Radiation simulation and planning is the careful mapping appointment that happens before radiation treatment for breast cancer begins. During a simulation session, your radiation oncology team positions you on a treatment table, takes detailed imaging scans, and creates marks or devices that make sure each future treatment lands in exactly the right place. The planning that follows turns those images into a customized plan shaped around your body — no two plans are alike, because individual cases differ and your care team designs your plan for your situation.
Key Takeaways
- Simulation is a dedicated planning appointment that happens before radiation starts — it is not treatment itself, and nothing therapeutic is delivered during the session.
- The goal is precision: your team records your exact position and uses imaging scans so treatment can be aimed consistently at the right area each time.
- You may get small alignment marks on your skin, and you may be fitted with positioning aids such as cushions or molds that help you stay comfortable and still.
- A planning team then uses your scans to design a customized plan around your anatomy; individual cases differ, so plans vary from person to person.
- Ask your care team anything ahead of time — knowing what to expect generally makes the appointment feel much more manageable.
What Is Radiation Simulation?
Radiation simulation — often just called “sim” — is a planning session that takes place after your cancer treatment team has recommended radiation and before any treatment is given. The word “simulation” can sound confusing, but it simply means rehearsing and recording the setup that will be used during treatment.
During the session you lie on a treatment table in the position you will use for your sessions. Your team takes detailed imaging scans, usually a CT scan, that show the area being treated and the nearby tissues and organs. These images become the blueprint your team works from.
It helps to think of simulation as the measuring and tailoring appointment — like being fitted for clothing before it is made. Nothing about it treats the cancer; it prepares the way so that treatment, when it begins, is as precise as possible.
Why Precise Planning Matters
Radiation for breast cancer is aimed at a specific area of the chest. Because breathing and small movements shift the body slightly, and because each person’s anatomy is different, the team needs an exact map to aim treatment the same way every time.
Careful planning serves two purposes at once:
- Aiming at the right target. The plan identifies the area your doctor wants treated and makes sure it is covered consistently from one session to the next.
- Limiting exposure elsewhere. The team maps nearby tissues — such as the lungs, heart, and skin — so the plan can be shaped to spare them as much as possible.
That second part is a big reason planning has become so detailed over the years. Modern planning tools let the team see your anatomy in three dimensions and shape treatment around it. The quality of this preparation is part of what makes the whole course of radiation safer and more reliable.
What Happens During a Simulation Appointment
Every center has its own routine, and individual cases differ, so the exact steps may vary. But in general, a simulation appointment tends to include the following:
Getting into position
You will be asked to lie on a flat treatment table. For breast treatment, this is usually on your back with your arms raised above your head and resting in supports. The staff will adjust your position carefully and take their time — being positioned well is the whole point of the visit.
You may be asked to hold still in a particular posture while they fine-tune it. This can feel a little awkward at first, but the team does this every day and will help you find a position you can hold comfortably and repeat.
Imaging scans
Once you are positioned, the team takes imaging scans — most often a CT scan — of the treatment area. The machine is a large ring or donut shape that moves around you; it does not touch you. You may be asked to breathe normally, or the team may ask you to hold your breath briefly for part of the scan.
These scans are for planning only. They give the team detailed cross-sectional pictures of your anatomy so the plan can be built around it.
Recording the setup
Throughout the appointment, the team notes the details of your position: the height of the table, the angle of the arm supports, the placement of cushions, and so on. Some of this is recorded with small marks on your skin or with positioning devices, described in the next section. The goal is a setup that can be reproduced exactly at each treatment session.
Appointments like this are usually done in the radiation department of a hospital or cancer center, and family members or companions can often wait nearby. If having someone with you would make you more comfortable, ask the center about its policy — many welcome it.
Skin Marks and Positioning Aids
One part of simulation that surprises people is the marking. Because the team needs to find the same position at every session, they often place tiny alignment marks on the skin. These are usually:
- Small ink marks or dots — temporary marks made with a special pen, or
- Tiny permanent dots — about the size of a freckle, sometimes called tattoos, that make long-term alignment reliable.
If permanent dots are suggested, your team should explain why they are recommended and ask for your agreement first. If you have concerns — for example about visibility on the skin — raise them. There are often alternatives, such as different marking methods, and your preferences matter.
You may also be fitted with positioning aids — custom-shaped cushions, molds, or supports that cradle your body in the right posture. These are made to fit you and are reused at each session so your position stays consistent. Some centers also use a light-based or surface-guided system that checks your position without any contact.
How Your Plan Is Built After the Appointment
Once your simulation scans are complete, the real design work begins — and it happens behind the scenes over the following days. A team that typically includes your radiation oncologist, a medical physicist, and a dosimetrist (a specialist who calculates treatment plans) works together on your case.
In broad terms, the process looks like this:
| Step | What happens |
|---|---|
| Outlining the targets | Your radiation oncologist marks the area to be treated and the nearby tissues to protect on your scan images. |
| Designing the beams | The team works out the angles and shapes of treatment that cover the target area while limiting exposure to surrounding tissue. |
| Checking and verifying | A medical physicist reviews the calculations independently to confirm the plan is safe and accurate before anything is scheduled. |
| Quality review | The whole plan is checked against the team’s standards, and only then is your first treatment appointment booked. |
This is one reason there is usually a gap of days between your simulation and your first treatment session. The time is not idle — it is spent making sure the plan is right. If your center has not given you a start date yet, that is normal; the plan has to be finished and checked first.
Your care team designs this for your situation. Factors like the type and stage of the cancer, the surgery you had, and your anatomy all feed into the plan. That is why a friend or family member’s radiation plan may look quite different from yours — even when the diagnosis sounds similar.
Common Questions and Worries
It is completely normal to feel nervous before a simulation appointment. Some of the worries people mention most often — and what is generally true about them:
- “Will it hurt?” Simulation itself is not painful. Lying still in one position can be uncomfortable, especially with arms raised, but the team will work with you to make it manageable.
- “Will I feel claustrophobic?” The scanner is an open ring, not an enclosed tube, and staff stay in contact with you throughout. Tell them beforehand if enclosed spaces bother you.
- “What if I move during the scan?” Small movements happen; the team watches and will simply repeat a scan if needed. You will not ruin anything.
- “Can I eat or take my medications beforehand?” There are usually no fasting requirements, but confirm with your center — instructions can differ.
- “What should I wear?” Comfortable clothing is fine; you will change into a gown for the appointment itself. Avoid lotions, creams, or deodorant on the treatment area that day unless your team says otherwise.
If any part of the process was not explained to your satisfaction, please ask for it to be explained again. A good team expects questions and would rather answer them now than have you worry later.
Questions to Ask Your Doctor
Bringing a short list of questions to your simulation visit or to the appointment where your plan is reviewed can help you feel more in control. Consider asking:
- What area will my radiation plan cover, and why was this area chosen for me?
- Will I have temporary marks, permanent dots, or another alignment method — and what are my options?
- How will nearby tissues, such as my heart and lungs, be protected in my plan?
- How long will the gap be between simulation and my first treatment?
- Who should I contact if I have skin changes, pain, or questions once treatment starts?
- Are there instructions I should follow between now and my first session — about skin care, for example?
- Can a family member or friend come with me to treatment sessions?
It can help to write down the answers, or to bring someone who can take notes while you listen. There is no such thing as a silly question when it is your own treatment being planned.
Frequently Asked Questions
Is simulation the same thing as treatment?
No. Simulation is the planning appointment where your position is set, scans are taken, and alignment marks or devices are created. No therapeutic radiation is delivered during simulation — it is preparation so that treatment, when it starts, is aimed precisely and consistently.
How should I prepare for my simulation appointment?
In general, no special preparation is needed: you can usually eat normally and take your regular medications. Wear comfortable clothes, since you will change into a gown. Your center may ask you to avoid lotions or deodorant on the chest area that day — follow whatever instructions they give you, since routines differ between centers.
Will the marks on my skin wash off?
Temporary ink marks can fade with washing, which is why some centers suggest being gentle with the marked area or why they may offer tiny permanent dots instead. If marks fade before treatment begins, the team can redo them — it is a routine part of the process, not a problem you caused.
Why is there a wait between simulation and my first treatment?
The days between simulation and treatment are when your customized plan is built: your scans are analyzed, the treatment is designed around your anatomy, and a physicist independently checks the calculations. This careful behind-the-scenes work is part of what keeps treatment accurate, so the wait is expected and purposeful.
Will my plan be the same as someone else’s with breast cancer?
Very unlikely. Plans are customized to each person’s anatomy, cancer type and stage, surgical history, and other individual factors. Your care team designs the plan for your situation, which is why comparing details with others can be misleading — even well-meaning comparisons.
Where can I learn more about radiation planning from trusted sources?
The National Cancer Institute (cancer.gov) and the American Cancer Society (cancer.org) both publish patient-friendly explanations of radiation therapy and how it is planned. Your own care team remains the best source for anything specific to your situation.
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- Proton therapy for breast cancer
- Showering during radiation therapy
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This article is for general educational purposes only and is not medical, legal, or financial advice. It does not replace professional care. Talk to your doctor or care team about your own situation. If you think you may have a medical emergency, call 911.