Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, SM2 5PT, UK.
Med Phys. 2020 Apr;47(4):1533-1544. doi: 10.1002/mp.14090. Epub 2020 Mar 3.
Several studies have demonstrated potential improvements in treatment time through the use of dynamic arcs for delivery of stereotactic body radiation therapy (SBRT) on Cyberknife. However, the delivery system has a finite accuracy, so that potential exists for dosimetric uncertainties. This study estimates the expected dosimetric accuracy of dynamic delivery of SBRT, based on realistic estimates of the uncertainties in delivery parameters.
Five SBRT patient cases (prostate A - conventional, prostate B - brachytherapy-type, lung, liver, partial left breast) were retrospectively studied. Treatment plans were produced for a fixed arc trajectory using fluence optimization, segmentation, and direct aperture optimization. Dose rate uncertainty was modeled as a smoothly varying random fluctuation of ± 1.0%, ±2.0% or ± 5.0% over a time period of 10, 30 or 60 s. Multileaf collimator uncertainty was modeled as a lag in position of each leaf up to 0.25 or 0.5 mm. Robot pointing error was modeled as a shift of the target location, with the direction of the shift chosen as a random angle with respect to the multileaf collimator and with a random magnitude in the range 0.0-1.0 mm at the delivery nodes and with an additional random magnitude of 0.5-1.0 mm in between the delivery nodes. The impact of the errors was investigated using dose-volume histograms.
Uncertainty in dose rate has the effect of varying the total monitor units delivered, which in turn produces a variation in mean dose to the planning target volume. The random sampling of dose rate error produces a distribution of mean doses with a standard deviation proportional to the magnitude of the dose rate uncertainty. A lag in multileaf collimator position of 0.25 or 0.5 mm produces a small impact on the delivered dose. In general, an increase in the PTV mean dose of around 1% is observed. An error in robot pointing of the order of 1 mm produces a small increase in dose inhomogeneity to the planning target volume, sometimes accompanied by an increase in mean dose by around 1%.
Based upon the limited data available on the dose rate stability and geometric accuracy of the Cyberknife system, this study estimates that dynamic arc delivery can be accomplished with sufficient accuracy for clinical application. Dose rate variation produces a change in dose to the planning target volume according to the perturbation of total monitor units delivered, while multileaf collimator lag and robot pointing error typically increase the mean dose to the planning target volume by up to 1%.
多项研究表明，在 Cyberknife 上使用动态弧形进行立体定向体部放射治疗（SBRT）可潜在地缩短治疗时间。然而，该输送系统的精度有限，因此存在剂量不确定性的潜在风险。本研究基于对输送参数不确定性的实际估计，估算了动态输送 SBRT 的预期剂量学准确性。
回顾性研究了 5 例 SBRT 患者（前列腺 A-常规，前列腺 B-近距离治疗型，肺，肝，左半乳）。使用通量优化、分割和直接孔径优化为固定弧形轨迹生成治疗计划。剂量率不确定性建模为在 10、30 或 60 s 的时间段内以±1.0%、±2.0%或±5.0%的平滑变化随机波动。多叶准直器不确定性建模为每个叶片位置的滞后，最大滞后为 0.25 或 0.5 mm。机器人指向误差建模为目标位置的移位，移位方向为多叶准直器的随机角度，在输送节点处的随机幅度为 0.0-1.0 mm，在输送节点之间的随机幅度为 0.5-1.0 mm。使用剂量-体积直方图研究了误差的影响。
剂量率的不确定性会影响输送的总监测单位数，从而导致计划靶区的平均剂量发生变化。剂量率误差的随机抽样产生了一个平均剂量分布，其标准差与剂量率不确定性的大小成正比。多叶准直器位置滞后 0.25 或 0.5 mm 会对输送剂量产生很小的影响。通常，观察到计划靶区的平均剂量增加约 1%。机器人指向误差在 1 mm 左右会导致计划靶区的剂量不均匀性略有增加，有时还会导致平均剂量增加约 1%。
基于 Cyberknife 系统的剂量率稳定性和几何精度的有限数据，本研究估计，动态弧形输送可以达到足够的精度，适用于临床应用。剂量率变化会根据输送的总监测单位数的扰动改变计划靶区的剂量，而多叶准直器滞后和机器人指向误差通常会使计划靶区的平均剂量增加高达 1%。
