Date of Graduation
Masters of Science (MS)
MR-guided radiation therapy (MRgRT) provides benefits such as superior soft tissue contrast, no imaging dose, and functional imaging capacities, but it also has concerns to be addressed. The electron return effect (ERE) refers to dose enhancement at the interface between different media, caused when radiation is delivered in the magnetic field. The ERE poses clinical concerns in MRgRT because it significantly enhances the dose at interfaces, and the intensity and pattern of the dose enhancement depends on many factors. The ERE results in a complex pattern of dose enhancement over a three-dimensional (3D) volume around tissue interfaces inside the patient body, giving rise to the need for 3D dosimetry. If proven reliable in the magnetic field, 3D dosimetry will provide a more rigorous means for the quality assurance (QA) of MRgRT than two-dimensional dosimetry. The 3D dosimeters can also be used to estimate the volumetric dose distribution in MRgRT treatment of heterogeneous treatment sites (e.g., lung), around simple structures such as the trachea and the esophagus, and in MR-guided brachytherapy.
In this thesis, the performance of PRESAGE® in the magnetic field was evaluated by investigating how well PRESAGE® could measure the ERE. The radiation response of Gafchromic® EBT3 film was shown to be unaffected by the magnetic field in a previous study, and the reliability of EBT3 film measurement in the magnetic field was independently verified in this thesis. As a result, the performance of PRESAGE® was evaluated by comparing against EBT3. Before measuring the volumetric ERE, the in-house made PRESAGE® formulation that was used throughout this thesis was tested for magnetic field effects. The formulation showed ~ 9 % under-response when irradiated in the magnetic field, but the response remained strictly linear and thus did not interfere with using PRESAGE® as a relative dosimeter. Finally, it was hypothesized that the measurement of the ERE using PRESAGE® would agree with EBT3 within 5 %/3 mm local gamma criteria. The EBT3 and PRESAGE® measurements agreed well with a passing rate over 90 %. The hypothesis was proven correct and showed that PRESAGE® is a promising material for the QA of MRgRT.
PRESAGE, 3D dosimetry, electron return effect, MR-guided radiation therapy, MRgRT, Gafchromic EBT3