| Summary: | 博士 === 國立清華大學 === 生醫工程與環境科學系 === 106 === In this study, we use PTSim (Particle Therapy Simulation) to develop a Monte Carlo simulation system for wobbling nozzle with layer stacking (LS) technique in Chang Gung Memorial Hospital (CGMH). A Monte Carlo simulation system of proton facility for clinical irradiation involves several physical components, like beam scatterer, ion chamber, collimator, range modular, beam modifier, and water phantom/CT image-based phantom. In addition, the incident particle parameters for Monte Carlo need to be optimized by comparing simulated depth doses with those of measurements using evaluation indices including range (R80), the width of Bragg peak (FWHM), distal dose falloff (W80-20%) and peak-to-entrance ratio (PE-Ratio). Proton depth doses of 70 to 230 MeV with three wobbling radius modes (S, M, L) for incident particle mean energy and energy spectrum spread were optimization. Good agreement between simulation and measurements was achieved. For all beam energy and wobbling radius, the maximum difference of R80, FWHM and W80-20% were less than 0.884 mm, and PE-Ratio maximum difference were under 0.144 in this study.
Besides, we also evaluated the impacts of fine degraders on proton beam quality. According to the results in this study, the trend of ΔFWHM and %FWHM demonstrated the fine degrader thickness may cause significant energy straggling effects. Moreover, the ΔDent also show the significant discrepancy, especially in the thicker fine degrader. Finally, the tissue heterogeneity effects may cause significant range variation. In this study, two major influencing factors of range perturbation had been investigated: the mass density and chemical composition. The density contribution was more significant, and the chemical composition was less significant unless the Z/A was very different, such as for cortical bone and air. If density scaling was applied, the electron density was a better factor than mass density for range scaling.
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