[论文解读] Optimization of patient-specific range modulators for conformal FLASH proton therapy

Sylvain Deffet, Kevin Souris+1 more|arXiv (Cornell University)|Mar 15, 2023

Radiation Therapy and DosimetryMedicine被引用 3

一句话总结

本文提出了一种新型逆向计划方法，联合优化患者特异性的体素化射程调制器和笔形束扫描点权重，以实现适形FLASH质子治疗。通过使用快速蒙特卡罗梯度估计器，在剂量约束下直接优化调制器几何形状和点权重，该方法在实现接近调强质子治疗（IMPT）的剂量分布的同时，实现了可3D打印、兼容CT的结构设计，并简化了模拟工作流程。

ABSTRACT

Purpose: A promising approach to enable FLASH conformal proton therapy is to passively degrade a single energy layer using a patient-specific range modulator. We propose an innovative method to directly optimize the geometrical characteristics of the range modulator and the treatment plan with respect to user defined constraints, similarly to state-of-the-art IMPT inverse planning. Methods: The kind of range modulators proposed in this study is a voxelized object which can be placed in the CT for dose computation, which simplifies the simulation pipeline. Both the geometrical characteristics of the range modulator and the weights of the PBS spots were directly optimized with respect to constraints on the dose using a first-order method. A modified Monte Carlo dose engine was used to provide an estimate of the gradient of the relaxed constraints with respect to the elevation values of the range modulator. Results: Assessed on a head and neck case, dose conformity logically appeared to be significantly degraded compared to IMPT. We then demonstrated that this degradation came mainly from the use of a large range shifter and therefore from physical limitations inherent in the passive degradation of beam energy. The geometry of the range modulator, on the other hand, was shown to be very close to being optimal. PBS dose rates were computed and discussed with respect to FLASH objectives. Conclusions: The voxelized range modulators optimized with the proposed method were proven to be optimal on a head and neck case characterized by two rather large volumes, with irregular contours and variable depths. The optimized geometry differed from conventional ridge filters as it was arbitrarily set by the optimizer. This kind of range modulators can be directly added in the CT for dose computation and is well suited for 3D printing.

研究动机与目标

• 解决现有FLASH质子治疗方法中缺乏对射程调制器的直接优化问题。
• 克服依赖预计算的IMPT计划和外部蒙特卡罗引擎的剂量模仿优化方法的局限性。
• 开发一种统一的优化框架，同时优化射程调制器几何形状和PBS点权重，类似于IMPT逆向计划。
• 通过可3D打印的体素化射程调制器实现实际应用，该调制器可直接嵌入CT用于剂量计算。
• 在FLASH剂量率约束下，评估该方法在复杂头颈部癌病例中的可行性与剂量适形性。

提出的方法

• 体素化射程调制器被定义为3D高程图（1×1×1 mm³分辨率），直接放置于CT中用于剂量计算。
• 使用一阶优化方法同时更新调制器的高程值和PBS点权重。
• 修改后的蒙特卡罗剂量引擎计算松弛剂量约束相对于每个体素高度的梯度。
• 优化过程在用户定义的靶区和危及器官剂量约束下进行，类似于IMPT逆向计划。
• 通过使用基于CT的调制器，避免了外部参数化几何引擎，简化了模拟流程。
• 该方法实现了调制器形状与点权重的直接联合优化，避免了顺序或解耦流程。

Figure 1: A 3D range modulator can be represented as a 2D elevation map. It is split into ’towers’ and a range shifter. A minimum thickness of plain material is left attached to the ’towers’ accordingly to 3D printing specification.

实验结果

研究问题

• RQ1 与顺序或剂量模仿方法相比，联合优化射程调制器几何形状和PBS点权重是否能提高FLASH质子治疗中的剂量适形性？
• RQ2 在单能层情况下，被动射束能量衰减的物理限制在多大程度上影响FLASH剂量适形性？
• RQ3 与传统脊状滤波器相比，体素化调制器设计在几何灵活性和3D可打印性方面表现如何？
• RQ4 使用基于CT的调制器能否简化模拟流程，从而替代具有参数化几何结构的复杂外部蒙特卡罗引擎？
• RQ5 优化后的FLASH计划可实现的剂量率是多少？是否达到与FLASH效应相关的40 Gy/s阈值？

主要发现

• 优化后的射程调制器几何形状非常接近最优，优化过程中自然出现了非对称金字塔形结构。
• 与完整IMPT相比，FLASH计划的剂量适形性显著下降，主要由于被动射束能量衰减的物理限制。
• 在评估的头颈部病例中，PBS剂量率未达到40 Gy/s阈值，扫描长度是主要限制因素。
• 体素化调制器设计可直接集成到CT中用于剂量计算，消除了对复杂外部蒙特卡罗模拟（含参数化几何结构）的需求。
• 该方法实现了与使用相同射程移位器的单场IMPT计划非常接近的剂量分布，表明剂量交付具有高保真度。
• 由于其1×1×1 mm³的粗分辨率，该方法非常适合3D打印，提高了机械稳定性和可制造性。

Figure 3: Experimental setup for the right part of the tumor. From nozzle to patient: range modulator (PMMA), range shifter (aluminium), and aperture (tungsten).