What is a simulation CT scan?


The simulation CT scan is performed before radiotherapy initiation. This scan is done in the radiotherapy department and allows to:

  • Identify the area to be treated and the position in which the patient must be placed at each session (tattoo marks on the skin);
  • Delineate the area to be treated and the organs to be protected for the dosimetry calculations.

The simulation CT scan is the preliminary step to any external radiotherapy treatment. This is a CT scan, similar to the scans performed in medical imaging departments, but dedicated to radiation therapy (and therefore performed in the radiation therapy department).


Depending on the tumor location, the regions explored will differ (e.g. head, thorax, abdomen, pelvis, limbs) and contrast medium may or may not be injected.  During this exam, the radiation oncologist will determine the treatment position, which will be the same at each radiotherapy session. The doctor will locate the area to be treated on the CT images, and then will send the coordinates of this area to the lasers of the machine. These lasers will project these coordinates onto the patient’s skin. Then, the radiation therapist will make small tattoo dots in the places indicated by the lasers (or will put marks on the mask in the case of neck-face or brain radiotherapy). These markers will be used to position the patient in exactly the same way at each session during which the same lasers and therefore the same coordinates will be used as during the simulation scan.

This exam also allows the radiation oncologist to delineate the area to be treated and the organs to be protected, which is called the contouring step.


Once the simulation scan is completed, dosimetry can be performed.