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Radiation Simulation

What is radiation simulation?

Radiation simulation is the set-up or planning process for radiation. The simulation visit will help your child get ready for the actual treatments. 

Watch the video: An Introduction to Radiation Therapy

During the simulation, the radiation oncologist will decide the best treatment position for your child. This is important because radiation can damage normal cells along with cancer cells. The correct body position can help kill more cancer cells while doing as little damage as possible to normal cells. A mask or body mold might be made to help hold your child’s body in the same position for each treatment.

Radiation therapist prepares patient for radiation therapy

During radiation simulation, your child will be fitted for devices such as a mask or body mold to make sure they stay in the same position during each treatment.

Getting ready for simulation

Before the simulation, you will meet with members of the radiation therapy care team. They will talk to you about what to expect and answer any questions. Your care team may also talk with you about whether your child will receive general anesthesia.

A child life specialist is one of the team members who might work closely with your child and prepare them for simulation. A child life specialist can show your child the imaging equipment and explain the process in a way that fits their age. Your child will learn what to expect, and if needed, the child life specialist will help develop a plan for coping.  

Imaging tests

Your child will have a Computed Tomography (CT) scan and sometimes Magnetic Resonance Imaging (MRI) to make sure their body is in the best position for treatments. When your child is in the correct position, the therapist will make some marks on their skin with a permanent marker or paint pen. The therapists will use these marks to help repeat the same treatment position each day.

Contrast for CT and MRI

In most cases, contrast will be used for either CT or MRI simulation. Contrast can make the images from scans appear more clear. The contrast agent is called gadolinium. This substance is given into a vein by IV. Your child’s schedule will have instructions about not eating or drinking before receiving the contrast. It is very important to follow the guidelines exactly. If you have questions about whether contrast will be used, please ask your child’s radiation oncology nurse.

Radiation tattoos and marks

In most cases, the therapist will place very small, freckle sized radiation tattoos on your child’s skin using a sterilized needle. Only the tip of the needle is used to make the mark. These marks are permanent and do not interfere with any other treatment your child will receive.

In some cases, the mark might be made with a permanent marker or paint pen instead. If the therapist makes a mask for your child, the marks will be on the mask. If your child does not need a mask and the marks are on the skin, the marks must remain on the skin until radiation therapy is complete. The therapist might touch up the marks to make sure they are easy to see.

Devices to help your child hold still


The therapists may use a mask (sometimes called an aquaplast mask) to hold your child’s head in the proper treatment position. The mask is made with a flat piece of plastic that has small holes in it, like mesh. When your child is in the correct position on the table, the plastic piece is placed in warm water. The therapist will give the plastic a few seconds to cool and then will place it on your child’s face and stretch it down over the chin. The warm water loosens the plastic, which helps the mask form over the face. It will feel very warm and wet but will cool quickly. Many patients say it feels like a warm washcloth on their face. As it dries, the plastic hardens to form a facemask. Your child will be able to breathe easily through the holes in the mask.


The therapist will also make a mold to go under your child’s head when they are lying on their back. The mold is a cloth bag filled with beads. The therapist adds water into the bag, and the beads harden so the mold forms into the shape of the back of your child’s head. This will make it more comfortable for your child to hold their head in the exact position needed for treatments. The mask and head mold will be used for each treatment to hold your child’s head in the correct treatment position.

If your child will be lying on their stomach during treatment, a similar facemask will be used. When your child is lying on their stomach with their face in the mask, the therapist will place an oxygen tube in the space between the table and the mask so that your child will be able to breathe easily.

Body mold

During simulation, staff may also make customized body molds, sometimes called a vac lock bag, to position your child. A vac lock bag can be used to support the head, abdomen, leg, or another part of the body. Your child will lie on their back on the bag. The vac lock bag feels squishy, like a beanbag. A vacuum will remove the air from the bag, so it molds around your child’s body.

Alpha cradle

An alpha cradle is used if your child needs Total Body Irradiation (TBI). If your child needs an alpha cradle for treatment, they will lie on the TBI table on top of a vac lock bag. The radiation therapist will pour a foaming solution inside a plastic bag called an alpha cradle and place it on top of your child like a blanket. The bag will harden and form the shape of their body. The bag will feel warm and will harden within 10 to 20 minutes. The bag will be used each time your child has radiation treatment to hold their body in the proper treatment position.

Reminders for parents and family caregivers

  • Your child may have general anesthesia for simulation and radiation treatments to help them stay still. Your medical team will make this decision. The anesthesia team members will give your child the anesthesia medicine and monitor your child the entire time. You or another family member may stay with your child until they fall asleep. Parents must wait in the waiting room, and a staff member will call you to the recovery room when your child wakes up.
  • If your child’s treatment requires a mask, their hair must be in the same style each day of the simulation and treatments. This will ensure that the mask holds their head in the correct position each time. Sometimes, your child may need a haircut before the simulation or treatments.
  • In most cases, the therapist will ask your child to remove some of their clothing during the simulation to perform scans and to place the marks on their skin. A hospital gown will be provided for your child. 
  • When your child dresses the morning of the simulation, try to avoid clothing with metal snaps, zippers, and buttons. The best clothing choices include loose, comfortable clothing such as t-shirts, sweatpants, or pajama bottoms, or other elastic waist pants. If your child’s clothing has metal pieces where the body will be scanned, they must change into a hospital gown before the simulation.
  • Your care team will let you know if a parent or family caregiver can be with your child for the simulation. You may be asked to return to the waiting area so that the staff can focus on the patient and simulation.   
  • For safety reasons, siblings must stay in the waiting area during the simulation. Make sure you have someone in the waiting area to watch siblings before you arrive for the simulation.

Your care team will let you know your schedule for simulation and radiation treatments. Your child may also have an appointment with a child life specialist. If you have questions about simulation or radiation therapy, talk to your care team. 

Key points

  • Radiation simulation is the set-up or planning process for radiation. The simulation visit will help your child get ready for the radiation treatments. 
  • During the simulation, the radiation oncologist will decide the best treatment position for your child. Imaging tests will be used to make sure your child’s body is in the correct position.
  • A face mask or body mold might be made to help hold your child’s body in the same position for each radiation treatment.
  • A child life specialist can help your child know what to expect during simulation and radiation therapy.   
  • If your child will have general anesthesia or CT or MRI with contrast, be sure your child follows the instructions for not eating and drinking before your appointment.

Reviewed: July 2023