Author ORCID Identifier

0000-0003-4346-1088

Date of Graduation

8-2018

Document Type

Dissertation (PhD)

Program Affiliation

Medical Physics

Degree Name

Doctor of Philosophy (PhD)

Advisor/Committee Chair

David Followill, Ph.D.

Committee Member

Laurence Court, Ph.D.

Committee Member

Clifton Fuller, M.D., Ph.D.

Committee Member

Gabriel Sawakuchi, Ph.D.

Committee Member

R. Jason Stafford, Ph.D.

Committee Member

Zhifei Wen, Ph.D.

Abstract

Introduction: Magnetic resonance (MR) guided radiation therapy (MRgRT) is an emerging field that integrates an MR imager with either a linear accelerator or three radioactive cobalt-60 sources. Before institutions participate in multi-institutional NCI-sponsored clinical trials, they are required to perform a credentialing test provided by IROC-Houston. During the credentialing test, end-to-end phantoms are used to evaluate the institution’s ability to perform consistent and accurate radiation treatments. IROC-Houston’s conventional anthropomorphic phantoms are not visible in MR, thus they are insufficient for MRgRT systems. The purpose of this work was to create an anthropomorphic thorax and a head and neck (H&N) phantom for MRgRT systems with magnetic fields ranging from 0.35T to 1.5T.

Methods: Over 80 synthetic materials were examined as potential materials used to construct the MRgRT thorax and H&N phantoms. Materials were characterized by: 1) measuring Hounsfield units, 2) visualizing in MR and CT imagers and 3) evaluating their dosimetric characteristics. Once materials were selected for the MRgRT phantoms, radiochromic film and double-loaded TLDs were then characterized in a 1.5T and a 0.35T MR environment. Reproducibility measurements on double-loaded TLDs were performed by using an acrylic block and irradiating it in 0T/1.5T and 0T/0.35T configurations on the Unity system and the MRIdian Cobalt 60 system, respectively. Geometrical thorax and H&N phantom slabs were designed to mimic similar interface conditions seen in anthropomorphic phantoms, but were simplified to reduce manufacturing time. The geometrical phantoms were designed with a rectangular tumor centrally located around surrounding tissue. These two phantoms were used to characterize radiochromic EBT3 film and TLDs by comparing beam profiles and point dose measurements irradiated with and without magnetic fields, respectively. GEANT4 Monte Carlo simulations validated the detectors in both Unity 0T/1.5T and MRIdian 0T/0.35T configurations. Two MRgRT anthropomorphic (H&N and thorax) phantoms were designed, manufactured and evaluated. A reproducibility and feasibility study was conducted to evaluate the phantom’s performance on MRgRT systems.

Results: This study found four materials which were tissue equivalent and visible on both MR and CT. Additionally, this study showed negligible difference in dose response between TLDs and radiochromic film when irradiated in 0.35T and 1.5T magnetic field environments. Two anthropomorphic phantoms were constructed and evaluated. The anthropomorphic thorax and H&N phantoms passed IROC-Houston’s 7%/5mm and 7%/4mm gamma passing criteria, respectively.

Conclusions: An anthropomorphic thorax and an H&N phantom were tissue equivalent, compatible with MR and CT workflows and could be used as end-to-end QA tools for MRgRT systems with magnetic fields ranging from 0.35T to 1.5T.

Keywords

MRgRT, MR Linac, MRgRT phantom, MR/CT phantom, IROC, anthropomorphic head and neck phantom, anthropomorphic thorax phantom, unity, mridian, end-to-end MRgRT phantom, MRIgRT

Available for download on Wednesday, June 26, 2019

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