目的:作为质子束治疗中调试和质量保证的一部分,侧向剂量曲线和输出因子必须被获取。这种测量可以使用点探测器进行,并且在小视场或陡峭的横向半影区域中尤其具有挑战性,因为探测器的体积效应可能导致扰动。为了解决这个问题,这项工作旨在量化和纠正这种扰动的六个点探测器在小质子场创建通过三种不同的传递技术。
方法:三种质子束传输技术的侧向剂量分布和输出测量(笔形束扫描,铅笔束扫描结合准直仪,使用准直器的被动散射)使用高分辨率EBT3薄膜进行,PinPoint3D31022电离室,一个微硅二极管60023和一个微钻石探测器60019(所有PTW弗莱堡,德国)。探测器特定侧向剂量响应函数K(x,y)作为卷积核变换未受干扰的剂量分布D(x,y)到测量的信号轮廓M(x,y)用于量化质子场中六个研究检测器的扰动并校正测量结果。剂量分布和检测器的K(x,y)的执行是为了帮助理解有关检测器选择和交付技术的组合的测量过程。
结果:场中心的横向半影增宽和信号减少的量化表明,即使是大体积电离室,笔形波束扫描场中的测量也仅受到轻微损害,横向半影的最大差异为0.25mm,场中心的信号减少为4%。相比之下,所研究的探测器不能准确地表示具有准直的辐射技术,如准直仪的被动散射半影展宽达1.6毫米,准直仪的笔形光束扫描半影展宽达2.2毫米。对于直径为3mm的准直器场,现场中心的信号减少在7.6%和60.7%之间被断言。横向剂量分布测量值已通过与相应的K(x,y)得到未受干扰的D(x,y)。为microDiamond获得的被动散射准直场的校正输出比,对于3mm的最小场尺寸,微硅和PinPoint3D显示出优于0.9%(一个标准偏差)的一致性。
结论:对三种递送技术产生的小质子场中的点探测器扰动进行了量化,发现对于具有陡峭剂量梯度的准直小质子场尤其明显。在所有调查的探测器中,微硅二极管显示出最小的扰动。基于检测器的K(x,y)被发现适用于获得不受干扰的侧向剂量分布和输出因子。K(x,y)通过仅考虑几何平均效应已被证明可以合理预测探测器的体积效应。这项工作的发现可用于指导各种质子场中点探测器的选择,并为开发小场质子剂量测定的实践规范做出贡献。
OBJECTIVE: As a part of the commissioning and quality assurance in proton beam therapy, lateral dose profiles and output factors have to be acquired. Such measurements can be performed with point detectors and are especially challenging in small fields or steep lateral penumbra regions as the detector\'s volume effect may lead to perturbations. To address this issue, this work aims to quantify and correct for such perturbations of six point detectors in small proton fields created via three different delivery techniques.
METHODS: Lateral dose profile and output measurements of three proton beam delivery techniques (pencil beam scanning, pencil beam scanning combined with collimators, passive scattering with collimators) were performed using high-resolution EBT3 films, a PinPoint 3D 31022 ionization chamber, a microSilicon diode 60023 and a microDiamond detector 60019 (all PTW Freiburg, Germany). Detector specific lateral dose response functions K(x,y) acting as the convolution kernel transforming the undisturbed dose distribution D(x,y) into the measured signal profiles M(x,y) were applied to quantify perturbations of the six investigated detectors in the proton fields and correct the measurements. A signal theoretical analysis in Fourier space of the dose distributions and detector\'s K(x,y) was performed to aid the understanding of the measurement process with regard to the combination of detector choice and delivery technique.
RESULTS: Quantification of the lateral penumbra broadening and signal reduction at the fields center revealed that measurements in the pencil beam scanning fields are only compromised slightly even by large volume ionization chambers with maximum differences in the lateral penumbra of 0.25 mm and 4% signal reduction at the field center. In contrast, radiation techniques with collimation are not accurately represented by the investigated detectors as indicated by a penumbra broadening up to 1.6 mm for passive scattering with collimators and 2.2 mm for pencil beam scanning with collimators. For a 3 mm diameter collimator field, a signal reduction at field center between 7.6% and 60.7% was asserted. Lateral dose profile measurements have been corrected via deconvolution with the corresponding K(x,y) to obtain the undisturbed D(x,y). Corrected output ratios of the passively scattered collimated fields obtained for the microDiamond, microSilicon and PinPoint 3D show agreement better than 0.9% (one standard deviation) for the smallest field size of 3 mm.
CONCLUSIONS: Point detector perturbations in small proton fields created with three delivery techniques were quantified and found to be especially pronounced for collimated small proton fields with steep dose gradients. Among all investigated detectors, the microSilicon diode showed the smallest perturbations. The correction strategies based on detector\'s K(x,y) were found suitable for obtaining unperturbed lateral dose profiles and output factors. Approximation of K(x,y) by considering only the geometrical averaging effect has been shown to provide reasonable prediction of the detector\'s volume effect. The findings of this work may be used to guide the choice of point detectors in various proton fields and to contribute toward the development of a code of practice for small field proton dosimetry.