Gray Laboratory, University of Oxford, Department of Oncology, Old Road Campus Research Building, Roosevelt Drive, Oxford OX3 7DQ, UK.
Phys Med. 2024 Aug;124:104488. doi: 10.1016/j.ejmp.2024.104488. Epub 2024 Jul 28.
To model relative biological effectiveness (RBE) differences found in two studies which used spread-out Bragg-peaks (SOBP) placed at (a) superficial depth and (b) at the maximum range depth. For pencil beam scanning (PBS), RBE at similar points within the SOBP did not change between the two extreme SOBP placement depths; in passively scattered beams (PSB), high RBE values (typically 1.2-1.3) were found within superficially- placed SOBP but reduced to lower values (1-1.07) at similar points within the extreme-depth positioned SOBP. The dose, LET (linear energy transfer) distributions along each SOBP were closely comparable regardless of placement depth, but significant changes in dose rate occurred with depth in the PSB beam.
The equations used allow α and β changes with falling dose rate (the converse to FLASH studies) in PSB, resulting in reduced α/β ratios, compatible with a reduction in micro-volumetric energy transfer (the product of Fluence and LET), with commensurate reductions in RBE. The experimental depth-distances, positions within SOBP, observed dose-rates and radiosensitivity ratios were used to estimate the changes in RBE.
RBE values within a 5 % tolerance limit of the experimental results for PSB were found at the deepest SOBP placement. No RBE changes were predicted for PBS beams, as in the published results.
Enhanced proton therapy toxicity might occur with PBS when compared with PSB for deeply positioned SOBP due to the maintenance of higher RBE. Scanned pencil beam users need to be vigilant about RBE and further research is indicated.
对两项研究中发现的相对生物效应（RBE）差异进行建模，这两项研究使用的是放置在（a）浅层和（b）最大射程深度的扩展布拉格峰（SOBP）。对于铅笔束扫描（PBS），在两个极端 SOBP 放置深度内，相似 SOBP 内的 RBE 没有变化；在被动散射束（PSB）中，在浅层放置的 SOBP 内发现了高 RBE 值（通常为 1.2-1.3），但在极端深度位置的 SOBP 内相似点的 RBE 值降低到较低值（1-1.07）。无论放置深度如何，每个 SOBP 沿剂量、线性能量传递（linear energy transfer，LET）分布都非常接近，但 PSB 光束的深度会导致剂量率发生显著变化。
所使用的方程允许 PSB 中的α和β随剂量率下降而变化（与 FLASH 研究相反），导致α/β比值降低，与微体积能量传递（通量和 LET 的乘积）减少一致，从而 RBE 降低。使用实验深度距离、SOBP 内的位置、观察到的剂量率和放射敏感性比来估计 RBE 的变化。
在 PSB 中，在最深 SOBP 放置处，发现 RBE 值在实验结果的 5%容差范围内。对于 PBS 光束，没有预测到 RBE 变化，这与已发表的结果一致。
与 PSB 相比，由于更高的 RBE 维持，当 SOBP 深度放置时，扫描铅笔束治疗可能会导致质子治疗毒性增强。使用 PBS 的扫描铅笔束用户需要警惕 RBE，并需要进一步研究。
