Smith Blake R, Hyer Daniel E
Department of Radiation Oncology, University of Iowa, Iowa City, Iowa, USA.
J Appl Clin Med Phys. 2025 Jan;26(1):e14477. doi: 10.1002/acm2.14477. Epub 2024 Dec 7.
To computationally characterize the LET distribution during dynamic collimation in PBS and quantify its impact on the resultant dose distribution.
Monte Carlo simulations using Geant4 were used to model the production of low-energy proton scatter produced in the collimating components of a novel PBS collimator. Custom spectral tallies were created to quantify the energy, track- and dose-averaged LET resulting from individual beamlet and composite fields simulated from a model of the IBA dedicated nozzle system. The composite dose distributions were optimized to achieve a uniform physical dose coverage of a cubical and pyramidal target, and the resulting dose-average LET distributions were calculated for uncollimated and collimated PBS deliveries and used to generate RBE-weighted dose distributions.
For collimated beamlets, the scattered proton energy fluence is strongly dependent on collimator position relative to the central axis of the beamlet. When delivering a uniform profile, the distribution of dose-average LET was nearly identical within the target and increased between 1 and within 10 mm surrounding the target. Dynamic collimation resulted in larger dose-average LET changes: increasing the dose-average LET between 1 and within 10 mm of a pyramidal target while reducing the dose-average LET outside this margin by as much as . Biological dose distributions are improved with energy-specific collimation in reducing the lateral penumbra.
The presence of energy-specific collimation in PBS can lead to dose-average LET changes relative to an uncollimated delivery. In some clinical situations, the placement and application of energy-specific collimation may require additional planning considerations based on its reduction to the lateral penumbra and increase in high-dose conformity. Future applications may embody these unique dosimetric characteristics to redirect high-LET portions of a collimated proton beamlet from healthy tissues while enhancing the dose-average LET distribution within target.
通过计算表征被动散射质子治疗（PBS）中动态准直过程中的传能线密度（LET）分布，并量化其对最终剂量分布的影响。
使用Geant4进行蒙特卡罗模拟，以对新型PBS准直器准直组件中产生的低能质子散射进行建模。创建自定义谱得分以量化从IBA专用喷嘴系统模型模拟的单个子野和复合野产生的能量、径迹平均LET和剂量平均LET。优化复合剂量分布以实现立方体和金字塔形靶区的均匀物理剂量覆盖，并计算未准直和准直PBS照射的最终剂量平均LET分布，并用于生成相对生物效应（RBE）加权剂量分布。
对于准直子野，散射质子能量注量强烈依赖于准直器相对于子野中心轴的位置。在进行均匀剂量分布照射时，靶区内剂量平均LET分布几乎相同，且在靶区周围10 mm范围内增加了1至 。动态准直导致更大的剂量平均LET变化：在金字塔形靶区10 mm范围内增加剂量平均LET，同时使该边缘外的剂量平均LET降低多达 。通过能量特异性准直改善生物剂量分布可减少侧向半影。
PBS中能量特异性准直的存在可能导致相对于未准直照射的剂量平均LET变化。在某些临床情况下，能量特异性准直的放置和应用可能需要基于其对侧向半影的减少和高剂量适形性的增加进行额外的计划考虑。未来的应用可能体现这些独特的剂量学特征，将准直质子子束的高LET部分从健康组织重新定向，同时增强靶区内的剂量平均LET分布。
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