Lloyd Samantha A M, Gagne Isabelle M, Bazalova-Carter Magdalena, Zavgorodni Sergei
Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada.
Phys Med Biol. 2016 Dec 21;61(24):8779-8793. doi: 10.1088/1361-6560/61/24/8779. Epub 2016 Nov 29.
To accurately simulate therapeutic electron beams using Monte Carlo methods, backscatter from jaws into the monitor chamber must be accounted for via the backscatter factor, S . Measured and simulated values of S for the TrueBeam are investigated. Two approaches for measuring S are presented. Both require service mode operation with the dose and pulse forming networking servos turned off in order to assess changes in dose rate with field size. The first approach samples an instantaneous dose rate, while the second approach times the delivery of a fixed number of monitor units to assess dose rate. Dose rates were measured for 6, 12 and 20 MeV electrons for jaw- or MLC-shaped apertures between [Formula: see text] and [Formula: see text] cm. The measurement techniques resulted in values of S that agreed within 0.21% for square and asymmetric fields collimated by the jaws. Measured values of S were used to calculate the forward dose component in a virtual monitor chamber using BEAMnrc. Based on this forward component, simulated values of S were calculated and compared to measurement and Varian's VirtuaLinac simulations. BEAMnrc results for jaw-shaped fields agreed with measurements and with VirtuaLinac simulations within 0.2%. For MLC-shaped fields, the respective measurement techniques differed by as much as 0.41% and BEAMnrc results differed with measurement by as much as 0.4%, however, all measured and simulated values agreed within experimental uncertainty. Measurement sensitivity was not sufficient to capture the small backscatter effect due to the MLC, and Monte Carlo predicted backscatter from the MLC to be no more than 0.3%. Backscatter from the jaws changed the electron dose rate by up to 2.6%. This reinforces the importance of including a backscatter factor in simulations of electron fields shaped with secondary collimating jaws, but presents the option of ignoring it when jaws are retracted and collimation is done with the MLC.
为了使用蒙特卡罗方法准确模拟治疗性电子束，必须通过反向散射因子(S)来考虑从准直器进入监测室的反向散射。研究了TrueBeam的(S)的测量值和模拟值。提出了两种测量(S)的方法。两种方法都需要在关闭剂量和脉冲形成网络伺服的服务模式下运行，以便评估剂量率随射野大小的变化。第一种方法对瞬时剂量率进行采样，而第二种方法对固定数量的监测单位的输送时间进行计时以评估剂量率。对能量为6、12和20 MeV的电子，在准直器或多叶准直器（MLC）形成的孔径大小在[公式：见正文]和[公式：见正文] cm之间时测量剂量率。测量技术得出的(S)值，对于由准直器准直的方形和非对称射野，其一致性在0.21%以内。测量得到的(S)值用于使用BEAMnrc在虚拟监测室中计算正向剂量分量。基于该正向分量，计算(S)的模拟值，并与测量值和瓦里安的VirtuaLinac模拟值进行比较。准直器形状射野的BEAMnrc结果与测量值以及VirtuaLinac模拟值的一致性在0.2%以内。对于MLC形状的射野，各自的测量技术差异高达0.41%，BEAMnrc结果与测量值的差异高达0.4%，然而，所有测量值和模拟值在实验不确定度范围内是一致的。测量灵敏度不足以捕捉由于MLC引起的小反向散射效应，蒙特卡罗预测来自MLC的反向散射不超过0.3%。准直器的反向散射使电子剂量率变化高达2.6%。这强化了在模拟由二级准直器准直的电子射野时纳入反向散射因子的重要性，但也提出了当准直器缩回且使用MLC进行准直时可忽略它的选择。
