Department of Radiation Oncology, Dongtan Sacred Heart Hospital, Hwaseong, Korea.
Department of Radiation Oncology, Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul, Korea.
Radiat Oncol. 2021 Aug 31;16(1):169. doi: 10.1186/s13014-021-01893-4.
To evaluate the combined effect of rotational error and dose gradient on target dose coverage in frameless stereotactic radiotherapy.
Three spherical targets of different diameters (1, 1.5, and 2 cm) were drawn and placed equidistantly on the same axial brain computed tomography (CT) images. To test the different isocenter-target distances, 2.5- and 5-cm configurations were prepared. Volumetric modulated arc therapy plans were created for different dose gradients from the target, in which the dose gradients were modified using the maximum dose inside the target. To simulate the rotational error, CT images and targets were rotated in two ways by 0.5°, 1°, and 2°, in which one rotation was in the axial plane and the other was in three dimensions. The initial optimized plan parameters were copied to the rotated CT sets, and the doses were recalculated. The coverage degradation after rotation was analyzed according to the target dislocation and 12-Gy volume.
A shallower dose gradient reduced the loss of target coverage under target dislocation, and the effect was clearer for small targets. For example, the coverage of the 1-cm target under 1-mm dislocation increased from 93 to 95% by increasing the Paddick gradient index from 5.0 to 7.9. At the same time, the widely accepted necrosis indicator, the 12-Gy volume, increased from 1.2 to 3.5 cm, which remained in the tolerable range. From the differential dose volume histogram (DVH) analysis, the shallower dose gradient ensured that the dose-deficient under-covered target volume received a higher dose similar to that in the prescription.
For frameless stereotactic brain radiotherapy, the gradient, alongside the margin addition, can be adjusted as an ancillary parameter for small targets to increase target coverage or at least limit coverage reduction in conditions with probable positioning error.
评估无框架立体定向放射治疗中旋转误差和剂量梯度对靶区剂量覆盖的综合影响。
在同一轴向脑部计算机断层扫描（CT）图像上绘制并等距放置三个不同直径（1、1.5 和 2cm）的球形靶区。为了测试不同等中心点-靶区距离，准备了 2.5 和 5cm 的配置。针对来自靶区的不同剂量梯度，为每个靶区创建容积调强弧形治疗计划，其中使用靶区内的最大剂量来修改剂量梯度。为了模拟旋转误差，将 CT 图像和靶区分别以 0.5°、1°和 2°的角度在两个平面上进行旋转，其中一个旋转在轴面，另一个在三维空间。将初始优化的计划参数复制到旋转的 CT 集，并重新计算剂量。根据靶区移位和 12-Gy 体积分析旋转后的覆盖度下降情况。
较浅的剂量梯度可减少靶区移位时靶区覆盖度的损失，对于较小的靶区效果更为明显。例如，通过将 Paddick 梯度指数从 5.0 增加到 7.9，1cm 靶区在 1mm 移位时的覆盖度从 93%增加到 95%。同时，被广泛接受的坏死指标 12-Gy 体积从 1.2cm 增加到 3.5cm，但仍在可接受范围内。从微分剂量体积直方图（DVH）分析可以看出，较浅的剂量梯度可确保剂量不足的未覆盖靶区体积获得更高的剂量，类似于处方剂量。
对于无框架立体定向脑部放射治疗，梯度可与边缘增量一起作为辅助参数，用于较小的靶区，以提高靶区覆盖率，或者至少在存在可能的定位误差的情况下限制覆盖率的降低。
