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，与传统的光子治疗相比，分次质子治疗提供了理论上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沉积高剂量，同时最佳地保留健康组织。然而，质子治疗的一致性增加也要求剂量传递的精确度提高。随着最近安装的几台锥束计算机断层扫描(CBCT)扫描仪的安装，体积图像引导正在进入质子治疗领域。虽然图像制导可以通过确保正确的靶对准来提高质子治疗的精度，但剂量分布对到达靶的放射厚度的依赖意味着仅有几何对准不足以确保精确的剂量传递。因此，在日常体积图像上计算目标的水当量厚度(WEW)是保证高精度的关键。理想的诊断质量的CT成像将被用于这一计算，因为与严重不均匀的CBCT图像相比，它们具有更好的完整性。不幸的是，由于空间的考虑，治疗位置的诊断性CT成像在质子治疗室中是不可能的，并且CT图像转换为必要的停止功率比(RSP)存在很大的不确定性(CBCT也是如此)。质子计算机断层扫描(PCT)通过使用治疗光束直接测量治疗位置的RSP，为上述问题提供了一种潜在的临床解决方案。通过使用单粒子跟踪和能量探测器，从原型PCT扫描仪获得了高精度的RSP图像。此外，质子相互作用的物理性质表明，在较低的成像辐射剂量下可以获得与CT等效的噪声水平的图像，这是实现频繁成像的关键。到目前为止，已经用广泛的质子束进行了PCT。我们的建议是与运营PCT原型的国际合作(我们是其成员)合作，开发铅笔束扫描PCT。这将打开我们研究建议背后的主要想法的大门：通量调制PCT(FMpCT)，其中调整铅笔束强度以产生尽可能低的成像剂量，同时在所有质子束相互作用的区域保持足够的图像质量，从而允许高精度、每日最新的湿估计。为了评估我们新型图像制导策略的性能，将在模拟环境和实验中与最先进的CBCT成像进行广泛的比较。因此，我们将致力于确定FMpCT在质子治疗影像指导中的价值。

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.