Bertholet Jenny, Worm Esben S, Fledelius Walther, Høyer Morten, Poulsen Per R
Department of Oncology, Aarhus University Hospital, Aarhus C, Denmark.
Department of Medical Physics, Aarhus University Hospital, Aarhus C, Denmark.
Int J Radiat Oncol Biol Phys. 2016 Jun 1;95(2):802-9. doi: 10.1016/j.ijrobp.2016.01.033. Epub 2016 Jan 22.
Image guided liver stereotactic body radiation therapy (SBRT) often relies on implanted fiducial markers. The target localization accuracy decreases with increased marker-target distance. This may occur partly because of liver rotations. The aim of this study was to examine time-resolved translations and rotations of liver marker constellations and investigate if time-resolved intrafraction rotational corrections can improve localization accuracy in liver SBRT.
Twenty-nine patients with 3 implanted markers received SBRT in 3 to 6 fractions. The time-resolved trajectory of each marker was estimated from the projections of 1 to 3 daily cone beam computed tomography scans and used to calculate the translation and rotation of the marker constellation. In all cone beam computed tomography projections, the time-resolved position of each marker was predicted from the position of another surrogate marker by assuming that the marker underwent either (1) the same translation as the surrogate marker; or (2) the same translation as the surrogate marker corrected by the rotation of the marker constellation. The localization accuracy was quantified as the root-mean-square error (RMSE) between the estimated and the actual marker position. For comparison, the RMSE was also calculated when the marker's position was estimated as its mean position for all the projections.
The mean translational and rotational range (2nd-98th percentile) was 2.0 mm/3.9° (right-left), 9.2 mm/2.9° (superior-inferior), 4.0 mm/4.0° (anterior-posterior), and 10.5 mm (3-dimensional). Rotational corrections decreased the mean 3-dimensional RMSE from 0.86 mm to 0.54 mm (P<.001) and halved the RMSE increase per millimeter increase in marker distance.
Intrafraction rotations during liver SBRT reduce the accuracy of marker-guided target localization. Rotational correction can improve the localization accuracy with a factor of approximately 2 for large marker-target distances.
图像引导下的肝脏立体定向体部放射治疗（SBRT）通常依赖于植入的基准标记物。随着标记物与靶区距离的增加，靶区定位精度会降低。这可能部分是由于肝脏的旋转所致。本研究的目的是检查肝脏标记物群组的时间分辨平移和旋转情况，并研究时间分辨的分次内旋转校正是否能提高肝脏SBRT的定位精度。
29例植入了3个标记物的患者接受了3至6次分割的SBRT。每个标记物的时间分辨轨迹是根据每天1至3次锥束计算机断层扫描的投影估计得出的，并用于计算标记物群组的平移和旋转。在所有锥束计算机断层扫描投影中，通过假设标记物经历以下两种情况之一来从另一个替代标记物的位置预测每个标记物的时间分辨位置：（1）与替代标记物相同的平移；或（2）经标记物群组旋转校正后的与替代标记物相同的平移。定位精度通过估计的标记物位置与实际标记物位置之间的均方根误差（RMSE）进行量化。为作比较，当将标记物的位置估计为其在所有投影中的平均位置时，也计算了RMSE。
平均平移和旋转范围（第2百分位数至第98百分位数）为2.0毫米/3.9°（左右）、9.2毫米/2.9°（上下）、4.0毫米/4.0°（前后）和10.5毫米（三维）。旋转校正使平均三维RMSE从0.86毫米降至0.54毫米（P<0.001），并且使标记物距离每增加1毫米时RMSE的增加减半。
肝脏SBRT过程中的分次内旋转会降低标记物引导的靶区定位精度。对于较大的标记物与靶区距离，旋转校正可将定位精度提高约2倍。
