External radiation therapy

External radiation (or external beam radiation) uses a machine that directs high-energy rays from outside the body into the tumor. Most people get external radiation therapy over many weeks. It’s done during outpatient visits to a hospital or treatment center.

How does your doctor plan your treatment?

After a physical exam and a review of your medical history and test results, the doctor will pinpoint the area to be treated. This is done a few days before starting radiation therapy 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 may be made to help you stay still during treatment. These will also help get you in the same position for each treatment. The radiation therapist may 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 doctor will decide how much radiation is needed, how it will be given, and how many treatments you should have.

External radiation therapy is usually given with a machine called a linear accelerator (often called a “linac” for short), which is described below.

How long does the 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 dose of radiation given all at once can cause more damage to nearby normal tissues. This can cause more side effects than giving the same dose over many treatments.

The total dose of external radiation therapy is usually divided into smaller doses called fractions. The most common way to give it is 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 your cancer
  • The type of cancer
  • The reason for the treatment
  • Your general health
  • Any other treatments you are getting

Other radiation schedules may be used in certain cases. For instance, radiation therapy may 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 (called hyperfractionated radiation). 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.

What happens during each treatment visit?

External radiation is a lot like having 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 position.

Depending on the area being treated, you may need to undress, so wear clothes that are easy to take off and put on. You’ll be asked to lie on a treatment table next to the radiation machine (the linear accelerator or linac). The machine has a wide arm that extends over the 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.

The radiation therapist may 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. You don’t have to hold your breath.

The linac will make clicking and whirring noises and may 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.

Newer forms of external radiation therapy

Newer, more precise ways of giving external radiation therapy can help better focus the radiation and do less damage to normal tissues. This allows doctors to use higher doses of radiation. Because these methods are newer, their long-term effects are still being studied.

Three-dimensional conformal radiation therapy (3D-CRT) delivers radiation beams shaped like the tumor at the cancer from different directions. Patients are fitted with a mold or cast to keep the body part still so the radiation can be aimed very accurately. This may make it possible to reduce radiation damage to normal tissues and better kill the cancer by increasing the radiation dose to the tumor.

Intensity modulated radiation therapy (IMRT) conforms to the tumor shape like 3D-CRT, but also changes the strength of the beams in some areas to give stronger doses to certain parts of the tumor and lessen damage to normal body tissues. IMRT gives even more control in limiting the radiation that reaches normal tissue and allows a higher dose to the tumor.

A form of IMRT called helical tomotherapy uses a linac 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.

Conformal proton beam radiation therapy is much like conformal therapy, but it uses proton beams instead of 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. This machine costs millions of dollars and requires expert staff. This is why proton beam therapy costs a lot and is only offered in a small number of treatment centers. More studies are needed to find out if proton radiation gives better results in certain cancers than other types of radiation treatment.

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. The surgeon finds the cancer while the patient is under anesthesia drugs are used to make the patient sleep and not feel pain). Normal tissues are moved out of the way and protected with special shields, so IORT lets the doctor give one large dose of radiation to the cancer and limit the effects on nearby tissues. IORT is usually given in a special operating room that has radiation-shielding walls.

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 mostly 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 CT or MRI 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.

There are 3 different 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. Several machines do stereotactic radiosurgery in this way, with names such as X-Knife, CyberKnife®, and Clinac®.
  • The Gamma Knife® uses about 200 small beams aimed at the tumor from different angles for a short period of time to deliver a large dose of radiation. It’s usually given in one treatment session. Again, it doesn’t use a knife and there’s no cutting.
  • A third type uses heavy charged particle beams (like protons or helium ion beams) to deliver radiation to the tumor. Also aimed at the tumor from different angles, the particles allow most of the radiation’s energy to be delivered to more precise depths, at the end of their paths. This limits damage to nearby healthy tissues or organs.

Most of the time, stereotactic radiosurgery uses one session to give the whole radiation dose. It may be repeated if needed. Sometimes doctors give the radiation in several smaller treatments to deliver the same or a slightly higher dose. (This is called fractionation.) This may be called fractionated radiosurgery or stereotactic radiotherapy.

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.

Last Medical Review: June 30, 2015 Last Revised: June 30, 2015

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