Graduation Semester and Year

2020

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Mingwu Jin

Second Advisor

Yujie Chi

Abstract

Carbon ion therapy is one of the advanced forms for radiotherapy and currently only available in few developed countries. Unlike conventional radiation therapy methods, carbon ion therapy has high potential in treating deep-seated and photon-radiation resistant tumors owing to its unique dose-conformality, higher relative biological effect and lower oxygen enhancement ratio. However, due to the lack of precise range verification tools in routine clinic, carbon ion therapy has not been used to its full potentiality. In this work, we used a Monte Carlo simulation tool, Geant4, to investigate range verification for carbon ion therapy. Geant4 is one of the well-stablished Monte Carlo simulation tools for the passage of particles through matter. Using Geant4, we have focused on exploring two of the key avenues in range verification for carbon ion therapy: positron emission tomography (PET) and prompt gamma imaging (PGI). These two methods have been extensively studied as possible solutions to the range uncertainty problem of carbon ion therapy to minimize the treatment margin and to lower the radiation to the organs at risk. In the first part of this work, we explored the potential of increasing the signals in PET through the use of radioactive carbon (C-11) ions instead of the stable carbon (C-12) ions at different incident energy levels. Their impact on PGI was also investigated if C-12 ions were replaced by C-11 ions. Prompt gammas (PGs) and annihilation gammas (AGs) were recorded for post-processing to mimic PGI and PET imaging, respectively. We used both time-of-flight (TOF) and energy selections for PGI, which boosted the ratio of PGs to background neutrons to 2.44, up from 0.87 without the selections. The ion inelastic process channel (for ions heavier than He2+) produced more PGs than the other channels, with a sharp drop in PG counts near the Bragg peak. AG yield from C-11 was 6~11 folds higher than from C-12 at low energies (penetration depth of several cm) and 30%~60% higher at high energies (penetration depth of dozens of cm) in PMMA. PG yield from C-11 is comparable to that from C-12 (0.87-fold in the worst scenario). Range verification in this regard can be benefited largely from PET signal boost while maintaining similar PG yield if C-12 ions were replaced by C-11 ions. These results demonstrate that using C-11 ion beams for potentially combined PGI and PET has great potential to improve online range verification accuracy and precision. In the second part of this work, we focused on the evaluation of the multi-slit camera, which is a mechanical collimation for PGI over a large field of view along the longitudinal beam axis, with the aim to explore the optimal setups of the camera relative to the phantom. Five parameters including slit and slab width, height of collimators and placement of detector as well as collimators were interrogated, which led to thousands of simulations. To facilitate the massive simulation, we first used an isotropic gamma source with an energy spectrum of interest and five evaluation metrics, including signal-to-background ratio, sensitivity, spatial resolution, peak-to-second peak ration, and slab-to-slit ratio to quantify the performance of each configuration. Afterward, a simulation with C-12 ion beam irradiation on a water phantom was used to assess the range verification performance of the top ranked configuration on each metric. Our preliminary results suggested that signal-to-background ratio and sensitivity outweigh other metrics for capturing the distal fall-off of PG in a uniform phantom. This work lays a strong foundation for future system design of PGI for real-time range verification and monitoring.

Keywords

particle therapy, carbon ion therapy, range verification, PET, PG

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Included in

Physics Commons

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