3-Dimensional Scintillation Dosimetry for Small Irradiation Fields Control in Protontherapy

by Erwan Mainguy

Abstract: The use of proton beams for radiotherapy has many benefits compared to « conventional » photon beams, especially in terms of ballistic properties, which make the treatment less radiotoxic.

However this technique remains limited in the case of small irradiation fields, due to a lack of tools for quality control. In order to compensate this, a new dosimetry system is being developped.

This system, based on the recording of the light emitted by a plastic scintillator thanks to a fast camera, should allow the reconstruction of a 3D map of the deposited dose.

The work done during this internship focused first on the analysis of the data collected during experiments at the CYCLHAD proton therapy center.

The first one allowed to study the « beam files » generated by the machine (containing the beams characteristics) and the images obtained by the camera, in order to determine the positions of the beam in the machine and the scintillator referentials. This led to an understanding of the parameters required to generate irradiation plans. Specifically customised irradiation plans were used in the next experiments to study the repeatability of the system at different beam intensities.

In parallel, measurements performed on the optical part of the system should lead to a better understanding of the vignetting and help to determine the most appropriate depth of field for our application.

Finally, by comparing the results of the Monte-Carlo simulations with the experimental results, it will be possible to determine the impact of the scintillation « quenching » on the dosimetric response. Indeed this phenomenon, which is similar to saturation, causes the decrease of the scintillation response. As a consequence, the emitted scintillation light is no longer proportional to the dose deposited by the beam.