Henry Thomas, Bassler Niels, Ureba Ana, Tsubouchi Toshiro, Valdman Alexander, Siegbahn Albert
a Medical Radiation Physics, Department of Physics , Stockholm University , Stockholm , Sweden.
b Department of Radiation Oncology, Graduate School of Medicine , Osaka University , Osaka , Japan.
Acta Oncol. 2017 Nov;56(11):1437-1443. doi: 10.1080/0284186X.2017.1350287. Epub 2017 Aug 22.
Grid therapy has in the past normally been performed with single field photon-beam grids. In this work, we evaluated a method to deliver grid therapy based on interlacing and crossfiring grids of mm-wide proton beamlets over a target volume, by Monte Carlo simulations.
Dose profiles for single mm-wide proton beamlets (1, 2 and 3 mm FWHM) in water were simulated with the Monte Carlo code TOPAS. Thereafter, grids of proton beamlets were directed toward a cubic target volume, located at the center of a water tank. The aim was to deliver a nearly homogeneous dose to the target, while creating high dose heterogeneity in the normal tissue, i.e., high gradients between valley and peak doses in the grids, down to the close vicinity of the target.
The relative increase of the beam width with depth was largest for the smallest beams (+6.9 mm for 1 mm wide and 150 MeV proton beamlets). Satisfying dose coverage of the cubic target volume (σ < ±5%) was obtained with the interlaced-crossfiring setup, while keeping the grid pattern of the dose distribution down to the target (valley-to-peak dose ratio <0.5 less than 1 cm before the target). Center-to-center distances around 7-8 mm between the beams were found to give the best compromise between target dose homogeneity and low peak doses outside of the target.
A nearly homogeneous dose distribution can be obtained in a target volume by crossfiring grids of mm-wide proton-beamlets, while maintaining the grid pattern of the dose distribution at large depths in the normal tissue, close to the target volume. We expect that the use of this method will increase the tumor control probability and improve the normal tissue sparing in grid therapy.
过去，格栅治疗通常使用单野光子束格栅进行。在本研究中，我们通过蒙特卡罗模拟评估了一种基于在靶区体积上交错和交叉发射毫米宽质子微束格栅来进行格栅治疗的方法。
使用蒙特卡罗代码TOPAS模拟了水中单个毫米宽质子微束（半高宽为1、2和3毫米）的剂量分布。此后，将质子微束格栅对准位于水箱中心的立方靶区体积。目的是在靶区内提供近乎均匀的剂量，同时在正常组织中产生高剂量不均匀性，即在格栅中的谷剂量和峰剂量之间产生高梯度，直至靶区附近。
对于最小的束流，束宽随深度的相对增加最大（1毫米宽、150兆电子伏质子微束增加6.9毫米）。通过交错交叉发射设置获得了立方靶区体积令人满意的剂量覆盖（σ<±5%），同时保持剂量分布的格栅模式直至靶区（靶区前1厘米内谷峰剂量比<0.5）。发现束流之间中心距约7 - 8毫米能在靶区剂量均匀性和靶区外低峰剂量之间取得最佳折衷。
通过交叉发射毫米宽质子微束格栅可在靶区内获得近乎均匀的剂量分布，同时在靠近靶区体积的正常组织深处保持剂量分布的格栅模式。我们预计该方法的使用将提高格栅治疗中的肿瘤控制概率并改善正常组织的保护。
