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External Beam Radiation

External beam radiation is the most common type of radiation therapy used to treat breast cancer.

External beam radiation is the most common type of radiation therapy used to treat breast cancer. A large machine called a linear accelerator aims a beam of high-energy X-ray radiation at either the whole breast or just the area of the breast affected by the cancer (partial breast radiation).

On this page, you can learn about the different types and techniques of external radiation therapy used to treat breast cancer and what to expect if you’re having radiation therapy as part of your breast cancer treatment plan.


Whole-breast radiation

For many years, external whole-breast radiation was given on an outpatient basis, usually for 5 days a week over 5 to 7 weeks.

A Gray is the way radiation oncologists measure the dose of radiation therapy. If you were on a 5-week treatment schedule, 45-50 Gray was the usual total amount given over 5 weeks (1.8 to 2 Gray at each of 25 treatments).

But a 5- to 7-week nearly daily commitment is difficult for many people, especially if they live far away from a treatment center.

So doctors developed a radiation therapy schedule that involves fewer treatments with higher doses of radiation at each treatment. This accelerated, or “hypofractionated,” radiation schedule puts about the same radiation dose into a 3- to 5-week schedule.

Hypofractionated Radiation Therapy

Dec. 5, 2020
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In 2017, the National Comprehensive Cancer Network (NCCN) updated its guidelines on whole-breast radiation to say that an accelerated schedule should be the standard of care. 1 In 2018, the American Society for Radiation Oncology (ASTRO) also updated its guidelines to say that an accelerated schedule should be the standard of care for whole-breast radiation therapy. The preferred hypofractionated dose schedule is 40 Gray in 15 doses or 42.5 Gray in 16 doses. This is often followed by a radiation boost dose to the area where the breast cancer used to be.

As shown below, external beam radiation is delivered from two different treatment beams, or “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 lymph nodes are also being treated, additional treatment fields may be added.


Accelerated partial-breast radiation

For certain women with early-stage breast cancer, doctors may consider external accelerated partial-breast radiation. This technique gives a larger dose of radiation over a shorter period of time to only the part of the breast where the cancer was, rather than the entire breast.

It’s important to know that external accelerated partial-breast radiation is not for everyone diagnosed with breast cancer. Both the NCCN and ASTRO guidelines on accelerated partial-breast radiation say that this technique may be offered to:

  • Women age 50 and older who do not have a BRCA mutation diagnosed with hormone-receptor-positive breast cancer that is 2 cm or smaller in size that has been removed with clear margins that are 2 mm or larger, and no cancer cells were found in the blood vessels or lymphatic system.

  • Women diagnosed with low- or intermediate-grade DCIS that is 2.5 cm or smaller in size that has been removed with clear margins that are 3 mm or larger.

External accelerated partial-breast radiation has mostly been studied in women so far and is not recommended for men diagnosed with breast cancer.


Radiation boost

No matter which radiation schedule (longer or accelerated) you receive, your doctor may recommend a radiation boost dose toward the end of your treatment schedule.

A radiation boost is a supplemental dose of radiation targeted directly at the area where the cancer was surgically removed. A separate planning session is usually required before the radiation boost can be given.

For people diagnosed with invasive breast cancer, ASTRO guidelines recommend a boost dose for:

  • cancers with positive margins after surgery; this means cancer cells came right up to the edge of the tissue removed

  • people age 50 and younger

  • people age 51 to 70 diagnosed with high-grade breast cancer

ASTRO guidelines say a boost isn’t needed for:

  • people older than 70 diagnosed with hormone-receptor-positive, low- to intermediate-grade breast cancer with negative margins wider than 2 mm; this means no cancer cells were found in the rim of healthy tissue removed with the cancer

For people diagnosed with DCIS, ASTRO guidelines recommend a boost dose for:

  • people age 50 and younger

  • people diagnosed with high-grade DCIS

  • DCIS with positive or close (less than 2 mm) margins

ASTRO guidelines say a boost isn’t needed for:

  • people older than 50 diagnosed with DCIS

  • DCIS that is small and low- to intermediate-grade

  • DCIS that has wide negative margins


Proton beam radiation therapy

Proton beam radiation uses beams of protons instead of X-rays. A proton is a particle with a positive electric charge that is in the nuclei (the core, or center) of all atoms.

X-rays release energy both before and after they hit their target. But protons release their energy only after traveling a certain distance. So doctors think protons may be able to deliver more radiation directly to the treatment area while possibly doing less damage to nearby healthy tissue. But this is still being studied.

Right now, proton beam radiation is only being used in clinical trials to treat breast cancer. The machines needed to deliver protons are very expensive and are not widely available.

If you’re interested in being treated with proton beam therapy, talk to your doctor to find a clinical trial that would be a good fit for your unique situation.


External radiation planning and treatment: What to expect

Daily external radiation treatments require careful planning to make sure the treatment area is mapped out as accurately as possible and that each day of your treatment goes smoothly.

Here’s what you can generally expect from the planning session through your daily treatment routine.

Planning and simulating external radiation treatment

Your first radiation therapy session is called a simulation. It is a planning and practice session, and you receive no radiation.

During the simulation session, your radiation oncology team maps out the area of the breast that needs treatment using imaging such as a CT scan, MRI, or X-ray.

Because it is so important to position the angles of radiation accurately, the simulation session can sometimes last up to an hour.

During the simulation session, your doctor will:

  • explain the pros and cons of radiation, the planning and treatment process, and answer any questions or concerns you may have

  • review the consent form and have you sign it

  • introduce you to the treatment team

  • precisely identify the area where you will receive radiation

Positioning is extremely important in radiation therapy. Your body will be positioned carefully so you get the best radiation treatment possible with the greatest benefits and the least side effects. You may lie on your back (supine) or lie face down (prone). You will be in the same position during every treatment, and you will have to remain still. To stabilize your position, you will probably be asked to lie in a special "immobilization device" on the treatment table. Unfortunately, no padding can be used on the treatment table or positioning devices because anything soft would make your treatment position less precise.

There are different kinds of immobilization devices. Some look like a cradle; others look like a foam box that is shaped to your form. You will not be trapped or closed in. You may be asked to lie down in a custom-shaped mold that just touches your back and sides, or your treatment center may use a "breast board" that places your head, arm, and hand in a fixed position. If you have left-sided breast cancer, you may receive special breathing instructions to help protect your heart during radiation treatment.

Pictures will be taken of the treatment area. The images are sent to the radiation planning computer, which will help set up the treatment fields (areas that determine what parts of your body will receive treatment and what parts will be avoided). Once the treatment fields are set, the team will place marks with small tattoos that are usually no bigger than the head of a pin or a freckle. The tattoo is a guide to help the technician line up the radiation treatment fields the same way each time you receive treatment. Tattoos are preferred because markers or pens are less precise and can fade or wash off.

A second planning session usually is needed to confirm the treatment and your positioning. Special X-rays are taken of each treatment field to make sure they are all set up correctly, and additional markings may be made to better define the confirmed treatment fields.

External radiation therapy routine

Here's what to expect during your visit to the radiation treatment center:

  • Your doctor will likely tell you to not wear deodorant, antiperspirant, lotion, powder, perfume, or oils to your appointment. They can interfere with the radiation treatment.

  • You'll change into a hospital gown or robe when you arrive at the treatment center. You'll also need to remove any jewelry that might get in the way of the treatment.

  • A radiation therapist will bring you to a treatment room where you will be placed in your treatment position. For breast radiation, you'll lie on your back or your stomach with the arm on the treatment side raised above your head. An immobilization device will be used to secure your position and make you more comfortable.

  • The technician will carefully line up the linear accelerator to treat the first treatment field. After the machine is positioned, the technician will leave the room. The technician can see you through a window or on a television screen and can hear through an intercom at all times.

  • The technician will then turn on the machine to deliver the radiation dose. Since you cannot feel radiation, the only way you will know when you are exposed is by the whirring or clicking sound of the machine. While the machine is running, you must remain completely still. It takes only between 30 seconds and a few minutes to deliver the radiation (depending on the type and dose of the radiation being used).

  • The technician will then come back into the room to find the position for the next field to be treated. If you are receiving radiation to lymph nodes, you may have extra fields during the session.

  • Each week, you will have special X-rays taken of the treatment field. These are called port films. Your doctor uses these to double-check that the radiation is precisely hitting the correct areas of your body. Because the tattoos or marks on your skin can shift with your skin, it's important to have one more way to make sure that the treatment is precise.

  1. Salerno, KE. NCCN Radiation Guideline Update. JNCCN. May, 2017. Available at:

  2. Shah C, et al. American Brachytherapy Society Consensus Statement on accelerated partial-breast radiation. Brachytherapy. 2017. Available at: 

Reviewed by 2 medical advisers
Chirag Shah, MD
Cleveland Clinic, Cleveland, OH
Marisa C. Weiss, MD
Lankenau Medical Center, Wynnewood, PA
Learn more about our advisory board

— Last updated on June 29, 2022, 3:06 PM

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