Getting External Beam Radiation Therapy
External radiation is the most common type of radiation therapy used for cancer treatment. You might also hear it called external beam radiation.
For this type of treatment, radiation (high-energy rays or particles) is aimed at the tumor from a machine outside your body. The machine focuses the radiation beam on the exact location of the tumor. It usually happens during outpatient visits to a hospital or treatment center.
Learn more about the types of external radiation therapy and what to expect during and after treatment.
- Beams used in external radiation therapy
- Types of external radiation therapy
- Photon beam radiation therapy
- Particle beam radiation therapy
- How long does external radiation treatment take?
- What happens during each treatment visit?
- Will I be radioactive during or after external radiation treatment?
- Learn more
Beams used in external radiation therapy
External radiation therapy is usually given as photon (x-ray) beams. Less often, particle beams (proton, neutron) or electron beams are used.
These beams are delivered very carefully. Most of the radiation reaches the cancer, and the healthy tissues in your body are damaged as little as possible. The beams are invisible and painless when they pass through your skin to the cancer.
Photon beams are the same type of radiation used during an x-ray, but the beams are much stronger. They can travel deep into your body to reach the tumor, but they can also damage healthy tissue around the tumor.
A machine called a linear accelerator delivers the photon beams. The radiation is released from the machine as a wave of energy.
Particle beams are made up of separate units of energy, such as protons or neutrons. They can travel deep into your body like photon beams, but their energy is only released at a certain distance.
This type of treatment can often deliver more radiation to the tumor while limiting damage to nearby normal tissues.
Special machines called particle accelerators (cyclotron or synchrotron) deliver the particle beams. The radiation is released from the machine as a stream of high-energy particles.
Electron beams are also separate units of energy. They can act like particle beams or be changed into photon beam radiation. Electron beams can be given from a linear accelerator or a particle accelerator.
Electrons don’t travel very far into your body, so they are most often used to treat cancers on the skin or near the body surface.
Types of external radiation therapy
External radiation therapy is either:
- Photon beam therapy or
- Particle beam therapy
?This depends on the kind of radiation beam used. Within each of these two categories, there are several types of radiation therapy.
Photon beam radiation therapy
One of the most common types of radiation treatment is photon beam radiation therapy. It can be given in many ways.
Types of photon beam radiation therapy:
3D-CRT delivers radiation beams from different directions designed to match the shape of the tumor. This helps reduce radiation damage to normal tissues and better kill the cancer cells by focusing the radiation dose on the tumor's exact shape and size.
IMRT is like 3D-CRT, but it also changes the strength (intensity) of some of the beams in certain areas. This allows stronger doses to be aimed at certain parts of the tumor and helps lessen damage to nearby healthy tissues.
- Helical tomotherapy is a type of IMRT that delivers many small beams of radiation at the tumor from different angles around the body. This may allow for even more precisely focused radiation.
SRS is a type of radiation treatment used for brain tumors and other tumors inside the head. A large dose of radiation is delivered to a small tumor area. Radiation is aimed at the cancer from many different angles so nearby tissues are affected as little as possible.
SRS is called "radiosurgery" because of how precisely the radiation is delivered, but there is no cutting or incision involved. It is usually done in one session.
Stereotactic body radiation therapy (SBRT): When SRS is given outside the brain, it is called stereotactic body radiation therapy (SBRT). SBRT can be used for some lung, spine, liver, kidney, and prostate cancers, and other tumors.
You might hear this technology called by a different name, depending on the type of machine your radiation clinic has:
- X-Knife or CyberKnife moves around to target the tumor from many angles. Other brand names include Synergy-S, Edge, Novalis, and TrueBeam.
- Gamma Knife uses about 200 small beams of radiation at one time, creating a very large dose. It’s usually given in one session. (There is no knife and no cutting.)
- Another type of machine aims 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 exact locations. This limits damage to nearby healthy tissues or organs.
Most people get the full radiation dose in one session with SRS, but it can be repeated if needed.
Sometimes, radiation is given in several smaller treatments to deliver the same or slightly higher dose. This is called fractionated radiosurgery or fractionated stereotactic radiotherapy.
Many of the treatment approaches above can be given using IGRT. You will get imaging scans (like a CT or MRI) before each IGRT treatment session.
A tumor might not be in exactly the same place in your body before each treatment. With IGRT, the radiation oncologist can adjust your position or the aim of the radiation beams to focus the radiation exactly on the tumor and limit exposure of normal tissues.
This approach combines some features of IMRT, IGRT, and SBRT. It is done with a machine known as an MRI-linac. This machine combines an MRI scanner with a linear accelerator (linac, the machine that delivers the radiation).
Like other types of IGRT, MRI images can be taken before each treatment. If the tumor has changed, the aim of the radiation can be adjusted.
MRI images can also be taken while the radiation is given.
If body functions (like breathing or digestion) cause the tumor to move out of the path of the radiation, the radiation stops until it is aimed correctly again. This helps limit radiation to nearby healthy tissues and organs as much as possible.
MRI-linac machines are made by MRIdian and Elekta.
This is radiation given directly to the tumor or tumors during surgery. It may be used for tumors that can’t be removed completely or when there’s a high risk the cancer will come back in the same area.
Learn more in Getting Intraoperative Radiation Therapy.
Particle beam radiation therapy
A common type of particle beam radiation therapy is proton therapy. Proton therapy focuses beams of protons on the cancer.
The beams used in proton therapy only travel a certain distance. The radiation beam stops at the tumor and doesn’t go beyond it, so the tissues behind the tumor are exposed to very little radiation.
This is different than the photons (x-rays) used in photon beam radiation therapy, which go through the body and expose tissues to radiation both before and after they hit the tumor.
Learn more in Getting Proton Therapy.
How long does external radiation treatment take?
Most people get daily radiation treatments every weekday (Monday–Friday) for 5 to 8 weeks. Weekend rest breaks give normal cells time to recover. Some people may have treatment twice a day.
Usually, the total dose of radiation needed to kill a tumor can’t be given all at once because a large dose all at one time does more damage to nearby healthy tissues. This can cause more side effects than giving the same dose, split up into many treatments over a period of days or weeks.
Your cancer care team will tell you the amount of radiation needed to treat your cancer and how often you need to get it.
How much radiation will I need?
The total dose of external radiation therapy is usually divided into smaller doses called fractions.
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
- Other treatments you’re getting
Is radiation always given daily?
Most people get radiation every weekday for several weeks, but not always. Other radiation schedules might be used in certain cases.
For example:
- You might get radiation therapy for only a few weeks (or less) when it’s used to relieve symptoms. This is because the overall dose of radiation needed is lower.
- In some cases, you might get 2 or more treatments each day.
- You might have several weeks off in the middle of treatments so your body can recover while the cancer shrinks.
Your cancer care team will talk to you about the best plan.
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. Each session lasts 15 to 30 minutes because of the time it takes to set up the equipment and put you in the right position.
Before starting radiation therapy, you will have a simulation or planning scan to pinpoint the exact treatment area. Once this plan is made, you will begin treatment.
In the treatment room
External radiation therapy is usually given with a machine called a linear accelerator. The machine delivers a beam (or multiple beams) of radiation. It 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 radiation, but it won’t touch you. The radiation beams are invisible, and you won’t feel anything.
Getting into position
Depending on the area being treated, you might need to undress. Wear clothes that are easy to take off and put on.
First, you’ll 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. This helps protect healthy tissues and organs.
When you’re in the right position, the radiation therapist goes into a shielded room nearby to safely operate the machine and watch you on a TV screen. You can talk with them over an intercom.
During treatment
You’ll need to lie still during the treatment.
The machine 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 the machine is working properly.
It’s OK to ask questions or voice concerns during the treatment.
- If you’re concerned about anything that happens, ask the therapist to explain.
- If you feel ill or uncomfortable, tell the therapist right away. The machine can be stopped at any time.
Will I be radioactive during or after external radiation treatment?
You are not radioactive at any time during or after external radiation treatment.
This is because external radiation therapy affects cells in your body only for a moment, and there is no radiation source in your body.
Talk to your cancer care team if you have questions about special safety measures.
Learn more
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- References
Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
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Last Revised: June 9, 2025
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