Fluence modulated proton computed tomography: a new approach for low-dose image guidance in particle therapy

由于质子与组织相互作用的物理性质，与常规光子治疗相比，分级质子治疗在理论上提供了有利的辐射剂量分布。质子束末端的尖锐剂量梯度可用于在肿瘤体积中存款高剂量，同时最佳地保留健康组织。然而，质子治疗的增加的一致性也需要增加剂量递送的精确度。随着最近安装的几台锥形束计算机断层扫描（CBCT）扫描仪，体积图像引导正在进入质子治疗。虽然图像引导可以通过确保正确的靶对准来提高质子治疗的精度，但是剂量分布对穿过以到达靶的放射学厚度的依赖性意味着仅几何对准不足以确保精确的剂量递送。因此，在日常体积图像上计算目标的所谓水当量厚度（WET）是确保高精度的关键。理想情况下，诊断质量CT成像将用于此计算，因为与CBCT图像相比，其具有上级完整性，CBCT图像具有严重的非均匀性。不幸的是，由于空间考虑，在质子治疗室中不容易实现治疗位置的诊断CT成像，并且CT图像转换为必要的停止功率比（RSP）具有显著的不确定性（CBCT也是如此）。质子计算机断层扫描（pCT）通过使用治疗射束直接测量治疗位置处的RSP，为上述问题提供了潜在的临床解决方案。通过使用单粒子跟踪和能量检测器，已经从原型pCT扫描仪获得了高精度RSP图像。此外，质子相互作用的物理性质表明，可以在低成像辐射剂量下获得CT等效噪声水平的图像，这对于频繁成像至关重要。我们的提案是与运营pCT原型的国际合作组织（我们是该组织的成员）合作开发笔形束扫描pCT。这将为我们的研究建议背后的主要思想打开大门：注量调制pCT（FMpCT），其中笔形射束强度被调整以产生尽可能低的成像剂量，同时在质子束相互作用的所有区域保持足够的图像质量，从而允许高精度、每日最新的WET估计。为了评估我们的新型图像引导策略的性能，将在模拟环境和实验环境中与现有技术的CBCT成像进行广泛的比较。因此，我们将致力于确定裂变材料条约在质子治疗图像引导方面的价值。

Fractionated proton therapy offers theoretically advantageous radiation dose distributions when compared to conventional photon therapy due to the physical nature of proton interactions with tissue. The sharp dose gradient at the end of a proton beam can be used to deposit a high dose in the tumour volume while optimally sparing healthy tissue. However, the increased conformity of proton therapy also requires increased precision in dose delivery. Volumetric image guidance is making its entry in proton therapy, with several cone beam com-puted tomography (CBCT) scanners installed recently. While image guidance may improve the precision of proton therapy by ensuring correct target alignment, the dependence of the dose distribution to the radiological thickness traversed to reach the target means that geometric alignment alone is not sufficient to ensure precise dose delivery. For this reason calculation of the so-called water equivalent thickness (WET) to the target on daily volumetric images is key to ensure high precision. Ideally diagnostic quality CT imaging would be employed for this calcula-tion due to their superior integrity when compared to CBCT images, which suffer from severe non-homogeneities. Diagnostic CT imaging in treatment position is unfortunately not readily pos-sible in a proton therapy treatment room due to space considerations, and CT image conversion to the necessary stopping power ratio (RSP) suffers from significant uncertainties (as does CBCT). Proton computed tomography (pCT) offers a potential clinical solution to the issues raised above by using the treatment beam to directly measure the RSP at the treatment position. By using single particle tracking and an energy detector, high accuracy RSP images have been obtained from prototype pCT scanners. Furthermore the physical nature of proton interactions suggests that images of CT-equivalent noise levels can be obtained at low imaging radiation doses, which is critical to enable frequent imaging.Up to now, pCT has been performed with broad proton beams. Our proposal is to develop pencil beam scanning pCT, in collaboration with the international collaboration operating the pCT pro-totype (of which we are members). This would open the door to the main idea behind our re-search proposal: fluence modulated pCT (FMpCT) where pencil beam intensities are adjusted to yield the lowest possible imaging dose while maintaining sufficient image quality in all areas where proton beams interact, thus allowing high accuracy, daily up-to-date WET estimates.To evaluate the performance of our novel image guidance strategy, an extensive comparison to state-of-the-art CBCT imaging will be performed both in a simulation environment and experi-mentally. We will thus aim at establishing the value of FMpCT for proton therapy image guidance.