Department of Radiation Oncology, University of Iowa Hospitals and Clinics, 200 Hawkins Drive, Iowa City, IA, 52242, USA.
Department of Medical Physics, University of Wisconsin, 1111 Highland Avenue, Madison, WI, 53705, USA.
Med Phys. 2020 Jul;47(7):2725-2734. doi: 10.1002/mp.14139. Epub 2020 Apr 6.
When designing a collimation system for pencil beam spot scanning proton therapy, a decision must be made whether or not to rotate, or focus, the collimator to match beamlet deflection as a function of off-axis distance. If the collimator is not focused, the beamlet shape and fluence will vary as a function of off-axis distance due to partial transmission through the collimator. In this work, we quantify the magnitude of these effects and propose a focused dynamic collimation system (DCS) for use in proton therapy spot scanning.
This study was done in silico using a model of the Miami Cancer Institute's (MCI) IBA Proteus Plus system created in Geant4-based TOPAS. The DCS utilizes rectangular nickel trimmers mounted on rotating sliders that move in synchrony with the pencil beam to provide focused collimation at the edge of the target. Using a simplified setup of the DCS, simulations were performed at various off-axis locations corresponding to beam deflection angles ranging from 0° to 2.5°. At each off-axis location, focused (trimmer rotated) and unfocused (trimmer not rotated) simulations were performed. In all simulations, a 4 cm water equivalent thickness range shifter was placed upstream of the collimator, and a voxelized water phantom that scored dose was placed downstream, each with 4 cm airgaps.
Increasing the beam deflection angle for an unfocused trimmer caused the collimated edge of the beamlet profile to shift 0.08-0.61 mm from the baseline 0° simulation. There was also an increase in low-dose regions on the collimated edge ranging from 14.6% to 192.4%. Lastly, the maximum dose, , was 0-5% higher for the unfocused simulations. With a focused trimmer design, the profile shift and dose increases were all eliminated.
We have shown that focusing a collimator in spot scanning proton therapy reduces dose at the collimated edge compared to conventional, unfocused collimation devices and presented a simple, mechanical design for achieving focusing for a range of source-to-collimator distances.
在设计铅笔束扫描质子治疗的准直系统时，必须决定是否旋转或聚焦准直器，以匹配射束偏转随离轴距离的函数。如果准直器未聚焦，则由于部分穿透准直器，射束束斑形状和剂量将随离轴距离而变化。在这项工作中，我们量化了这些影响的大小，并提出了一种用于质子治疗点扫描的聚焦动态准直系统（DCS）。
本研究在使用基于 Geant4 的 TOPAS 构建的迈阿密癌症研究所（MCI）IBA Proteus Plus 系统模型的计算机上进行。DCS 利用安装在旋转滑台上的矩形镍修剪器，与铅笔束同步移动，在靶区边缘提供聚焦准直。使用 DCS 的简化设置，在对应于从 0°到 2.5°的射束偏转角度的各种离轴位置进行了模拟。在每个离轴位置，都进行了聚焦（修剪器旋转）和非聚焦（修剪器未旋转）模拟。在所有模拟中，在准直器上游放置了 4cm 水等效厚度的移频器，在下游放置了一个对剂量进行评分的体素化水体模，两者都有 4cm 的气隙。
对于非聚焦修剪器，增加射束偏转角度会导致射束束斑轮廓的准直边缘从基线 0°模拟的位置偏移 0.08-0.61mm。在准直边缘的低剂量区域也有增加，范围从 14.6%到 192.4%。最后，非聚焦模拟的最大剂量 增加了 0-5%。使用聚焦修剪器设计，可以消除轮廓偏移和剂量增加。
我们已经表明，在点扫描质子治疗中聚焦准直器可以降低与传统非聚焦准直器相比的准直边缘剂量，并提出了一种简单的机械设计，用于在源到准直器距离范围内实现聚焦。
