Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305-5847, USA.
Med Phys. 2010 Oct;37(10):5287-97. doi: 10.1118/1.3488887.
To implement and evaluate clinic-ready adaptive imaging protocols for online patient repositioning (motion tracking) during prostate IMRT using treatment beam imaging supplemented by minimal, as-needed use of on-board kV.
The authors examine the two-step decision-making strategy: (1) Use cine-MV imaging and online-updated characterization of prostate motion to detect target motion that is potentially beyond a predefined threshold and (2) use paired MV-kV 3D localization to determine overthreshold displacement and, if needed, reposition the patient. Two levels of clinical implementation were evaluated: (1) Field-by-field based motion correction for present-day linacs and (2) instantaneous repositioning for new-generation linacs with capabilities of simultaneous MV-kV imaging and remote automatic couch control during treatment delivery. Experiments were performed on a Varian Trilogy linac in clinical mode using a 4D motion phantom programed with prostate motion trajectories taken from patient data. Dosimetric impact was examined using a 2D ion chamber array. Simulations were done for 536 trajectories from 17 patients.
Despite the loss of marker detection efficiency caused by the MLC leaves sometimes obscuring the field at the marker's projected position on the MV imager, the field-by-field correction halved (from 23% to 10%) the mean percentage of time that target displacement exceeded a 3 mm threshold, as compared to no intervention. This was achieved at minimal cost in additional imaging (average of one MV-kV pair per two to three treatment fractions) and with a very small number of repositionings (once every four to five fractions). Also with low kV usage (approximation 2/fraction), the instantaneous repositioning approach reduced overthreshold time by more than 75% (23% to 5%) even with severe MLC blockage as often encountered in current IMRT and could reduce the overthreshold time tenfold (to < 2%) if the MLC blockage problem were relieved. The information acquired for repositioning using combined MV-kV images was found to have submillimeter accuracy.
This work demonstrated with a current clinical setup that substantial reduction of adverse targeting effects of intrafraction prostate motion can be realized. The proposed adaptive imaging strategy incurs minimal imaging dose to the patient as compared to other stereoscopic imaging techniques.
实施并评估在使用治疗束成像补充最小限度、按需使用机载千伏（kV）的情况下，用于在线患者重新定位（运动跟踪）的前列腺调强放疗的临床就绪自适应成像方案。
作者研究了两步决策策略：（1）使用电影-MV 成像和前列腺运动的在线更新特征来检测潜在超出预设阈值的靶区运动，（2）使用 MV-kV 三维定位来确定超过阈值的位移，如果需要，则重新定位患者。评估了两种临床实施水平：（1）基于场的运动校正，用于目前的直线加速器，（2）用于具有同时 MV-kV 成像和治疗过程中远程自动床控制能力的新一代直线加速器的即时重新定位。实验在配备有 4D 运动体模的瓦里安 Trilogy 直线加速器上以临床模式进行，该体模编程了来自患者数据的前列腺运动轨迹。使用二维电离室阵列检查了剂量学影响。对来自 17 名患者的 536 条轨迹进行了模拟。
尽管多叶准直器（MLC）叶片有时会遮挡 MV 成像仪上标记物投影位置的视野，导致标记物检测效率降低，但与无干预相比，场校正将靶区位移超过 3 毫米阈值的平均时间百分比降低了一半（从 23%降至 10%）。这是在额外成像（每两个至三个治疗部分平均一个 MV-kV 对）的成本最小化和很少的重新定位（每四个至五个部分一次）的情况下实现的。即使在当前调强放疗中经常遇到的严重 MLC 遮挡的情况下，使用低千伏（近似每部分 2 次）的即时重新定位方法也可以将超过阈值的时间减少 75%以上（从 23%降至 5%），如果 MLC 遮挡问题得到缓解，超过阈值的时间可以减少十倍（降至<2%）。发现使用 MV-kV 组合图像进行重新定位所获得的信息具有亚毫米级精度。
这项工作使用当前的临床设置证明，前列腺运动的分次内靶区不良效应可以得到显著降低。与其他立体成像技术相比，所提出的自适应成像策略对患者的成像剂量最小。
