使用蒙地卡羅方法模擬電腦刀治療機最佳化之電子入射參數與計算效率

OPTIMIZATION OF THE INITIAL PARAMETERS AND CALCULATION EFFICIENCY IN MONTE CARLO SIMULATION FOR CYBERKNIFE

林妙蓉(Miao-Jung Lin) ； 李宗其(Chung-Chi Lee) ； 陳彥超(Yen-Chao Chen) ； 張東杰(Tung-chieh Chang) ； 趙興隆(Hsing-Lung Chao) ； 范兆岳(Chao-Yueh Fan) ； 詹頂立(Din-Li Tsan) ； 趙自強(Tsi-chian Chao)

《放射治療與腫瘤學》 24卷1期 (2017/03) Pp. 49-56

https://doi.org/10.6316/TRO/201724(1)49

摘要

目的：本研究為應用蒙地卡羅來模擬三軍總醫院立體定位放射手術治療中心所使用的電腦刀系統 （G3 Cyberknife system, Accuray Inc., Sunnyvale, CA）機頭的結構，利用百分深度劑量曲線與 側向劑量剖面曲線之測量值與蒙地卡羅模擬得到之結果進行比較，出最佳化之入射電子參數。 材料與方法：研究中應用蒙地卡羅模擬程式BEAMnrc09 來建立之機頭模組設定與射源參數，再將 此機頭模組射源輸入DOSXYZnrc09 程式計算水假體百分深度劑量（percentage depth dose）、側向劑量曲線（lateral profile）及劑量輸出因子並與實際量測值做比對。 結果：研究中以最佳化之電子初始參數所計算之結果與實際量測結果做比較，百分深度劑量的 差異約 1%，80% 照野寬度內之模擬剖面劑量分佈也與測量結果只有 1.5% 的劑量差異。兩側在 20% 及 80% 劑量處模擬與測量的距離差異在 0.5 mm 以內。在劑量輸出因子的部分，最小的三 個準直儀差異為 2.8%-5%，主要原因為所使用的 diode detector 材質仍然為非組織等效，所以會 有高估的狀況。 結論：以最佳化的模擬參數所得蒙地卡羅模擬結果與實驗結果是吻合的。而在模擬的過程中需要 花費大量的嘗試錯誤的方式來得到最佳化參數，但在此研究中利用了一些方法減少的模擬的時 間，可提供日後模擬電腦刀儀器時使用。

關鍵字

電腦刀 ； 蒙地卡羅模擬方法

並列摘要

Purpose : The study aims to optimize the initial parameters and calculating efficiency in Monte Carlo simulation for Cyberknife G3 system. Materials and Methods : BEAM09 Monte Carlo codes were applied in this study. The BEAMnrc code was used to simulate the treatment head and generate the phase space files. The DOSXYZnrc code was used to calculate the depth dose curves (percentage depth dose, PDD), lateral profiles and the output factors. Mean energy and the FWHMR were used to determine initial electron parameters. For calculating dose in the region of interest, the use of smaller voxel size may increase the calculation time; conversely, the adaption of larger voxel size may cause a higher partial volume effect. This study aims to investigate the optimal voxel size for dose calculation in a water phantom to achieve reasonable simulation efficiency and an acceptable accuracy. The Kα method was used for the optimization by comparing the differences between diode measurement data and MC simulations in PDDs and profiles. Disagreement between simulation and measurement were evaluated through the dose differences of PDDs from depths of 1.5 to 20 cm, and the lateral profiles of 80% field width. Distance to agreement (DTA) at lateral positions of 20% to 80% dose profiles of penumbra region were also used for the comparisons. Result : For the efficiency of dose calculation, setting the voxel size equal to one tenth of field width would produce optimal simulation efficiency and acceptable accuracy. According to these parameters, the dose differences of the PDDs were about 1% from depths of 1.5 to 20 cm, the dose differences for lateral profiles within 80% field width were also within 1.5%, and the disagreements of DTA were less than 0.5 mm. The discrepancies of output factor were from 2.8 to 5% for the three smallest cones, which were possibly caused by the effect of electron scattering at the metallic parts of the detector shielding. Conclusion : For Monte Carlo simulations of LINAC and dose calculations, it is important to accurately determine the initial electron beam. These parameters, mean energy and FWHM of incidence electron, have been determined by matching the calculated dose with the measured dose through a trial and error process. This study also applied some methods to increase simulation efficiency which could be the reference of future research.

並列關鍵字

Cyberknife ； Monte Carlo simulation