Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italy.
Massachusetts General Hospital, Department of Physics, Boston, USA.
Phys Med. 2019 Jan;57:58-64. doi: 10.1016/j.ejmp.2018.12.011. Epub 2018 Dec 21.
In scanned proton beam therapy systematic deviations in spot size at iso-center can occur as a result of changes in the beam-line optics. There is currently no general guideline of the spot size accuracy required clinically. In this work we quantify treatment plan robustness to systematic spot size variations as a function of spot size and spot spacing, and we suggest guidelines for tolerance levels for spot size variations.
Through perturbation of spot size in treatment plans for 7 patients and a phantom, we evaluated the dose impact of systematic spot size variations of 5% up to 50%. We investigated the dependence on nominal spot size by studying scenarios with small, medium and large spot sizes for various inter-spot spacings. To come to tolerance levels, we used the Γ passing rate and dose-volume-histograms.
Limits on spot size accuracy were extracted for 8 sites, 3 different spot sizes and 3 different inter-spot spacings. While the allowable spot size variation strongly depends on the spot size, the inter-spot spacing turned out to be only of limited influence.
Plan robustness to spot size variations strongly depend on spot size, with small spot plans being much more robust than larger spots plans. Inter-spot spacing did not influence plan robustness. Combining our results with existing literature, we propose limits of ±25%, ±20% and ±10% of the spot width σ, for spots with σ of 2.5, 5.0 and 10 mm in proton therapy spot scanning facilities, respectively.
在扫描质子束治疗中，由于束线光学元件的变化，等中心处的光斑尺寸会出现系统偏差。目前临床上并没有关于光斑尺寸精度要求的一般准则。在这项工作中，我们定量评估了光斑尺寸变化对治疗计划的稳健性，研究了光斑尺寸和光斑间距对治疗计划的影响，并提出了光斑尺寸变化容限的建议。
通过对 7 名患者和一个体模的治疗计划中的光斑尺寸进行微扰，我们评估了系统光斑尺寸变化 5%至 50%的剂量影响。我们通过研究小、中、大光斑尺寸以及不同光斑间距的情况，研究了光斑尺寸的依赖关系。为了确定容限水平，我们使用了 Γ 通过率和剂量体积直方图。
为 8 个部位、3 种不同光斑尺寸和 3 种不同光斑间距提取了光斑尺寸精度的限制。虽然允许的光斑尺寸变化强烈依赖于光斑尺寸，但光斑间距的影响有限。
治疗计划对光斑尺寸变化的稳健性强烈依赖于光斑尺寸，小光斑计划比大光斑计划更稳健。光斑间距对计划稳健性没有影响。结合我们的结果和现有文献，我们建议在质子治疗点扫描设备中，光斑宽度为 2.5、5.0 和 10mm 的光斑，其允许的光斑尺寸变化分别为±25%、±20%和±10%。
