Impact of the MRI Restraint System on the Quality of the Synthetic Scan for Intracranial Radiotherapy Treatment

In radiotherapy, CT scans are the reference imaging tool for treatment planning, as they provide the electron density of the tissues required to calculate the dose. Magnetic Resonance Imaging (MRI) is used as a complement to CT to contour target volumes and organs at risk. While the properties of CT allow us to determine the electron density of tissues, the properties of MRI allow us to determine the proton density of tissues. MRI therefore provides better tissue contrast, but cannot be used directly to calculate the dose. In recent years, MRI-only protocols have been introduced to remove the need for a CT examination and avoid possible MRI-CT registration errors. Dose calculations are then performed on MRI images converted into synthetic CT images (sCT) using software based on artificial intelligence (for the most recent). Various commercial solutions are available for generating these sCTs, and the ICO already has experience of evaluating them.

While MRI scans for diagnostic purposes are performed in the free position, the generation of an sCT for treatment preparation requires the MRI scan to be performed in the radiotherapy position. For cranial lesions, a thermoformed mask fixed to the table is made for each patient. Depending on the type of treatment and its associated degree of precision, different types of mask exist at the ICO: '3-point' for normo-fractionated treatments and 'stereo' for treatments of 1 to 3 fractions. The use of these masks during the MRI examination can interfere with the correct positioning of the antennae and thus lead to a deterioration in the quality of the images obtained (MRI and, by repercussion, sCT).

The aim of this study was to evaluate the impact of the 2 thermoformed masks used at the ICO on the quality of a synthetic scan for 3 healthy volunteers.