Division Medical Radiation Physics, Department of Radiotherapy, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna/AKH Wien, Währinger Gürtel 18-20, 1090, Vienna, Austria.
MedAustron Ion Therapy Centre/EBG MedAustron, Marie-Curie-Straße 5, 2700, Wiener Neustadt, Austria.
Med Phys. 2019 May;46(5):2444-2456. doi: 10.1002/mp.13472. Epub 2019 Apr 15.
The dose core of a proton pencil beam (PB) is enveloped by a low dose area reaching several centimeters off the central axis and containing a considerable amount of the dose. Adequate modeling of the different components of the PB profile is, therefore, required for accurate dose calculation. In this study, we experimentally validated one electromagnetic and two nuclear scattering models in GATE/Geant4 for dose calculation of proton beams in the therapeutic energy window (62-252 MeV) with and without range shifter (RaShi).
The multiple Coulomb scattering (MCS) model was validated by lateral dose core profiles measured for five energies at up to four depths from beam plateau to Bragg peak region. Nuclear halo profiles of single PBs were evaluated for three (62.4, 148.2, and 252.7 MeV) and two (97.4 and 124.7 MeV) energies, without and with RaShi, respectively. The influence of the dose core and nuclear halo on field sizes varying from 2-20 cm was evaluated by means of output factors (OFs), namely frame factors (FFs) and field size factors (FSFs), to quantify the relative increase of dose when increasing the field size.
The relative increase in the dose core width in the simulations deviated negligibly from measurements for depths until 80% of the beam range, but was overestimated by up to 0.2 mm in σ toward the end of range for all energies. The dose halo region of the lateral dose profile agreed well with measurements in the open beam configuration, but was notably overestimated in the deepest measurement plane of the highest energy or when the beam passed through the RaShi. The root-mean-square deviations (RMSDs) between the simulated and the measured FSFs were less than 1% at all depths, but were higher in the second half of the beam range as compared to the first half or when traversing the RaShi. The deviations in one of the two tested hadron physics lists originated mostly in elastic scattering. The RMSDs could be reduced by approximately a factor of two by exchanging the default elastic scattering cross sections for protons.
GATE/Geant4 agreed satisfyingly with most measured quantities. MCS was systematically overestimated toward the end of the beam range. Contributions from nuclear scattering were overestimated when the beam traversed the RaShi or at the depths close to the end of the beam range without RaShi. Both, field size effects and calculation uncertainties, increased when the beam traversed the RaShi. Measured field size effects were almost negligible for beams up to medium energy and were highest for the highest energy beam without RaShi, but vice versa when traversing the RaShi.
质子铅笔束（PB）的剂量核心被一个低剂量区域包围，该区域在中心轴几厘米之外，并包含相当数量的剂量。因此，为了准确计算剂量，需要对 PB 轮廓的不同分量进行充分的建模。在这项研究中，我们使用 GATE/Geant4 实验验证了一个电磁和两个核散射模型，用于计算治疗能窗（62-252 MeV）中有无射程位移器（RaShi）的质子束的剂量。
通过在从射束平台到布拉格峰区域的五个深度处测量五个能量的横向剂量核心轮廓，验证了多次库仑散射（MCS）模型。对于三个（62.4、148.2 和 252.7 MeV）和两个（97.4 和 124.7 MeV）能量，评估了无 RaShi 和有 RaShi 时的单个 PB 的核晕轮廓。通过输出因子（OFs），即框架因子（FFs）和场尺寸因子（FSFs），评估了剂量核心和核晕对从 2-20 cm 变化的射野大小的影响，以量化增加射野大小时剂量的相对增加。
在所有能量的情况下，模拟中的剂量核心宽度的相对增加在 80%的射程深度内与测量值相差可以忽略不计，但在射程末端，在 σ 方向的测量值高估了 0.2mm。在开放束配置中，横向剂量分布的剂量晕区域与测量值吻合良好，但在最高能量的最深测量平面或在通过 RaShi 时，剂量晕区域被显著高估。在所有深度处，模拟和测量的 FSF 之间的均方根偏差（RMSD）均小于 1%，但在束的后半部分比前半部分或在通过 RaShi 时更高。在两个测试的强子物理列表之一中，偏差主要源于弹性散射。通过为质子交换默认的弹性散射横截面，RMSD 可以减少约两倍。
GATE/Geant4 与大多数测量量吻合良好。MCS 在射束末端的系统高估。当射束穿过 RaShi 或在没有 RaShi 的情况下接近射束末端的深度时，核散射的贡献被高估。当射束穿过 RaShi 时，场尺寸效应和计算不确定性都会增加。当射束穿过 RaShi 时，测量的场尺寸效应几乎可以忽略不计，而对于中等能量的射束，最高的是没有 RaShi 的最高能量射束，但反之亦然。
