PROton Stopping power estimation in Ion Therapy (PROSIT)

带电粒子治疗（CPT）是一种以致死剂量照射肿瘤而不危及健康组织和邻近器官的有效方法。单束的剂量谱给出了CPT的强度：一个明显的最大值，即布拉格峰，然后急剧下降。这一特点也使得CPT对实际停止功率（SP）与用于治疗计划的计算SP之间的小不匹配非常敏感。因此，引入安全边际是为了防止可能的计划错误和患者形态学变化的影响。然而，监测和验证实际剂量沉积（或至少粒子范围）是必要的，以充分利用光束的物理特性。目前还没有一种监测方法达到临床常规，但许多技术正在研究中。其中，即时伽马时序（PGT）显示出巨大的潜力。PGT依赖于探测到的提示伽马（PG）射线的时间分布，这些射线起源于光束路径上的去激发。在我们与INFN - Turin的联合工作中，我们已经证明，使用铅笔质子束的计算机数据，不仅可以从不同探测器的PGT剖面重建粒子范围，而且可以重建PG发射顶点的时空分布。我们的方法，SER-PGT，通过最大化泊松似然和测量过程的建模来重建时空PG发射分布的估计。在PROSIT中，我们将首先改进和扩展我们的模型和算法，以便更好地重建，包括对背景信号和数据截断的补偿，以及参数优化。在对均匀目标和计算机数据的初步研究的支持下，我们计划更进一步，从重构的时空PG分布中获得阻止力，因为后者包含了目标内粒子运动的信息。为此，我们利用盖革值域规则对波特菲尔德提出的方程进行解析反演，以建立粒子运动学模型。然后，将方程参数拟合到SER-PGT的后处理输出中。我们将把这一过程扩展到非均匀目标，同时考虑到在铅笔束扫描中，不同能量和方向的几束束靶向肿瘤体积。最后，我们设想开发一种能够跳过中间SER-PGT重建的程序，使用PG发射模型直接从测量的PGT数据中估计SP。我们的长期目标是在比较理论和实际停止功率的基础上进行体内治疗验证和个性化的治疗计划调整，后者通过多探测器PGT系统结合我们的模型和程序进行估计。

Charged particle therapy (CPT) is an effective procedure for irradiating tumors with lethal doses while sparing healthy tissue and neighboring organs at risk. The dose profile of single beams gives CPT its strength: a pronounced maximum, the Bragg peak, followed by a steep drop. This feature also makes CPT very sensitive to small mismatches between the actual stopping power (SP) and the computed SP used for therapy planning. Safety margins are thus introduced to prevent the effects of possible planning errors and changes in patient’s morphology. However, monitoring and verification of the actual dose deposition (or at least the particle range) are necessary to take full advantage of the beam physical properties. At present no monitoring method has reached clinical routine, but many techniques are under investigation. Among them, prompt-gamma timing (PGT) shows great potential. PGT relies on the time profiles of detected prompt-gamma (PG) rays which originate from de-excitations along the beam path. In our joint work with INFN - Turin, we have shown using in-silico data of a pencil proton beam that not only the particle range but also the spatiotemporal distribution of the PG emission vertices can be reconstructed from PGT profiles from various detectors. Our method, SER-PGT, reconstructs estimates of the spatiotemporal PG emission distribution through maximization of the Poisson likelihood and the modeling of the measurement process. Within PROSIT we will first improve and extend our models and algorithms for a better reconstruction, including compensation for background signals and data truncation, as well as parameter optimization. Supported by preliminary works with homogeneous targets and in-silico data, we plan to go beyond and obtain the stopping power from the reconstructed spatiotemporal PG distribution, as the latter contains information about the particle motion within the target. To this aim, we analytically invert the equation proposed by Bortfeld to model the particle kinematics using Geiger’s range rule. Then, the parameters of the equation are fitted to the post-processed output of SER-PGT. We will extend this procedure to non-uniform targets, also taking into account that in pencil-beam scanning several beams of different energies and directions target the tumor volume. Finally, we envision developing a procedure able to skip the intermediate SER-PGT reconstruction to directly estimate the SP from measured PGT data using models of the PG emission. Our long-term goal is to allow for in-vivo treatment verification and the personalized readjustment of the treatment plan based on the comparison of the theoretical and the actual stopping power, the latter estimated through a multi-detector PGT system in combination with our models and procedures.