質子治療，主要是利用質子束的深度劑量曲線前低後面急劇上升後又急劇下降的優點，而質子束高劑量區的深度位置分布與質子束的能量及能量的不準度 E有關，這在治療癌症上來說可以利用控制能量大小將劑量準確的投擲在入射方向上腫瘤所在位置；但因腫瘤是立體的其周遭都會有正常的細胞，因此對垂直入射方向上的橫向劑量的分布情況也是在治療上必須考量的因素之一。本研究主要目的是建立一套模型，可以精確描述質子束在水中的深度劑量曲線及橫向劑量隨深度變化情形。所使用的方法是在質子束出口處及水箱內各放置一個自製的具有空間解析度為一毫米的條狀探測器，分別作為監控及量測使用。首先利用監控探測器得知在該位置質子束的大小及形狀，再由探測器在水中量測深度劑量曲線與Geant4模擬交叉比對得知質子束的能量。接著利用探測器在空氣中不同深度下量測橫向寬度分佈結果再與Geant4模擬交叉對照，找出質子束的發散角。最後再將所得到的能量與發散角模擬質子束在水中不同深度下的橫向寬度分佈並與實驗量測結果相互比較。利用此方法分別對在日本大阪大學核物理研究中心(Reaserch Center for Nuclear Physics，RCNP)質子束能量100MeV及200MeV兩次實驗結果比較。這兩次實驗，在實驗上的架設、質子束形狀寬度、加速器設定皆不同的情況下，在水中橫向寬度分佈演變的趨勢實驗結果與模擬甚為吻合。證明了此套方法的正確性及模擬系統的整體的物理機制及參數設定適用於深度劑量曲線及質子束橫向寬度變化模擬的需求。Proton therapy takes the advantage of Bragg curve which delivers higher dose at depth before it stops abruptly. The less addressed point is its lateral dose distribution which is relevant to the healthy cells surround tumor. The main purpose of this study is to establish a model that can accurately estimate depth dose distribution and lateral dose distribution of a narrow proton beam in different depths. Parameters of proton beam are obtained by comparing experimental measurements with Geant4 simulations. Beam size and shape are measured by monitoring detector and proton beam energy can be learnt from depth-dose curve in water, by comparing measurement and simulation. Results from experiments prefer that proton beam propagates with a dispersion angle. Such dispersion angle is obtained by comparing measurement and simulation result of lateral dose development in air. Finally, these parameters are fed into simulation of proton beam propagation in water. Development of lateral dose in water at different depth is compared with experimental result. They agree well with proton energies of 100 and 200 MeV respectively. A reliable model has been established to estimate depth dose distribution and lateral dose distribution of a narrow proton beam in water. At the same time, previous study claims discrepancy between Geant4 simulation and experiment in lateral dose development in water. Our study shows such discrepancy disappears if dispersion angle has been introduced in proton beam propagation.
