Author ORCID Identifier
Date of Graduation
Doctor of Philosophy (PhD)
Geoffrey S. Ibbott, Ph.D.
Laurence E. Court, Ph.D.
Brian P. Hobbs, Ph.D.
Gabriel O. Sawakuchi, Ph.D.
Keila E. Torres, M.D., Ph.D.
Jihong Wang, Ph.D.
VOLUMETRIC, MAGNETIC RESONANCE-VISIBLE, AND RADIATION-SENSITIVE DETECTORS FOR MAGNETIC RESONANCE IMAGE-GUIDED RADIATION THERAPY
Hannah Jungeun Lee
Advisory Professor: Geoffrey S. Ibbott, Ph.D.
Due to the superior soft-tissue contrast of magnetic resonance imaging (MRI) compared to conventional computed tomography (CT) and other on-board imaging techniques, several groups have integrated MRI and radiation treatment machine systems. The advent of MR image-guided radiation therapy (MR-IGRT) using systems, such as the 1.5 MRI – 7 MV linear accelerator (MR-Linac), now allow for improved soft-tissue on-board imaging for patient position and tumor target localization verification and the ability to assess functional biological tissue characteristics with MRI, all without increasing the patient radiation burden.
However, with the advantages of MRI guidance in MR-IGRT came the dosimetric challenges of the presence of a strong magnetic field. When the magnetic field is oriented perpendicular to the radiation beam, Lorentz forces act on secondary electrons causing hot and cold spots at tissue transition areas. These interactions with the magnetic field cause perturbations of the dose distribution in three dimensions. Current vendor-supplied electronic quality assurance tools can provide at best quasi-3D sampling of the dose distribution and cannot be MR imaged. As a result, there was a growing need for volumetric, MR-visible, and radiation-sensitive detectors for MR-IGRT applications. To fill this need for volumetric dose quality assurance, this dissertation work investigated existing and novel formulations of radiochromic gel dosimeters. After the optimal radiochromic gel formulation was identified, it was characterized for dose linearity, radiological properties, reproducibility, time stability, energy dependence, reusability, dose rate dependence, fractionation dependence, gel matrix dependence, and diffusion. Next, strong magnetic field and gradient field/radiofrequency effects on the response of 3D dosimeters were assessed along with other MR considerations that were and were not specific to MR-IGRT systems. Finally, heterogeneous and homogeneous 3D dosimeters were used for end-to-end testing with a variety of Monaco TPS plans.
This dissertation work contributed significantly to the fields of 3D dosimetry, MR-IGRT, and radiation physics: the first proof of concept of real-time 2D and 3D dose acquisition during irradiation was presented, a novel radiochromic gel dosimeter and its reusable version were presented and characterized, and the first full end-to-end testing including adaptive planning using daily MR images of the 3D dosimeters was presented. Overall, the feasibility and benefit of MR-visible and radiation-sensitive 3D dosimeters were presented in this dissertation work for MR-IGRT applications.
MR-IGRT, MRgRT, MR image guidance, MR-Linac, 3D dosimetry