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, and it can feel unfamiliar the first time you go through it. It works by using a machine that aims radiation at a specific part of your body from outside. Most people have treatment once a day, Monday through Friday, usually as an outpatient, and the number of weeks depends on your specific cancer.
Before your first treatment: the planning visit
Before you start, you have a meeting that lasts about one to two hours. This includes a physical exam, a review of your medical history, and sometimes imaging. You also have a planning session called simulation, where the team maps exactly where your treatment area is. During simulation, the team may make tiny, freckle-sized skin marks — sometimes tattooed — to help position you the same way each time, and for some areas, such as the head or neck, they may make a body mold or fitted mask to help keep you still and in the right spot.
Arriving for a treatment session
You may be asked to change into a gown. The treatment room itself is usually cool. You will lie on a treatment table, or sit, depending on where your cancer is. The therapist positions you carefully using your skin marks and, if you have one, your mold or mask, so that the treatment lines up with your simulation plan.
During the session
You may see harmless colored positioning lights as the machine and therapist line everything up. Once you're in position, you stay very still and breathe normally while the machine delivers radiation, typically for one to five minutes.
The therapist leaves the room to control the machine but stays close by, watching you through a window or on a screen, and talking with you through a speaker the whole time. They can stop the machine at any point if needed. You won't feel, hear, see, or smell the radiation itself, though you will hear and see the machine moving around you as it works.
Most visits last about thirty minutes to an hour in total, but most of that time is spent on careful positioning rather than the treatment itself.
What to wear and what to avoid
Wear comfortable, soft clothing to your appointments. It's best to avoid jewelry, adhesive bandages, or powder in the area being treated, since these can interfere with treatment or irritate the skin.
About radiation safety
A common worry is whether radiation therapy makes you radioactive or unsafe to be around. It does not. External beam radiation therapy does not make you radioactive, so you can safely be around other people afterward, including pregnant women and children.
Building the routine
Because treatment usually happens daily on weekdays over a period of weeks, many people find it helps to build radiation therapy into a predictable part of their routine — the same time of day, the same way of getting there, and a plan for 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'll need and why, what to expect at your simulation visit, and whether you'll need a mold or mask. During treatment, it's fair to ask anything that comes up — from what to wear to whether you can be around family members afterward.
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
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Your next step
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