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. It's painless, takes roughly 30–60 minutes, and no radiation treatment is given that day.
Simulation is a planning session, not a treatment — no radiation is delivered.
A CT scan in your treatment position maps where radiation should go.
Small skin marks or a custom mold help line you up precisely each time.
It's painless and usually takes about 30–60 minutes.
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The full explanation.
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 a CT scan is taken. The team may add small skin marks or a custom mold, so you can be lined up precisely each time. It's painless, takes about 30 to 60 minutes, and 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 a CT scan, sometimes with other imaging added.
- A radiation therapist may tattoo or draw small dots of colored ink on your skin. These marks, sometimes temporary and sometimes tiny permanent dots, 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 overly precise, even snug, is doing its job.
How simulation compares to your actual treatment visits
Simulation itself usually takes about 30 to 60 minutes, most of it spent getting your position exactly right. Once treatment begins, most individual visits are shorter, often 30 minutes to an hour, and the vast majority of that time is still spent on positioning. The radiation itself, once you're lined up, typically lasts only one to five minutes.
What happens after simulation
Your treatment team uses the simulation scan to design your specific treatment plan, a process that can take several days to complete before your first actual radiation session. 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 many days after simulation will my first treatment happen?
- What will each treatment visit actually involve, once I start?
Sources
Words to know
Tap any term to see what it means.
Common questions
Does simulation hurt?
No. It mainly involves lying still while a scan is done. The hardest part for some people is holding the position, which is why molds or supports are used to help.
Are the skin marks permanent?
Sometimes. Some centers use tiny permanent dots (like small tattoos) for accuracy; others use temporary marks. Ask your team which they use so you know what to expect.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Your next step
Get ready with a checklist and questions for the visit.
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Last updated: 2026-08-11Next planned review: 2027-07-12
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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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