Department of Radiation Oncology, Princess Margaret Cancer Centre, Toronto, Ontario, Canada.
Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada.
Med Phys. 2024 May;51(5):3635-3647. doi: 10.1002/mp.17034. Epub 2024 Mar 22.
Dynamic treatment in Gamma Knife (GK) radiosurgery systems delivers radiation continuously with couch movement, as opposed to stationary step-and-shoot treatment where radiation is paused when moving between isocenters. Previous studies have shown the potential for dynamic GK treatment to give faster treatment times and improved dose conformity and homogeneity. However, these studies focused only on computational simulations and lack physical validation.
This study aims conduct dynamic treatment dosimetric validation with physical experimental measurements. The experiments aim to (1) address assumptions made with computational studies, such as the validity of treating a continuous path as discretised points, (2) investigate uncertainties in translating computed plans to actual treatment, and (3) determine ideal treatment planning parameters, such as interval distance for the path discretization, collimator change limitations, and minimum isocenter treatment times.
This study uses a GK ICON treatment delivery machine, and a motion phantom custom-made to attach to the machine's mask adapter and move in 1D superior-inferior motion. Phantom positioning is first verified through comparisons against couch motion and computed doses. For dynamic treatment experiments, the phantom is moved through a program that first reads the desired treatment plan isocenters' position, time, and collimator sizes, then carries out the motion continuously while the treatment machine delivers radiation. Measurements are done with increasing levels of complexity: varying speed, varying collimator sizes, varying both speed and collimator sizes, then extends the same measurements to simulated 2D motion by combining phantom and couch motion. Dose comparisons between phantom motion radiation measurements and either couch motion measurements or dose calculations are analyzed with 2 mm/2% and 1 mm/2% gamma indices, using both local and global gamma index calculations.
Phantom positional experiments show a high accuracy, with global gamma indices for all dose comparisons 99%. Discretization level to approximate continuous path as discrete points show the good dose matches with dose calculations when using 1 and 2-mm gaps. Complex 1D motion, including varying speed, collimator sizes, or both, as well as 2D motion with the same complexities, all show good dose matches with dose calculations: the scores are 92.0% for the strictest 1 mm/2% local gamma index calculation, 99.8% for 2 mm/2% local gamma index, and 97.0% for all global gamma indices. Five simulated 2D treatments with optimized plans scored highly as well, with all gamma index scores 95.3% when compared to stationary treatment, and scores 97.9% when compared to plan calculated dose.
Dynamic treatment computational studies are validated, with dynamic treatment shown to be physically feasible and deliverable with high accuracy. A 2-mm discretization level in treatment planning is proposed as the best option for shorter dose calculation times while maintaining dose accuracy. Our experimental method enables dynamic treatment measurements using the existing clinical workflow, which may be replicated in other centers, and future studies may include 2D or 3D motion experiments, or planning studies to further quantify potential indication-specific benefits.
与固定的步进式放射治疗相比，伽玛刀（GK）放射外科系统中的动态治疗在治疗过程中伴随着治疗床的移动持续输送辐射，而在步进式放射治疗中，当在等中心点之间移动时会暂停辐射。之前的研究表明，动态 GK 治疗具有缩短治疗时间、提高剂量适形性和均匀性的潜力。然而，这些研究仅关注于计算模拟，缺乏物理验证。
本研究旨在通过物理实验测量进行动态治疗剂量学验证。实验旨在：（1）解决计算研究中提出的假设，例如将连续路径离散化为离散点的有效性；（2）研究将计算计划转化为实际治疗的不确定性；（3）确定理想的治疗计划参数，例如路径离散化的间隔距离、准直器变化限制以及最小等中心点治疗时间。
本研究使用 GK ICON 治疗输送机器和一个专门制作的运动体模，该体模可连接到机器的面罩适配器上，并在一维上下方向上移动。通过与治疗床运动和计算剂量的比较，首先验证体模的定位。对于动态治疗实验，体模按照程序移动，该程序首先读取所需治疗计划等中心点的位置、时间和准直器大小，然后在治疗机器输送辐射的同时连续执行运动。测量的复杂性逐渐增加：改变速度、改变准直器大小、同时改变速度和准直器大小，然后通过组合体模和治疗床运动将相同的测量扩展到模拟二维运动。通过使用局部和全局伽马指数计算，用 2mm/2%和 1mm/2%伽马指数分析体模运动辐射测量值与治疗床运动测量值或剂量计算值之间的剂量比较。
体模位置实验显示出高度的准确性，所有剂量比较的全局伽马指数均 99%。离散化水平以近似连续路径作为离散点，当使用 1mm 和 2mm 间隙时，与剂量计算具有很好的剂量匹配。包括速度、准直器大小或两者同时变化的复杂一维运动，以及具有相同复杂性的二维运动，与剂量计算都显示出很好的剂量匹配：最严格的 1mm/2%局部伽马指数计算得分为 92.0%，2mm/2%局部伽马指数得分为 99.8%，所有全局伽马指数得分为 97.0%。五个经过优化的模拟二维治疗也获得了很高的分数，与固定治疗相比，所有伽马指数得分 95.3%，与计算剂量相比，得分 97.9%。
动态治疗的计算研究得到了验证，表明动态治疗在物理上是可行的，可以实现高精度的治疗。在治疗计划中提出了 2mm 的离散化水平作为缩短剂量计算时间同时保持剂量准确性的最佳选择。我们的实验方法可以在现有的临床工作流程中进行动态治疗测量，这可以在其他中心复制，未来的研究可以包括二维或三维运动实验，或规划研究以进一步量化潜在的特定适应证的获益。
