4D cone beam computed tomography as a novel tool for accurate time-resolved dose calculation in particle therapy

锥束计算机断层扫描(CBCT)扫描仪越来越多地安装在最先进的龙门式质子治疗设施中。这些扫描仪采用平板探测器技术，只需一次机架旋转即可获得三维(3D)体积。在过去的三年里，包括我们小组在内的几个国际研究小组已经开发出在强度校正的CBCT图像上计算质子剂量分布的能力。由于与CT相比，CBCT的图像质量较低，因此这项任务不是微不足道的。基于CBCT的剂量计算可以用来检测由间隔变化引起的剂量传递错误，例如体重减轻。这支持实施自适应放射治疗(ART)，在治疗过程中采用纠正措施，以恢复最初的计划质量。质子疗法现在开始通过位于腹胸区域(肺或肝脏肿瘤)的呼吸内运动来应对肿瘤带来的额外挑战。四维CT扫描(4D-CT)捕捉患者的平均呼吸周期，用于治疗计划。针对移动目标的最终治疗计划的准确性很容易因呼吸模式、幅度或基线的变化而降低。特别是，质子对这种变化的高度敏感性表明，在治疗部位进行4D成像将是可取的，因为它将允许ART用于移动目标。由于旋转速度慢，临床CBCT扫描仪本质上可以通过跟踪患者横隔膜的运动来进行4D图像重建。然而，如果不进行强度校正，4D-CBCT的图像质量不足以用于质子剂量计算。因此，将3D-CBCT图像校正方法扩展到4D-CBCT图像具有明显的潜力。为此，我们将开发一种4D-CBCT校正协议，包括优化的CBCT采集、图像配准和迭代重建算法。这些方法将在一项模式化研究中进行评估，并追溯应用于患者数据集。修正的4D-CBCT数据集将允许仔细评估为处理移动目标而提出的各种策略--例如，不同的扫描方案和边缘概念--以及它们对分馏质子治疗过程中变化的稳健性。拟议项目的目的是开发产生修正的4D-CBCT数据集的方法，该方法允许基于当前的呼吸模式和当天的解剖来计算时间分辨的质子剂量，并基于这些方法来评估ART。这一结果有望进一步研究和开发其他移动肿瘤的ART方法，最终目标是将移动肿瘤的最好的高精度质子疗法引入临床。

Cone beam computed tomography (CBCT) scanners are increasingly installed in state-of-the-art gantry-equipped proton therapy facilities. These scanners employ flat panel detector technology and allow the acquisition of a three dimensional (3D) volume in a single gantry rotation. They are currently used in image guided photon and proton therapy to correct interfractional patient positioning errors.In the past three years, several international research groups, including our group, have developed the ability to compute proton dose distributions on intensity corrected CBCT images. This task is non-trivial due to the lower image quality of CBCT compared to CT. CBCT based dose calculations can be used to detect errors in dose delivery caused by interfractional changes such as weight loss. This supports the implementation of adaptive radiation therapy (ART) where corrective measures are applied during the treatment course to restore the initial plan quality. Proton therapy is now beginning to tackle the additional challenges raised by tumours with respiratory intrafractional movement located in the abdominothoracic region (lung or liver tumours). A four dimensional CT scan (4D-CT), which captures the patient’s average breathing cycle, is used for treatment planning. The accuracy of the resulting treatment plan for moving targets can easily be degraded by changes in the breathing pattern, amplitude or baseline. Especially the high sensitivity of protons to such changes suggests that 4D imaging at the treatment site would be desirable, since it would allow ART for moving targets. With their slow rotation speed, clinical CBCT scanners intrinsically allow 4D image reconstruction by tracking the motion of the patient’s diaphragm. However, without intensity correction, the image quality of 4D-CBCT is insufficient for proton dose calculation.There is thus a clear potential for extending 3D-CBCT image correction methods to 4D-CBCT images. For this purpose we will develop a 4D-CBCT correction protocol including optimised CBCT acquisition, image registration and iterative reconstruction algorithms. The methods will be evaluated in a phantom study and retrospectively applied to patient datasets. Corrected 4D-CBCT datasets would allow the careful evaluation of the various strategies proposed to handle moving targets – e.g. different scanning schemes and margin concepts – and their robustness to changes during the course of fractionated proton therapy.The aim of the proposed project is to develop methods for generating corrected 4D-CBCT datasets that allow time-resolved proton dose calculation based on the current breathing pattern and anatomy of the day, and to evaluate ART based upon these methods. The results are expected to enable further research and developments on other ART methods for moving tumours, with the ultimate goal to introduce the best possible high precision proton therapy of moving tumours into the clinic.