Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI, 53705, USA.
Division of Medical Physics, Northwestern Medicine Chicago Proton Center, 4455 Weaver Parkway, Warrenville, IL, 60555, USA.
Med Phys. 2020 Oct;47(10):5343-5356. doi: 10.1002/mp.14453. Epub 2020 Sep 9.
There has been a growing interest in the development of energy-specific collimators for low-energy pencil beam scanning (PBS) to reduce the lateral penumbra. One particular device that has been the focus of several recent published works is the dynamic collimation system (DCS), which provides energy-specific collimation by intercepting the scanned proton beam as it nears to target edge with a set of orthogonal trimmer blades. While several computational studies have shown that this dynamic collimator can provide additional healthy tissue sparing, there has not been any rigorous experimental work to benchmark the theoretical models used in these initial studies. Therefore, it was the purpose of this work to demonstrate an experimental method that could integrate an experimental prototype with a clinical PBS system and benchmark the Monte Carlo methods that have been used to model the DCS.
An experimental DCS prototype was designed and built in house to actively collimate individual proton beamlets during PBS within a well-characterized experimental setup. Monte Carlo methods were initially used to assess construction tolerances and later benchmarked against measurements, including integral depth dose and lateral asymmetric beamlet profiles. The experimental apparatus and measurement geometry were modeled using MCNP6 benchmarked from measurements performed at the Northwestern Chicago Proton Center.
Gamma analysis tests were used to evaluate the agreement between the measured and simulated profiles with a strict 1 mm/1% criteria and 5% dose threshold. Excellent agreement was observed between the simulated and measured profiles, which included 1 mm/1% gamma analysis pass rates of at least 100% and 95% for the integral depth dose (IDD) profiles and lateral profiles, respectively. Differences in the relative profile shape were observed experimentally between beamlets collimated on- and off-axis, which was attributed to the partial transmission of the beam through an unfocused collimator. Exposure rates resulting from the activation of the device were monitored with survey meter measurements and were found to agree with Monte Carlo estimates of the exposure rate to within 20%.
A DCS prototype was constructed and integrated into a clinical dose delivery system. While the results of this work are not exhaustive, they demonstrate the effects of beam source divergence, device activation, and beamlet deflection during scanning, which were found to be successfully modeled using Monte Carlo methods and experimentally benchmarked. Excellent agreement was achieved between the simulated and measured lateral spot profiles of collimated beamlets delivered on- and off-axis in PBS. The Monte Carlo models adequately predicted the measured elevated plateau region in the integral depth-dose profiles from the low-energy scatter off the collimators.
人们对开发用于低能铅笔束扫描（PBS）的特定能量准直器越来越感兴趣，以减小横向半影。最近的一些出版物特别关注一种特定的设备，即动态准直系统（DCS），它通过用一组正交的微调叶片在接近目标边缘的扫描质子束上进行拦截，提供特定能量的准直。虽然有几项计算研究表明，这种动态准直器可以提供额外的健康组织保护，但还没有任何严格的实验工作来基准这些初步研究中使用的理论模型。因此，本工作的目的是展示一种实验方法，该方法可以将实验原型与临床 PBS 系统集成，并基准用于模拟 DCS 的蒙特卡罗方法。
设计并构建了一个内部的实验 DCS 原型，以在一个经过充分表征的实验设置中，在 PBS 期间主动准直单个质子束。最初使用蒙特卡罗方法评估结构公差，然后根据测量结果进行基准测试，包括积分深度剂量和横向不对称束斑轮廓。实验设备和测量几何形状使用 MCNP6 进行建模，该模型通过在西北芝加哥质子中心进行的测量进行了基准测试。
使用严格的 1mm/1%标准和 5%剂量阈值的伽马分析测试评估了测量和模拟轮廓之间的一致性。模拟和测量轮廓之间的一致性非常好，其中包括至少 100%和 95%的积分深度剂量（IDD）轮廓和横向轮廓的 1mm/1%伽马分析通过率。在轴上和轴外准直的束斑之间，实验中观察到相对轮廓形状的差异，这归因于光束通过未聚焦准直器的部分透射。使用表面剂量计测量监测了设备激活产生的照射率，并发现与蒙特卡罗估计的照射率相差在 20%以内。
构建了一个 DCS 原型并将其集成到临床剂量输送系统中。虽然这项工作的结果并不是详尽无遗的，但它们展示了束源发散、设备激活和扫描期间束斑偏转的影响，这些影响使用蒙特卡罗方法成功建模并进行了实验基准测试。在 PBS 中轴上和轴外准直的束斑的模拟和测量横向光斑轮廓之间实现了极好的一致性。蒙特卡罗模型充分预测了来自准直器的低能散射的积分深度剂量轮廓中的测量升高平台区域。
