Department of Radiotherapy, Medical University of Vienna/AKH, Vienna, Austria.
Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna, Austria.
Med Phys. 2020 Jan;47(1):223-233. doi: 10.1002/mp.13883. Epub 2019 Nov 13.
Magnetic resonance guidance in proton therapy (MRPT) is expected to improve its current performance. The combination of magnetic fields with clinical proton beam lines poses several challenges for dosimetry, treatment planning and dose delivery. Proton beams are deflected by magnetic fields causing considerable changes in beam trajectories and also a retraction of the Bragg peak positions. A proper prediction and compensation of these effects is essential to ensure accurate dose calculations. This work aims to develop and benchmark a Monte Carlo (MC) beam model for dose calculation of MRPT for static magnetic fields up to 1 T.
Proton beam interactions with magnetic fields were simulated using the GATE/Geant4 toolkit. The transport of charged particle in custom 3D magnetic field maps was implemented for the first time in GATE. Validation experiments were done using a horizontal proton pencil beam scanning system with energies between 62.4 and 252.7 MeV and a large gap dipole magnet (B = 0-1 T), positioned at the isocenter and creating magnetic fields transverse to the beam direction. Dose was measured with Gafchromic EBT3 films within a homogeneous PMMA phantom without and with bone and tissue equivalent material slab inserts. Linear energy transfer (LET) quenching of EBT3 films was corrected using a linear model on dose-averaged LET method to ensure a realistic dosimetric comparison between simulations and experiments. Planar dose distributions were measured with the films in two different configurations: parallel and transverse to the beam direction using single energy fields and spread-out Bragg peaks. The MC model was benchmarked against lateral deflections and spot sizes in air of single beams measured with a Lynx PT detector, as well as dose distributions using EBT3 films. Experimental and calculated dose distributions were compared to test the accuracy of the model.
Measured proton beam deflections in air at distances of 465, 665, and 1155 mm behind the isocenter after passing the magnetic field region agreed with MC-predicted values within 4 mm. Differences between calculated and measured beam full width at half maximum (FWHM) were lower than 2 mm. For the homogeneous phantom, measured and simulated in-depth dose profiles showed range and average dose differences below 0.2 mm and 1.2%, respectively. Simulated central beam positions and widths differed <1 mm to the measurements with films. For both heterogenous phantoms, differences within 1 mm between measured and simulated central beam positions and widths were obtained, confirming a good agreement of the MC model.
A GATE/Geant4 beam model for protons interacting with magnetic fields up to 1 T was developed and benchmarked to experimental data. For the first time, the GATE/Geant4 model was successfully validated not only for single energy beams, but for SOBP, in homogeneous and heterogeneous phantoms. EBT3 film dosimetry demonstrated to be a powerful dosimetric tool, once the film response function is LET corrected, for measurements in-line and transverse to the beam direction in magnetic fields. The proposed MC beam model is foreseen to support treatment planning and quality assurance (QA) activities toward MRPT.
质子治疗中的磁共振引导有望提高其现有性能。磁场与临床质子束线的结合对剂量学、治疗计划和剂量输送带来了一些挑战。质子束会受到磁场的偏转，导致束流轨迹发生很大变化，同时布拉格峰位置也会回缩。为了确保准确的剂量计算，对这些效应进行适当的预测和补偿是至关重要的。本工作旨在开发和基准测试用于静态磁场高达 1 T 的磁共振引导质子治疗的蒙特卡罗（MC）束流模型。
使用 GATE/Geant4 工具包模拟质子束与磁场的相互作用。首次在 GATE 中实现了对定制 3D 磁场图中带电粒子传输的模拟。使用能量在 62.4 和 252.7 MeV 之间的水平质子铅笔束扫描系统以及位于等中心点的大型间隙偶极磁铁（B=0-1 T）进行了验证实验，该磁铁产生横向于束流方向的磁场。在没有和有骨和组织等效材料片插入的同质 PMMA 体模中，使用 Gafchromic EBT3 胶片测量剂量。EBT3 胶片的线性能量传递（LET）淬灭使用剂量平均 LET 方法上的线性模型进行校正，以确保模拟与实验之间的真实剂量比较。使用胶片在两种不同配置下测量平面剂量分布：平行于和横向于束流方向，使用单能场和扩展布拉格峰。MC 模型与 Lynx PT 探测器测量的单个光束的空气侧向偏转和光斑尺寸以及 EBT3 胶片的剂量分布进行了基准测试。比较实验和计算的剂量分布以测试模型的准确性。
在穿过磁场区域后，在等中心点后 465、665 和 1155 mm 处测量的质子束在空气中的偏转与 MC 预测值相差 4 mm 以内。测量的束流全宽度半最大值（FWHM）与计算值之间的差异小于 2 mm。对于同质体模，测量和模拟的深度剂量分布显示出在 0.2 mm 和 1.2%以内的射程和平均剂量差异。模拟的中央束位置和宽度与胶片测量值相差<1 mm。对于两种非同质体模，测量和模拟的中央束位置和宽度之间的差异均在 1 mm 以内，证实了 MC 模型的良好一致性。
开发并基准测试了用于质子与高达 1 T 的磁场相互作用的 GATE/Geant4 束流模型。首次成功验证了 GATE/Geant4 模型不仅适用于单能束，也适用于 SOBP，适用于同质和非同质体模。EBT3 胶片剂量学证明是一种强大的剂量学工具，一旦对胶片响应函数进行 LET 校正，即可在磁场中沿束流方向和横向进行测量。所提出的 MC 束流模型有望支持磁共振引导质子治疗的治疗计划和质量保证（QA）活动。
