質子治療為放射治療方式之一，與一般光子治療相比它具有更精確劑量投擲的特性。 質子治療在劑量分布末端急速下降提供緊致外廓的同時，卻對射程不準度造成相對敏 感，此不準度容易傷害到與腫瘤接近的敏感器官；因此，在治療過程中對質子射程作 即時監控是有其必要性。藉由監控質子與人體組織發生核反應所產生二次光子可達成 此目的。本研究採用 GEANT4 模擬軟體版本 9.4.p02，相關物件包括 PMMA 靶材及其 左右各一塊 LYSO 探測器，入射輻射源為 130 MeV 單一能量質子束，所有條件均與在 林口長庚醫院實驗條件相同，最後實驗結果與模擬結果相比。在模擬數據所觀察到光 子能量都落在 0 – 10 MeV之間，它們主要來自正子煙沒的 511 KeV光子(PAG)及各元 素激發產生的即時光子(PG)。我們以在 PMMA 靶材中產生的同位素作為估算 PAG 及 PG 數目依據。實驗中光子能譜因束流是否通過 Beam-ON 或 Beam-OFF 而不同。模擬 結果顯示 PG 或 PAG 在深度分布都可以拿來估算質子治療射程，只是兩者所測得射程 深度與布拉格峰分別相距為 1 mm 或 8 mm。比較 Beam-OFF 數據中 PAG 與模擬數據 中正子發射源在縱向深度分布，兩者分布大致相同。另外，假設 15O 及 11C之半生期為 已知，由實驗數據可推導出 15O 與 11C 兩者的比例為 1.42 : 1，此比例與模擬預測值相 近。最後我們結論 GEANT4 是一套強而有力的模擬工具，它適合模擬質子與人體組織 所發生核反應及光子產生;Proton therapy is a treatment modality which can deliver dose precisely to cancer tumor in comparison to photon therapy. However, the sharp distal fall-off in proton therapy provides compact comformity, yet it is sensitive to uncertainty in dose delivery during treatment. The uncertainty can be risky for sensitive organ close to the tumor. Therefore, it necessary to monitor and verify the range of proton during treatment. In-beam range monitoring can be achieved by detecting secondary gamma emitted from proton interaction with human body. This study is performed using GEANT4 monte carlo toolkit version 9.4.p02. The geometry involves a PMMA slab as the target and two detectors made of LYSO placed on either side of PMMA target. Monoenergetic proton beam with energy 130 MeV is used as radiation sources in this study. Results of simulation are compared with experiment conducted at Chang Gung Memorial Hospital in Taiwan under the same conditions as simulation. The energies of the gammas detected in simulation were in the range of 0 – 10 MeV. In this simulation study, major peaks from 511 KeV gamma, or positron annihilation gamma (PAG), and other peaks correspond to the characteristic energy of prompt gamma (PG) were observed. Isotopes generated in PMMA target are used as a reference for estimating the numbers of PAG or PG created. In experiment measurement, two types of gamma spectra are obtained from two different durations depending on proton irradiation is ON (Beam-ON) or OFF (Beam-OFF). Simulation result shows that longitudinal distribution of both PG sources and positron emitter sources are adequate to verify the range of proton treatment by a difference of 1 mm or 8 mm from Bragg peak respectively. The comparison of Beam-OFF data and simulation data of positron emitter shows that they are in good agreement in longitudinal distribution. Ratio of positron emitters in the measured data is 1.42 : 1 for 15O : 11C positron emitters, which is the same as simulation. At the end we conclude that GEANT4 monte carlo simulation toolkit is a powerful and suitable tool for simulation of nuclear interactions and gamma production from proton interaction in tissues.
