What to Know About Your Sim Appointment

What to Know About Your Sim Appointment

Reviewed by Jasper Clinical Board

Last updated 5/24/21

If you’re getting external radiation therapy, one of the first major steps is your simulation appointment—aka “SIM.” Simulation is one of those mysterious pieces of medical jargon that can make cancer treatment so confusing (and sometimes frustrating). 
Fortunately, you can get prepared with just a quick primer. Here’s what you should know about simulation—and some important questions to ask your healthcare team as you go.

What is a simulation appointment?

Radiation therapy kills cancer cells by shooting them with precise, high-energy beams of radiation. To make sure everything goes smoothly, your healthcare team needs to A) learn the best way to position your body during treatment, B) map the location of your cancer, and C) determine how much radiation is needed. And that’s exactly what happens during a simulation session. 
This appointment is called a “simulation” because you don’t actually receive any radiation during one of these sessions, but your health team may simulate a treatment using an X-ray machine or other medical device. The whole process—which usually takes about an hour—is a bit like a dress rehearsal. The goal is to gather information and set the stage for a smooth treatment course over the coming weeks.

What happens during simulation?

Body positioning

One major goal of simulation is learning how to position your body and hold it completely still. During this session, your healthcare team may fit you for an “immobilization device” to help you do this. That could mean medical tape, a plastic mold, foam padding, or a mesh face mask to gently hold in place the part of your body that’s being treated. These devices aren’t always fun to wear, but they ensure that the energy beam only hits cancerous cells (which is very important because radiation can damage healthy cells, too).
You’ll probably be lying still for this process, perhaps for up to an hour. If you think this might be difficult for you, consider taking a pain reliever like Tylenol ahead of time. Also let your healthcare team know if you think being immobile will make you anxious. Getting a detailed preview of the process can be helpful, and your oncologist may also be able to prescribe you an anti-anxiety medication or allow you to listen to a guided meditation via headphones or earbuds (if you’re allowed to bring your own, Calm, Headspace, and free-of-cost Insight Timer have several to choose from).

Cancer imaging

The next important step is to map your cancer. Your healthcare team needs to know exactly where it is, so they can precisely target it and make sure it’s shrinking. During your simulation appointment, they’ll create this map using imaging technology like X-ray or CT scans. These scans may also help your doctor learn key details about your cancer and choose the appropriate radiation dose.

Skin marking

Finally, your healthcare team will mark your skin to guide the radiation treatment. They may use a semi-permanent marker (which will wash off over the course of a few weeks) or create small, permanent tattoos (which can be removed through a simple procedure, though typically they’re so unnoticeable that they’re left alone). These are tiny marks—smaller than a freckle—which most people hardly notice. During the simulation, you’ll see “reference lasers” projected onto your body and the room around you. By aligning these lasers with your skin markings, the team can target cancer with incredible precision—down to the millimeter.

What to ask your doctor

Needless to say, you should feel comfortable asking your doctor anything. If you’re confused, hesitant, or worried about any part of your simulation appointment, speak up! Your healthcare team is there to help you—and that’s easiest when they know what you’re thinking.
Beyond that, here are a few questions that are useful to ask during a simulation appointment:

1. “How should I prepare for my first radiation treatment?”

You may need to eat a special diet, avoid certain medicines, and follow other treatment guidelines. Your team will probably send you home with helpful literature that you can review at your leisure.

2. “What side effects should I expect?”

Depending on the part of your body being treated, you might experience sore throat (for head and neck treatment), diarrhea (for lower abdomen treatment), or cough (for chest treatment). These are just some examples, so keep an eye out and mention any unusual symptoms or sensations to your team.

3. “What’s the best way to manage my discomfort?”

Many people find it tough to stay immobilized for long periods of time. As we mentioned above, relaxation techniques (meditation, deep breathing) can be very helpful during these sessions. The treatment center may also offer to play music for you during your appointments, or even read you a guided meditation. If all else fails, your doctor may prescribe you anti-anxiety medication.

4. “Who are all these people?”

A whole team of healthcare workers will be on hand during your simulation appointment. Don’t be shy about asking for introductions! You’ll probably see your radiation oncologist (whom you already know), as well as radiation oncology nurses, a radiation therapist (a team member who performs the treatment), and a dosimetrist (an analytical team member who helps determine and oversee your medication dosages). 

5. “When will I see you next?”

This question is for your radiation oncologist—the doctor who’s overseeing your radiation therapy. They won’t be at every treatment, but you’ll see them regularly (once per week is common) to assess your progress and discuss any side effects. Make sure you have this meeting on your calendar and are prepared for it!

The content on this website is intended to provide the best possible information for you, but should not be considered—or used as a substitute for—medical advice. If you have questions about your diagnosis or treatment, please contact your health care provider(s). For questions or comments about this content, please email us at