Department of Radiation Oncology, Amsterdam UMC, Cancer Center Amsterdam, Amsterdam, the Netherlands.
Varian Medical Systems, Inc, Palo Alto, California, USA.
Med Phys. 2024 Oct;51(10):7580-7588. doi: 10.1002/mp.17185. Epub 2024 May 25.
High-energy transmission beams (TBs) are currently the main delivery method for proton pencil beam scanning ultrahigh dose-rate (UHDR) FLASH radiotherapy. TBs place the Bragg-peaks behind the target, outside the patient, making delivery practical and achievement of high dose-rates more likely. However, they lead to higher integral dose compared to conventional intensity-modulated proton therapy (IMPT), in which Bragg-peaks are placed within the tumor. It is hypothesized that, when energy changes are not required and high beam currents are possible, Bragg-peak-based beams can not only achieve more conformal dose distributions than TBs, but also have more FLASH-potential.
This works aims to verify this hypothesis by taking three different Bragg-peak-based delivery techniques and comparing them with TB and IMPT-plans in terms of dosimetry and FLASH-potential for single-fraction lung stereotactic body radiotherapy (SBRT).
For a peripherally located lung target of various sizes, five different proton plans were made using "matRad" and inhouse-developed algorithms for spot/energy-layer/beam reduction and minimum monitor unit maximization: (1) IMPT-plan, reference for dosimetry, (2) TB-plan, reference for FLASH-amount, (3) pristine Bragg-peak plan (non-depth-modulated Bragg-peaks), (4) Bragg-peak plan using generic ridge filter, and (5) Bragg-peak plan using 3D range-modulated ridge filter.
Bragg-peak-based plans are able to achieve sufficient plan quality and high dose-rates. IMPT-plans resulted in lowest OAR-dose and integral dose (also after a FLASH sparing-effect of 30%) compared to both TB-plans and Bragg-peak-based plans. Bragg-peak-based plans vary only slightly between themselves and generally achieve lower integral dose than TB-plans. However, TB-plans nearly always resulted in lower mean lung dose than Bragg-peak-based plans and due to a higher amount of FLASH-dose for TB-plans, this difference increased after including a FLASH sparing-effect.
This work indicates that there is no benefit in using Bragg-peak-based beams instead of TBs for peripherally located, UHDR stereotactic lung radiotherapy, if lung dose is the priority.
高能传输束（TBs）目前是质子铅笔束扫描超高剂量率（UHDR）FLASH 放疗的主要输送方法。TBs 将布拉格峰放置在目标后面，在患者外部，使输送成为可能，并更有可能实现高剂量率。然而，与将布拉格峰放置在肿瘤内的传统强度调制质子治疗（IMPT）相比，它们会导致更高的积分剂量。人们假设，当不需要能量变化且可以实现高束流时，基于布拉格峰的射束不仅可以实现比 TB 更适形的剂量分布，而且还具有更大的 FLASH 潜力。
本研究旨在通过采用三种不同的基于布拉格峰的输送技术，并在单次分割肺立体定向体部放疗（SBRT）中，从剂量学和 FLASH 潜力方面将其与 TB 和 IMPT 计划进行比较，验证这一假设。
对于大小不同的外周肺靶区，使用“matRad”和内部开发的用于点/能量层/束减少和最小监测单位最大化的算法，为五种不同的质子计划进行了制作：（1）IMPT 计划，为剂量学提供参考，（2）TB 计划，为 FLASH 量提供参考，（3）原始布拉格峰计划（非深度调制布拉格峰），（4）使用通用脊滤波器的布拉格峰计划，以及（5）使用 3D 调强脊滤波器的布拉格峰计划。
基于布拉格峰的计划能够实现足够的计划质量和高剂量率。与 TB 计划和基于布拉格峰的计划相比，IMPT 计划导致最低的 OAR 剂量和积分剂量（FLASH 节省效应为 30%后也是如此）。基于布拉格峰的计划彼此之间差异很小，通常比 TB 计划的积分剂量低。然而，TB 计划几乎总是导致比基于布拉格峰的计划更低的平均肺剂量，并且由于 TB 计划的 FLASH 剂量较高，在包括 FLASH 节省效应后，这种差异会增加。
如果肺剂量是优先考虑的因素，那么对于外周 UHDR 立体定向肺放疗，使用基于布拉格峰的射束而不是 TB 没有优势。
