External Beam Radiation Therapy

External radiation (or external beam radiation) is the most common type of radiation therapy used for cancer treatment. A machine is used to aim high-energy rays (or beams) from outside the body into the tumor. (The machine most commonly used is called a linear accelerator or “linac.”)

Radiation technology allows the precise delivery of external beam radiation therapy. Modern machines better focus the radiation and do less damage to normal tissues, so doctors can use higher doses of radiation.

External radiation is usually done during outpatient visits to a hospital or treatment center. Most people get external radiation therapy in multiple sessions over many weeks.

Types of external radiation therapy

Three-dimensional conformal radiation therapy (3D-CRT) delivers radiation beams from different directions designed to match the shape of the tumor. This helps to reduce radiation damage to normal tissues and better kill the cancer by focusing the radiation dose on the tumor.

Image guided radiation therapy (IGRT) is a form of 3D-CRT where imaging scans (like a CT scan) are done before each treatment. This allows the radiation oncologist to adjust the position of the patient or re-focus the radiation as needed to hit the tumor and limit other damage.

Intensity modulated radiation therapy (IMRT) is like 3D-CRT, but it also changes the strength of some of the beams in certain areas. This gets stronger doses to certain parts of the tumor and helps lessen damage to nearby normal body tissues.

Helical-tomotherapy a form of IMRT delivers radiation inside a large “donut.” For this treatment, you lie on a table that slowly slides through the donut as the machine spirals around you. It delivers many small beams of radiation at the tumor from different angles around the body. This may allow for even more precisely focused radiation.

Proton beam radiation therapy uses proton beams instead of electrons or x-rays. Protons are parts of atoms that cause little damage to tissues they pass through but are very good at killing cells at the end of their path. This means that proton beam radiation may be able to deliver more radiation to the tumor while reducing side effects on normal tissues. Protons can only be put out by a special machine called a cyclotron or synchrotron.

Stereotactic radiosurgery isn’t really surgery, but a type of radiation treatment that gives a large dose of radiation to a small tumor area, usually in one session. It’s used for brain tumors and other tumors inside the head. In some cases, a head frame or shell may be used to help keep the patient’s head still. Once the exact location of the tumor is known from brain scans, radiation is sent to the area from many different angles. The radiation is very precisely aimed to affect nearby tissues as little as possible.

Treatment outside the brain is called stereotactic body radiation therapy (SBRT). SBRT may be used for certain lung, spine, and liver tumors.

In many radiation therapy clinics this technology is called by the name of the vendor that makes the machine.

There are 3 main ways stereotactic radiosurgery can be given:

  • The most common type uses a movable linac that’s controlled by a computer. The machine moves around to target the tumor from many different angles. X-Knife, CyberKnife®, and Clinac® all work this way.
  • The Gamma Knife® uses about 200 small beams aimed at the tumor from different angles for a short period to deliver a large dose of radiation. It’s usually given in one treatment session. Again, this is a type of radiation therapy – it doesn’t use a knife and there’s no cutting.
  • Another type aims heavy charged particle beams (like proton or helium ion beams) at the tumor from different angles. These particles release most of the radiation’s energy at the end of their paths, at more precise depths. This limits damage to nearby healthy tissues or organs.

Although most patients will be given the full radiation dose in one session with stereotactic radiosurgery, it may be repeated if needed. Sometimes doctors give the radiation in several smaller treatments to deliver the same or slightly higher dose. This may be called fractionated radiosurgery or fractionated stereotactic radiotherapy.

Intraoperative radiation therapy (IORT) is external radiation given directly to the tumor or tumors during surgery. It may be used if the tumors can’t be removed completely or if there’s a high risk the cancer will come back in the same area. While you are asleep, the surgeon moves normal tissues away from the tumor and protects them with special shields. This lets the doctor give one large dose of radiation to the cancer and limit the effects on nearby tissues. IORT is given in a special operating room that has radiation-shielding walls.

How does your doctor plan your radiation treatment?

Radiation is planned and given by a team of trained health care providers. The radiation oncologist is a doctor who treats cancer with radiation and oversees the care of each patient getting radiation. Working closely with the radiation oncologist, the radiation therapist gives the daily radiation treatment and positions patients for each treatment. Other professionals include the medical physicist and dosimetrist who plan and calculate the doses of radiation.

Before starting radiation therapy, your radiation oncologist will examine you, review your medical history and test results, and pinpoint the exact area to be treated in a process called simulation. You’ll be asked to lie still on a table while the radiation therapist uses imaging scans (like a CT scan or MRI) to define your treatment field (also called the treatment port). These are the exact places on your body where the radiation beams will be aimed.

Radiation beams are aimed very precisely. A special mold, mask, or cast of a body part might be made to make sure you are in the same position for each treatment and to help you stay still during treatment. The radiation therapist might mark the treatment field with freckle-sized dots of semi-permanent ink. The marks will likely fade away over time, but they’re needed until your treatment is finished. Don’t use soap on or scrub these marks. Sometimes the area may be marked with permanent dots like a tattoo. (These can later be removed with a laser.)

Based on the simulation, other tests, and your cancer type, the radiation oncologist will decide how much radiation is needed, how it will be given, and how many treatments you should have.

How long does external radiation treatment take?

In most cases the total dose of radiation needed to kill a tumor can’t be given all at once. This is because a large dose given one time can cause more damage to nearby normal tissues. This can cause more side effects than giving the same dose over spread out into many treatments.

The total dose of external radiation therapy is usually divided into smaller doses called fractions. Most patients get radiation treatments daily, 5 days a week (Monday through Friday) for 5 to 8 weeks. Weekend rest breaks allow time for normal cells to recover. The total dose of radiation and the number of treatments is based on:

  • The size and location of the cancer
  • The type of cancer
  • The reason for the treatment
  • Your general health
  • Any other treatments you’re getting

Other radiation schedules might be used in certain cases. For instance, radiation therapy might last only a few weeks (or less) when it’s used to relieve symptoms, because the overall dose of radiation is lower. In some cases, radiation might be given as 2 or more treatments each day. Or it might be given as split-course therapy, which allows for several weeks off in the middle of treatments so the body can recover while the cancer shrinks. Your doctor will talk to you about the best plan in your case.

What happens during each treatment visit?

External radiation is a lot like getting a regular x-ray. The treatment itself is painless and takes only a few minutes. But each session can last 15 to 30 minutes because of the time it takes to set up the equipment and put you in the right position.

External radiation therapy is usually given with a linear accelerator (or linac) which delivers a beam (or multiple beams) of radiation. The machine has a wide arm that extends over the treatment table. The radiation comes out of this arm. The machine can move around the table to change the angle of the radiation, if needed, but it won’t touch you.

Depending on the area being treated, you might need to undress, so wear clothes that are easy to take off and put on. You’ll be asked to lie on the treatment table next to the radiation machine.

The radiation therapist might put special heavy shields between the machine and parts of your body that aren’t being treated to help protect normal tissues and organs.

Once you’re in the right position, the radiation therapist will go into a nearby room to operate the machine and watch you on a TV screen. The room is shielded, or protected from the radiation so that the therapist isn’t exposed to it. You can talk with the therapist over an intercom. You’ll be asked to lie still during the treatment, but you won’t have to hold your breath.

The linac will make clicking and whirring noises and might sometimes sound like a vacuum cleaner as it moves to aim the radiation beam from different angles. The radiation therapist controls the movement and checks to be sure it’s working properly. If you’re concerned about anything that happens in the treatment room, ask the therapist to explain. If you feel ill or uncomfortable during the treatment, tell the therapist right away. The machine can be stopped at any time.

Will I be radioactive during or after external radiation treatment?

External radiation therapy affects cells in your body only for a moment. Because there’s no radiation source in your body, you are not radioactive at any time during or after treatment.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

American College of Radiology and the Radiological Society of North America. External Beam Therapy (EBT). Accessed at www.radiologyinfo.org/en/info.cfm?pg=ebt on April 1, 2016.

American Society for Radiation Oncology. Treatment Types: External Beam Radiation Therapy. Accessed at www.rtanswers.org/treatmentinformation/treatmenttypes/externalbeamradiation/ on February 8, 2016.

Halperin EC, Perez CA, Brady LW (Eds). Principles and Practice of Radiation Oncology. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2013.

National Cancer Institute. Radiation Therapy for Cancer. 2010. Accessed at www.cancer.gov/about-cancer/treatment/types/radiation-therapy/radiation-fact-sheet on March 21, 2016.

Last Medical Review: February 10, 2017 Last Revised: February 10, 2017

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