Department of Radiation Oncology, Stanford University, Stanford, CA 94305-5847, USA.
Med Phys. 2012 May;39(5):2686-93. doi: 10.1118/1.4704729.
To evaluate the geometric accuracy of beam targeting in external surrogate-based gated volumetric modulated arc therapy (VMAT) using kilovoltage (kV) x-ray images acquired during dose delivery.
Gated VMAT treatments were delivered using a Varian TrueBeam STx Linac for both physical phantoms and patients. Multiple gold fiducial markers were implanted near the target. The reference position was created for each implanted marker, representing its correct position at the gating threshold. The gating signal was generated from the RPM system. During the treatment, kV images were acquired immediately before MV beam-on at every breathing cycle, using the on-board imaging system. All implanted markers were detected and their 3D positions were estimated using in-house developed software. The positioning error of a marker is defined as the distance of the marker from its reference position for each frame of the images. The overall error of the system is defined as the average over all markers. For the phantom study, both sinusoidal motion (1D and 3D) and real human respiratory motion was simulated for the target and surrogate. In the baseline case, the two motions were synchronized for the first treatment fraction. To assess the effects of surrogate-target correlation on the geometric accuracy, a phase shift of 5% and 10% between the two motions was introduced. For the patient study, intrafraction kV images of five stereotactic body radiotherapy (SBRT) patients were acquired for one or two fractions.
For the phantom study, a high geometric accuracy was achieved in the baseline case (average error: 0.8 mm in the superior-inferior or SI direction). However, the treatment delivery is prone to geometric errors if changes in the target-surrogate relation occur during the treatment: the average error was increased to 2.3 and 4.7 mm for the phase shift of 5% and 10%, respectively. Results obtained with real human respiratory curves show a similar trend. For a target with 3D motion, the technique is able to detect geometric errors in the left-right (LR) and anterior-posterior (AP) directions. For the patient study, the average intrafraction positioning errors are 0.8, 0.9, and 1.4 mm and 95th percentile errors are 1.7, 2.1, and 2.7 mm in the LR, AP, and SI directions, respectively.
The correlation between external surrogate and internal target motion is crucial to ensure the geometric accuracy of surrogate-based gating. Real-time guidance based on kV x-ray images overcomes the potential issues in surrogate-based gating and can achieve accurate beam targeting in gated VMAT.
评估在千伏 (kV) X 射线图像引导的外部基准门控容积旋转调强放疗 (VMAT) 中，使用在剂量输送期间获取的千伏 X 射线图像进行光束靶向的几何精度。
使用瓦里安 TrueBeam STx 直线加速器为物理体模和患者进行门控 VMAT 治疗。在靶区附近植入多个金基准标记物。为每个植入的标记物创建参考位置，代表其在门控阈值下的正确位置。门控信号由 RPM 系统生成。在治疗过程中，使用机载成像系统在每个呼吸周期的 MV 射束开启前立即获取千伏图像。使用内部开发的软件检测所有植入的标记物并估计其 3D 位置。标记物的定位误差定义为标记物在每一帧图像中与其参考位置的距离。系统的整体误差定义为所有标记物的平均值。对于体模研究，目标和替代物的模拟了正弦运动（1D 和 3D）和真实人体呼吸运动。在基线情况下，两种运动在第一个治疗分数中是同步的。为了评估替代物-目标相关性对几何精度的影响，引入了两种运动之间 5%和 10%的相位偏移。对于患者研究，对五个立体定向体部放疗 (SBRT) 患者的一个或两个部分进行了分次内千伏图像采集。
在体模研究中，在基线情况下实现了高精度的几何精度（在上下或 SI 方向上的平均误差为 0.8 毫米）。然而，如果在治疗过程中目标-替代物关系发生变化，治疗输送容易出现几何误差：相位偏移为 5%和 10%时，平均误差分别增加到 2.3 和 4.7 毫米。使用真实人体呼吸曲线得到的结果显示出相似的趋势。对于具有 3D 运动的目标，该技术能够检测到左右 (LR) 和前后 (AP) 方向的几何误差。对于患者研究，在 LR、AP 和 SI 方向上，分次内定位误差的平均值分别为 0.8、0.9 和 1.4 毫米，95%的误差分别为 1.7、2.1 和 2.7 毫米。
外部替代物和内部目标运动之间的相关性对于确保基于替代物的门控的几何精度至关重要。基于千伏 X 射线图像的实时引导克服了基于替代物的门控中的潜在问题，可以在门控 VMAT 中实现精确的光束靶向。
