Electromagnetic guided couch and multileaf collimator tracking on a TrueBeam accelerator
Electromagnetic guided couch and multileaf collimator tracking on a TrueBeam accelerator
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DOI：
10.1118/1.4946815
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发表时间：
2016-05-01
期刊：
影响因子：
3.8
通讯作者：
Poulsen, Per Rugaard
中科院分区：
文献类型：
--
作者：
Hansen, Rune;Ravkilde, Thomas;Poulsen, Per Rugaard
目的：Couch和MLC跟踪是两种很有前途的放射治疗实时运动补偿方法。到目前为止，couch和MLC跟踪实验主要是由不同的研究小组进行的，没有直接比较体积调制电弧治疗（VMAT）方案的couch和MLC跟踪的文章。瓦里安TrueBeam 2.0加速器包括一个原型跟踪系统，可选择沙发或MLC补偿。本研究提供了两种跟踪类型与其他相同设置的直接比较。方法：在配备Millennium MLC的TrueBeam加速器上，对电磁引导沙发和MLC跟踪的几何和剂量学性能进行了实验表征。跟踪系统的延迟是在没有运动预测的情况下确定的，作为正弦目标运动与沙发或MLC补偿运动之间的时间滞后，由连续的MV门脉成像记录。几何和剂量学的跟踪精度测量跟踪实验与运动幻影再现四个前列腺和四个肺肿瘤轨迹。光束视场的几何跟踪误差由连续MV图像中嵌入的金标记与圆形MLC孔径之间的距离确定。以静态剂量分布为参考，将剂量跟踪误差量化为低调制和高调制VMAT计划在8个运动轨迹下的2%/ 2mm伽马失败率。结果：在所有正弦周期长度下，MLC跟踪延迟约为146 ms，而由于沙发加速的限制，随着周期长度的减少，MLC跟踪延迟从187 ms增加到246 ms。平均均方根几何误差分别为0.80 mm（沙发跟踪）、0.52 mm （MLC跟踪）和2.75 mm（无跟踪）平行于MLC叶片，0.66 mm（沙发跟踪）、1.14 mm （MLC跟踪）和2.41 mm（无跟踪）垂直于叶片。运动诱导的伽马失败率平均为0.1%（沙发跟踪），8.1% （MLC跟踪）和30.4%（无跟踪）前列腺运动，2.9%（沙发），2.4% （MLC）和41.2%（无跟踪）肺肿瘤运动。残留的跟踪误差主要是由于对快速的肺肿瘤运动适应不足造成的，对垂直于MLC叶片的前列腺运动适应不足造成的。结论：Couch和MLC跟踪显著提高了VMAT递送的几何和剂量学准确性。然而，这两种跟踪类型有不同的优缺点。虽然沙发跟踪可以完美地纠正缓慢移动的目标，如前列腺，但对于垂直于MLC叶片的持续目标移动，MLC跟踪可能会有相当大的剂量误差。MLC跟踪的优点包括更快的动态，更好地适应快速移动的目标，避免移动患者，以及跟踪目标旋转和变形的潜力。(C) 2016年美国医学物理学家协会。
Purpose: Couch and MLC tracking are two promising methods for real-time motion compensation during radiation therapy. So far, couch and MLC tracking experiments have mainly been performed by different research groups, and no direct comparison of couch and MLC tracking of volumetric modulated arc therapy (VMAT) plans has been published. The Varian TrueBeam 2.0 accelerator includes a prototype tracking system with selectable couch or MLC compensation. This study provides a direct comparison of the two tracking types with an otherwise identical setup.Methods: Several experiments were performed to characterize the geometric and dosimetric performance of electromagnetic guided couch and MLC tracking on a TrueBeam accelerator equipped with a Millennium MLC. The tracking system latency was determined without motion prediction as the time lag between sinusoidal target motion and the compensating motion of the couch or MLC as recorded by continuous MV portal imaging. The geometric and dosimetric tracking accuracies were measured in tracking experiments with motion phantoms that reproduced four prostate and four lung tumor trajectories. The geometric tracking error in beam's eye view was determined as the distance between an embedded gold marker and a circular MLC aperture in continuous MV images. The dosimetric tracking error was quantified as the measured 2%/2 mm gamma failure rate of a low and a high modulation VMAT plan delivered with the eight motion trajectories using a static dose distribution as reference.Results: The MLC tracking latency was approximately 146 ms for all sinusoidal period lengths while the couch tracking latency increased from 187 to 246 ms with decreasing period length due to limitations in the couch acceleration. The mean root-mean-square geometric error was 0.80 mm (couch tracking), 0.52 mm (MLC tracking), and 2.75 mm (no tracking) parallel to the MLC leaves and 0.66 mm (couch), 1.14 mm (MLC), and 2.41 mm (no tracking) perpendicular to the leaves. The motion-induced gamma failure rate was in mean 0.1% (couch tracking), 8.1% (MLC tracking), and 30.4% (no tracking) for prostate motion and 2.9% (couch), 2.4% (MLC), and 41.2% (no tracking) for lung tumor motion. The residual tracking errors were mainly caused by inadequate adaptation to fast lung tumor motion for couch tracking and to prostate motion perpendicular to the MLC leaves for MLC tracking.Conclusions: Couch and MLC tracking markedly improved the geometric and dosimetric accuracies of VMAT delivery. However, the two tracking types have different strengths and weaknesses. While couch tracking can correct perfectly for slowly moving targets such as the prostate, MLC tracking may have considerably larger dose errors for persistent target shift perpendicular to the MLC leaves. Advantages of MLC tracking include faster dynamics with better adaptation to fast moving targets, the avoidance of moving the patient, and the potential to track target rotations and deformations. (C) 2016 American Association of Physicists in Medicine.