OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, 01307, Germany.
Institute of Radiooncology - OncoRay, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, 01328, Germany.
Med Phys. 2018 Jul;45(7):3429-3434. doi: 10.1002/mp.12961. Epub 2018 Jun 3.
Given its sensitivity to anatomical variations, proton therapy is expected to benefit greatly from integration with magnetic resonance imaging for online anatomy monitoring during irradiation. Such an integration raises several challenges, as both systems mutually interact. The proton beam will experience quasi-continuous energy loss and energy-dependent electromagnetic deflection at the same time, giving rise to a deflected beam trajectory and an altered dose distribution with a displaced Bragg peak. So far, these effects have only been predicted using Monte Carlo and analytical models, but no clear consensus has been reached and experimental benchmark data are lacking. We measured proton beam trajectories and Bragg peak displacement in a homogeneous phantom placed inside a magnetic field and compared them to simulations.
Planar dose distributions of proton pencil beams (80-180 MeV) traversing the field of a 0.95 T NdFeB permanent magnet while depositing energy in a PMMA slab phantom were measured using EBT3 radiochromic films and simulated using the Geant4 toolkit. Deflected beam trajectories and the Bragg peak displacement were extracted from the measured planar dose distributions and compared against the simulations.
The lateral beam deflection was clearly visible on the EBT3 films and ranged from 1 to 10 mm for 80 to 180 MeV, respectively. Simulated and measured beam trajectories and Bragg peak displacement agreed within 0.8 mm for all studied proton energies.
These results prove that the magnetic field-induced Bragg peak displacement is both measurable and accurately predictable in a homogeneous phantom at 0.95 T, and allows Monte Carlo simulations to be used as gold standard for proton beam trajectory prediction in similar frameworks for MR-integrated proton therapy.
鉴于质子治疗对解剖结构变化的敏感性，预计将其与磁共振成像（MRI）集成，以便在照射过程中进行在线解剖监测，这将大大受益。这种集成带来了一些挑战，因为这两个系统相互作用。质子束会同时经历准连续的能量损失和与能量相关的电磁偏转，从而导致射束轨迹偏转和布拉格峰位移的剂量分布改变。到目前为止，这些影响仅通过蒙特卡罗和分析模型进行预测，但尚未达成明确共识，并且缺乏实验基准数据。我们在置于磁场中的均质体模内测量了质子束轨迹和布拉格峰位移，并将其与模拟结果进行了比较。
使用 EBT3 光致变色胶片测量了穿过 0.95T NdFeB 永磁体场的 80-180MeV 质子铅笔束在 PMMA 平板体模中沉积能量时的平面剂量分布，并使用 Geant4 工具包进行了模拟。从测量的平面剂量分布中提取了偏转射束轨迹和布拉格峰位移，并与模拟结果进行了比较。
EBT3 胶片上可以清楚地看到横向束偏转，对于 80 到 180MeV 的质子能量，分别为 1 到 10mm。对于所有研究的质子能量，模拟和测量的射束轨迹和布拉格峰位移的吻合度在 0.8mm 以内。
这些结果证明，在 0.95T 的均质体模中，磁场诱导的布拉格峰位移既可以测量，也可以准确预测，并且可以将蒙特卡罗模拟用作类似的磁共振集成质子治疗框架中质子束轨迹预测的金标准。
