The short answer
External beam radiation aims radiation at a specific part of your body from a machine, usually once a day, Monday through Friday, as an outpatient. Before starting, you have a planning visit called simulation. During a session, you lie still while a therapist positions you and leaves the room to run the machine, watching and talking with you the whole time. It doesn't make you radioactive.
External beam radiation comes from a machine that aims radiation at a specific part of your body.
Most people have treatment once a day, Monday through Friday, usually as an outpatient.
Before starting, you have a planning visit and a simulation session that maps your treatment area.
During a session, you stay still while the machine delivers radiation for one to five minutes.
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The full explanation.
The short version
External beam radiation therapy is one of the most common cancer treatments. It can feel unfamiliar the first time you go through it. A machine outside your body aims radiation at one specific part of you. Most people have treatment once a day, Monday through Friday, usually as an outpatient. How many weeks you need depends on your cancer.
Before your first treatment: the planning visit
Before you start, you have a meeting that lasts one to two hours. It includes a physical exam, a talk about your medical history, and sometimes imaging tests. You then have a planning session called simulation. At simulation, the team works out exactly where your treatment area is.
The therapist marks that area with small dots of ink on your skin, about the size of a freckle. The dots may be tattooed, and they are used to place you the same way every session. For some areas, such as the head or neck, the team may also make a body mold or a fitted mask to keep you still.
Arriving for a treatment session
You may be asked to change into a gown. The treatment room itself is usually very cool. You will lie on a treatment table, or sit, depending on where your cancer is. The therapist uses your skin dots, and your mold or mask if you have one, to place you in the right position.
During the session
You may see harmless colored lights pointed at your skin marks as the team lines everything up. Once you are in position, you stay very still and breathe normally. Radiation is delivered for 1 to 5 minutes.
The therapist leaves the room to control the machine, but stays close by. They watch you through a window or on a screen and talk with you through a speaker. They can stop the machine at any time. You will not feel, hear, see, or smell the radiation, though you will hear and see the machine moving around you.
Most visits last from 30 minutes to an hour. Most of that time goes on careful positioning, not on the treatment itself.
What to wear and what to avoid
Wear comfortable clothes made of soft fabric. Choose clothes that are easy to take off. Do not wear jewelry, adhesive bandages, or powder in the treatment area.
About radiation safety
A common worry is whether radiation therapy makes you radioactive. It does not. External beam radiation therapy will not make you radioactive. You may safely be around other people, including pregnant women, babies, and young children.
Building the routine
Treatment usually happens every weekday over a period of weeks. Many people find it helps to make it a fixed part of the day. Go at the same time, travel the same way, and plan what happens right afterward. Knowing the pattern in advance can make each visit feel more routine and less draining.
Questions for your team
Before you start, ask how many weeks of treatment you need and why. Ask what happens at your simulation visit. Ask whether you will need a mold or a mask. During treatment, it is fair to ask anything that comes up, from what to wear to whether you can be around family afterward.
When to get help sooner
- Call your care team the same day if the treated skin turns wet, raw or painfully broken open. This is called a moist reaction. It can become infected, and your nurse may want to start a special dressing or an antibiotic rather than wait for your next session.
- Call your care team within a day or two if skin over the treatment area becomes red, darker, swollen or itchy, or if a mask or mold that fitted fine at first starts to rub or press. Both are usually manageable, and both are simpler to fix early in a course than late.
Sources for this section: NCI — Skin and Nail Changes During Cancer Treatment.
Words to know
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Common questions
How often will I have radiation therapy?
Most people have external beam radiation therapy once a day, Monday through Friday, usually as an outpatient. The number of weeks of treatment depends on your specific cancer.
What happens before treatment starts?
Before starting, you have a meeting that lasts one to two hours, including a physical exam, medical history, and possibly imaging. You also have a planning session called simulation, where the team maps your treatment area and may make small skin marks or a body mold or mask to help position you the same way each time.
What happens during a session?
You may change into a gown, then lie on a treatment table (or sit, depending on the area being treated). The therapist positions you using the skin marks and any mold or mask, then leaves the room to run the machine. You stay very still and receive radiation for one to five minutes while breathing normally. Most visits last thirty minutes to an hour, mostly for positioning.
Will I feel the radiation during treatment?
No. You won't feel, hear, see, or smell the radiation itself, though you will hear and see the machine move around you.
Will radiation therapy make me radioactive?
No. External beam radiation therapy does not make you radioactive. You can safely be around others afterward, including pregnant women and children.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
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Your next step
Get ready with a checklist and questions for the visit.
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Last updated: 2026-08-11Next planned review: 2027-07-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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