本研究將利用PTSim建立林口長庚醫院質子治療中心第二間治療室(PG2)蒙地卡羅機頭模擬系統並著重於擾動式技術(Wobbling)及漸層堆積法(LS)的初始質子射束參數最佳化。蒙地卡羅模擬系統建立需要完整的機頭組件建模及初始射束參數調整，經由改變蒙地卡羅初始入射射束參數使其所計算的深度劑量分佈與實際量測吻合。而深度劑量分佈評估則會採用一系列的布拉格尖峰特性指標，如：射程（R80）、布拉格峰尖峰寬度(FWHM)及尖峰後劑量衰減距離(W80-20%)、尖峰劑量與入射點劑量的比值(PE-Ratio)。而此套系統將會尋找70至230 MeV質子射束在三種不同射束掃描半徑下的最佳化初始平均能量及能譜分佈。在各個能量射束不同掃描半徑下，其最佳化參數所計算出來的深度劑量分佈可發現R80、FWHM及W80-20%和實際量測相比最大差異皆小於0.884 mm，而PE-ratio最大差異也僅0.144，皆低於臨床量測誤差，因此本研究已成功地完成蒙地卡羅最佳化參數調整。
此外，本研究也探討利用微型降能器進行射程調節(Range shift)對於射束特性的影響。在臨床治療計畫系統中，此方法將會忽略穿過降能器後的能譜分佈用以計算人體體內劑量分佈。從ΔFWHM及%FWHM的趨勢來看，微型降能器的厚度將會明顯改變能譜分佈，而表面劑量(ΔDent)也有顯著的差異。因此當臨床需利用微型降能器進行射程調節時需要特別注意，尤其是使用較厚的微型降能器。
而人體組織非均質性對於質子射束射程的影響，在本研究主要探討兩個影響劑量變化因素：物質密度及其化學組成成分。根據實驗結果顯示，密度的影響較為顯著而物質化學組成影響相對較小，而當物質的Z/A值和其他物質差異過大時則會增加劑量差異。此外，當各物質中的深度劑量分佈需做射程修正時，使用電子密度能提供較好的修正效果。

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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