Centre for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland.
Department of Physics, ETH Zürich, Switzerland.
Br J Radiol. 2020 Mar;93(1107):20190919. doi: 10.1259/bjr.20190919. Epub 2020 Feb 6.
Monte Carlo (MC) simulations substantially improve the accuracy of predicted doses. This study aims to determine and quantify the uncertainties of setting up such a MC system.
Doses simulated with two Geant4-based MC calculation codes, but tuned to the beam data, have been compared. Different methods of MC modelling of a pre-absorber have been employed, either modifying the beam source parameters (descriptive) or adding the pre-absorber as a physical component (physical).
After the independent beam modelling of both systems in water (resulting in excellent range agreement) range differences of up to 3.6/4.8 mm (1.5% of total range) in bone/brain-like tissues were found, which resulted from the use of different mean water ionisation potentials during the energy tuning process. When repeating using a common definition of water, ranges in bone/brain agreed within 0.1 mm and gamma-analysis (global 1%,1mm) showed excellent agreement (>93%) for all patient fields. However, due to a lack of modelling of proton fluence loss in the descriptive pre-absorber, differences of 7% in absolute dose between the pre-absorber definitions were found.
This study quantifies the influence of using different water ionisation potentials during the MC beam modelling process. Furthermore, when using a descriptive pre-absorber model, additional Faraday cup or ionisation chamber measurements with pre-absorber are necessary.
This is the first study quantifying the uncertainties caused by the MC beam modelling process for proton pencil beam scanning, and a more detailed beam modelling process for MC simulations is proposed to minimise the influence of critical parameters.
蒙特卡罗（MC）模拟大大提高了预测剂量的准确性。本研究旨在确定和量化建立这样的 MC 系统的不确定性。
比较了两个基于 Geant4 的 MC 计算代码模拟的剂量，但对束流数据进行了调整。采用了不同的 MC 建模方法来模拟预吸收器，要么修改束源参数（描述性），要么将预吸收器添加为物理组件（物理性）。
在对两个系统在水中进行独立的束流建模后（导致射程非常吻合），在骨/脑样组织中发现了高达 3.6/4.8mm 的射程差异（占总射程的 1.5%），这是由于在能量调整过程中使用了不同的平均水离化势能。当使用共同的水定义重复时，骨/脑的射程一致，伽玛分析（整体 1%，1mm）显示所有患者场的吻合度非常高（>93%）。然而，由于在描述性预吸收器中缺乏质子通量损失的建模，在预吸收器定义之间发现了 7%的绝对剂量差异。
本研究量化了在 MC 束流建模过程中使用不同水离化势能的影响。此外，当使用描述性预吸收器模型时，需要进行带有预吸收器的附加法拉第杯或电离室测量。
这是第一项量化质子铅笔束扫描 MC 束流建模过程引起的不确定性的研究，并提出了更详细的 MC 模拟束流建模过程，以最小化关键参数的影响。
