基于第三方软件建立射波刀蒙特卡罗模型的研究

Research of establishing Monte Carlo model of Cyber Knife based on third party software

目的:利用蒙特卡罗方法模拟射波刀的射线输运过程,并与实际测量结果相比较,验证所建模型的准确性。方法:应用BEAMnrc与DOSXYZnrc程序,建立射波刀物理结构模型,包括12个不同尺寸的准直器系统,通过调整电子束流的平均能量、强度分布半高宽参数,得到水模体中剂量场分布的百分深度剂量、离轴比、输出因子、射线质以及修正因子等数据,并与实际测量数据对比,从而确定所模拟模型中电子束流参数。结果:射线束流百分深度剂量数据对比中超过90%的点误差<2%;离轴比数据中超过90%的点误差<2%;射野输出因子在准直器尺寸≥20 mm时基本重合,但是在准直器尺寸≤15 mm时存在一定误差;射线质的值为0.639,与测量结果0.635相近,最终确定了电子束流平均能量为6.9 Me V,强度分布半高宽为0.4 cm。结论:所建立的蒙特卡罗模型能够准确地模拟射波刀射线输运过程,为后续研究奠定了基础。

Objective To verify the accuracy of the established model through using Monte Carlo methods to simulate radial transmission process of CyberKnife and comparing it with the results of actual detective results. Methods： The BEAMnrc and DOSXYZnrc programs were used to establish the physical structure model of CyberKnife （included collimator systems of 12 different sizes）. Through regulated the electron beam parameters （average energy and full width at half maximum of strength distribution） to obtain a series of data about distribution of dosage field that included of percent depth dose, off axial ratio, output factor, beam quality and correction factor in the water phantom. And then, all of them were compared with the actual measurement data so as to determine the electron beam parameter of the simulated model. Results： In the comparison of percent depth dose for radial beam, the point errors of more than 90% were less than 2%. In the data about off axial ratio, the point errors of more than 90% were less than 2%. The output factors of field was basically coincident when the collimator size was greater than or equal to 20 mm, while there was smaller error when it was less than or equal to 15 mm. Besides, the value of radial quality was 0.639 which closed to the measurement result （0.635）. Finally, the 6.9 MeV was confirmed as the average energy of electron beam and 0.4cm was confirmed as full width at half maximum of strength distribution. Conclusion： The established Monte Carlo model can accurately simulate radial transmission process of CyberKnife, and it lays a foundation for further research.