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

锥形束计算机断层扫描（CBCT）扫描仪越来越多地安装在最先进的机架配备质子治疗设施。这些扫描仪采用平板探测器技术，并允许在单次机架旋转中采集三维（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了在强度校正的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，因此该任务并非微不足道。基于CBCT的剂量计算可用于检测由分次间变化（如体重减轻）引起的剂量输送误差。这支持自适应放射治疗（ART）的实施，其中在治疗过程中应用校正措施以恢复初始计划质量。质子治疗现在开始解决位于腹胸区的具有呼吸分次内运动的肿瘤（肺或肝肿瘤）所带来的额外挑战。四维CT扫描（4D-CT）可捕获患者的平均呼吸周期，用于治疗计划。所得到的针对移动目标的治疗计划的准确性可以容易地由于呼吸模式、幅度或基线的变化而降低。特别是质子对这种变化的高灵敏度表明，治疗部位的4D成像将是期望的，因为它将允许ART用于移动靶。由于其旋转速度慢，临床CBCT扫描仪本质上允许通过跟踪患者横膈膜的运动来进行4D图像重建。然而，如果没有强度校正，4D-CBCT的图像质量不足以进行质子剂量计算，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，我们将开发一个4D-CBCT校正协议，包括优化CBCT采集，图像配准和迭代重建算法。将在体模研究中评价这些方法，并将其回顾性应用于患者数据集。校正后的4D-CBCT数据集将允许仔细评估处理移动目标的各种策略-例如不同的扫描方案和边缘概念-以及它们对分割质子治疗过程中变化的鲁棒性。拟议项目的目的是开发用于生成校正后的4D-CBCT数据集的方法，基于当前的呼吸模式和当天的解剖结构，解析质子剂量计算，并基于这些方法评估ART。这些结果有望进一步研究和开发用于移动肿瘤的其他ART方法，最终目标是将移动肿瘤的最佳高精度质子治疗引入临床。

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.