医用直线加速器入射电子束能量的快速模拟确定

Fast confirmation of the incident electron energy by simulation for medical linac

寻找一种医用直线加速器模拟时入射电子打靶能量的快速确定方法,从而节省调试所需的时间。利用蒙特卡罗软件包EGSnrc/BEAMnrc针对Varian 600C、Trilogy和Edge无均整模式FFF(Flattening Filter Free),标称能量均为6 MV的情况,模拟计算射野分别为3 cm×3 cm、10 cm×10 cm、40 cm×40 cm时,不同能量的入射电子打靶产生的X射线在水体模中的剂量分布;通过分析模拟所得到的结果,寻找确定入射电子打靶能量的方法。当入射电子打靶能量在5.5~6.5 MeV范围内时,不同射野的百分深度剂量(Percentage Depth Dose,PDD)对入射电子打靶能量“不敏感”;3 cm×3 cm和10 cm×10 cm的离轴比(Off Axis Ratio,OAR)对入射电子打靶能量“不敏感”;40 cm×40 cm的OAR对入射电子打靶能量十分“敏感”,具体表现为:在离轴距离为14.5~19 cm这一区间内水下5 cm处的OAR平均值,随着打靶能量的上升而下降,即每提高0.1 MeV,Varian 600C、Trilogy、Edge FFF分别下降0.82%、0.98%、0.47%。通过对在离轴距离为14.5~19 cm这一区间内水下5 cm处的OAR平均值和入射电子打靶能量进行拟合,由拟合结果和OAR测量值反推模拟所需的入射电子打靶能量,利用反推的打靶能量作为输入,模拟产生的X射线在水体模中的PDD、OAR,与测量值相比,差异均在1%之内。

[Background]Monte Carlo method provides a powerful tool for the simulation of particle transport in radiotherapy,hence can be used for determination of the target energy of the incident electron in medical linac simulation.[Purpose]This study aims to find a way that can confirm the incident electron energy accurately and quickly,and save time for Monte Carlo linac simulation.[Methods]First of all,Monte Carlo program package EGSnrc/BEAMnrc was employed to build up the beam models of Varian 600C,Trilogy and Edge FFF(Flattening Filter Free)model with the same nominal energy of simulation of 6 MV.Then,the dose distributions in different fields(3 cm×3 cm,10 cm×10 cm,40 cm×40 cm)using different incident electron energies in water phantom were calculated.Finally simulation results were compared and analyzed to find out a way to ensure the incident electron energy accurately and quickly.[Results]When the incident electron energy varies from 5.5 MeV to 6.5 MeV,percentage depth dose(PDD)is insensitive to the incident electron energy;the off axis ratio(OAR)profiles of 3 cm×3 cm and 10 cm×10 cm are insensitive to the incident electron energies,the OAR profiles of different incident electron energies are almost the same.The OAR profile of 40 cm×40 cm is very sensitive to the incident electron energy.Detailed results at 5 cm underwater show that when the incident electron energy increases by every 0.1 MeV,the average of OAR between the off axis distance from 14.5 cm to 19 cm decreases:0.82%for Varian 600C 6 MV,0.98%for Trilogy 6 MV,0.47%for Edge 6 MV FFF,respectively.Combining the measured OAR profiles of large field(40 cm×40 cm)with the linear relationship between average OAR and the incident electron energy,the incident electron energy can be determined quickly.Compared with the measured values,the PDD and OAR using the fitting energy as input in water phantom,profiles differences are both within 1%.[Conclusions]This proposed method provides a way to ensure the incident electron energy accurately and quickly for medical linac.