from 2 different treatment fields. The two 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 two 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 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.
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