Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin, 53705, USA.
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA, 94305, USA.
Med Phys. 2021 Apr;48(4):1967-1982. doi: 10.1002/mp.14761. Epub 2021 Mar 4.
The aim of this work was to develop and benchmark a magnetic resonance (MR)-guided linear accelerator head model using the GEANT4 Monte Carlo (MC) code. The validated model was compared to the treatment planning system (TPS) and was also used to quantify the electron return effect (ERE) at a lung-water interface.
The average energy, including the spread in the energy distribution, and the radial intensity distribution of the incident electron beam were iteratively optimized in order to match the simulated beam profiles and percent depth dose (PDD) data to measured data. The GEANT4 MC model was then compared to the TPS model using several photon beam tests including oblique beams, an off-axis aperture, and heterogeneous phantoms. The benchmarked MC model was utilized to compute output factors (OFs) with the 0.35 T magnetic field turned on and off. The ERE was quantified at a lung-water interface by simulating PDD curves with and without the magnetic field for 6.6 × 6.6 and 2.5 × 2.5 field sizes. A 2%/2 mm gamma criterion was used to compare the MC model with the TPS data throughout this study.
The final incident electron beam parameters were 6.0 MeV average energy with a 1.5 MeV full width at half maximum (FWHM) Gaussian energy spread and a 1.0 mm FWHM Gaussian radial intensity distribution. The MC-simulated OFs were found to be in agreement with the TPS-calculated and measured OFs, and no statistical difference was observed between the 0.35 T and 0.0 T OFs. Good agreement was observed between the TPS-calculated and MC-simulated data for the photon beam tests with gamma pass rates ranging from 96% to 100%. An increase of 4.3% in the ERE was observed for the 6.6 × 6.6 field size relative to the 2.5 × 2.5 field size. The ratio of the 0.35 T PDD to the 0.0 T PDD was found to be up to 1.098 near lung-water interfaces for the 6.6 × 6.6 field size using the MC model.
A vendor-independent Monte Carlo model has been developed and benchmarked for a 0.35 T/6 MV MR-linac. Good agreement was obtained between the GEANT4 and TPS models except near heterogeneity interfaces.
本研究旨在利用 GEANT4 蒙特卡罗（MC）代码开发并验证一款适用于磁共振（MR）引导直线加速器头模型。将验证后的模型与治疗计划系统（TPS）进行比较，并用于量化在肺水界面处的电子返回效应（ERE）。
为了使模拟束流分布和百分深度剂量（PDD）数据与实测数据相匹配，我们迭代优化了入射电子束的平均能量，包括能量分布的离散度，以及径向强度分布。然后，我们使用几种光子束测试，包括斜射束、离轴孔径和不均匀体模，对 GEANT4 MC 模型与 TPS 模型进行了比较。使用带有和不带有 0.35 T 磁场的 MC 模型计算了输出因子（OFs）。通过模拟有和没有磁场的 PDD 曲线，在肺水界面处量化了 ERE，模拟的射野大小分别为 6.6×6.6 和 2.5×2.5。在整个研究过程中，我们使用了 2%/2mm 的伽马准则来比较 MC 模型与 TPS 数据。
最终入射电子束的参数为 6.0 MeV 的平均能量，半高全宽（FWHM）为 1.5 MeV 的高斯能量展宽和 1.0mm 的 FWHM 高斯径向强度分布。MC 模拟的 OFs 与 TPS 计算和实测的 OFs 一致，并且在 0.35 T 和 0.0 T OFs 之间没有观察到统计学差异。光子束测试的 TPS 计算数据与 MC 模拟数据吻合良好，伽马通过率在 96%至 100%之间。对于 6.6×6.6 的射野大小，ERE 增加了 4.3%，而对于 2.5×2.5 的射野大小，ERE 增加了 4.3%。在 MC 模型中，在肺水界面附近，0.35 T 的 PDD 与 0.0 T 的 PDD 的比值高达 1.098。
我们开发并验证了一款适用于 0.35 T/6 MV MR-直线加速器的与厂商无关的蒙特卡罗模型。除了在不均匀界面附近，GEANT4 和 TPS 模型之间的一致性较好。
