The short answer
Radiation simulation is the planning appointment that maps exactly where your radiation will go. You'll usually lie on a table while a CT scan is taken in your treatment position; the team may make small skin marks (sometimes tiny permanent dots) or a custom mold to keep you still and precise. No radiation treatment is given that day. Ask your team how long your appointment will take.
Simulation is a planning session, not a treatment — no radiation is delivered.
X-rays or scans taken in your treatment position map where radiation should go.
Small skin marks or a custom mold help line you up precisely each time.
Ask your team how long your simulation visit will take.
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The short answer
Radiation simulation is the planning appointment that maps exactly where your treatment will be aimed. You lie in your treatment position while scans are taken. The team may add small skin marks or a custom mold, so you can be lined up precisely each time. No radiation is given that day.
Before the visit
Wear comfortable, easy-to-change clothing. Ask ahead whether to avoid lotions or deodorant on the day, and whether anything else applies to the specific area being treated.
What commonly happens, step by step
- You lie on a table in the exact position you'll be in for treatment.
- The team figures out your treatment area using x-rays or scans while you lie very still.
- A radiation therapist may tattoo or draw small dots of colored ink on your skin. These marks, either ink that fades or permanent dots about the size of a freckle, help line you up the same way every session.
- For some treatment areas, the team custom-builds a positioning device, such as a plastic or plaster body mold, or a fitted mask with air holes for treatment near the head or neck. These devices help keep you in exactly the same position each time.
Why so much care goes into this one appointment
Precision is the whole point of simulation. The careful setup here is what lets your daily treatments target the cancer accurately, while sparing as much healthy tissue around it as possible. A mold or mask that feels snug is meant to keep you still.
How simulation compares to your actual treatment visits
NCI says most treatment visits last from 30 minutes to an hour, with most of that time spent placing you in the correct position, and that you get radiation for 1 to 5 minutes. Ask your team how long the simulation visit will take.
What happens after simulation
Your treatment team uses the simulation to plan your treatment. Ask when your first treatment will be scheduled, so the gap doesn't feel unexplained.
What to ask your team
- Will my skin marks be temporary, or tiny permanent dots?
- Do I need a custom mold or mask, and what is that like?
- What should I wear, or avoid using on my skin, on simulation day?
- How long after simulation will my first treatment happen?
- What will each treatment visit actually involve, once I start?
Sources
Common questions
Does simulation hurt?
It mainly involves lying still while scans are done. Holding the position can be hard for some people, which is why molds or supports may be used. Tell your team if you are uncomfortable or in pain.
Are the skin marks permanent?
Sometimes. NCI says the therapist may tattoo or draw small dots of colored ink on your skin. Permanent dots are about the size of a freckle and stay for life, while ink marks fade. Ask which your team uses.
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: 2026-10-16Next planned review: 2027-07-12
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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, and this 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 then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. 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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