Dosimetry for online monitored MR-Linac radiotherapy

In radiotherapy a key goal is to ascertain that tumors are sufficiently covered with radiation dose while healthy tissue is spared as much as possible. Conventionally, a mix of image modalities such as CT, PET, and MR are used to give the optimal description of the tumor. Recently the combination of megavoltage linear accelerators and MR-scanners in MR-Linacs has become commercially available. These new machines allow for daily adaptions of treatments plans through MR-images taken immediately before or during patient treatment. Furthermore, some of these machines can process images so fast that they can turn the beam on and off based on the tumor’s position extracted from the MR-images. Three Danish Hospitals have purchased MR-Linacs and they were installed in 2019 at: Copenhagen University hospital Rigshospitalet, Copenhagen university Hospital Herlev and Gentofte, and Odense University Hospital.
To fully exploit the features of these new machines the dosimetry must be tested and validated preferably by independent methods. A detector system that can provide time-resolved dosimetry without image distortions is essential. Many conventional detector systems contain materials, such as metals or graphite that creates susceptibility artifacts in MR-images or in other ways disturb the magnetic field. This PhD project details an investigation of a plastic scintillator system for use in MR-Linacs.
It was found that the scintillator system can provide time-resolved dosimetry, with dose per pulse resolution in MR-Linacs without image distortion, and a protocol for measuring absolute dose through an alanine calibration was made. The dose per pulse measurements revealed machine specific dose-rate transients on multiple MR-Linacs. The effect of these dose-rate transients was in some cases a 1 % reduction in dose measured in the central beam. The PhD project played into an ongoing project towards measuring small field output factors in MR-Linacs; the current code of practice TRS483 does not include measurements in magnetic fields. A protocol for measuring output factors in MR-Linacs was developed using plastic scintillation detectors including a framework to evaluate and create correction factors for light filtration in optical fibers and a uncertainty budget.
This thesis concludes that plastic scintillation detectors have a significant potential for dosimetry in MR-Linacs. Their MR compability and high time-resolution provides highly detailed data relevant for gated treatments and the low perturbation makes them excellent detectors for small field measurement in MR-Linacs.