Department of Radiation Oncology, Washington University in St. Louis, MO, 63110, USA.
Department of Physics, Washington University in St. Louis, MO, 63110, USA.
Med Phys. 2021 Apr;48(4):1941-1955. doi: 10.1002/mp.14748. Epub 2021 Feb 19.
To investigate the feasibility of using the high Z storage phosphor material BaFBrI:Eu in conjunction with the low Z storage phosphor material KCl:Eu for simultaneous proton dose and linear energy transfer (LET) measurements by (a) measuring the fundamental optical and dosimetric properties of BaFBrI:Eu , (b) evaluating its compatibility in being readout simultaneously with KCl:Eu dosimeters, and (c) modeling and validating its LET dependence under elevated proton LET irradiation.
A commercial BaFBrI:Eu storage phosphor detector (Model ST-VI, Fujifilm) was characterized with energy dispersive x-ray spectroscopy (EDS) analysis to obtain its elemental composition. The dosimeters were irradiated using both a Mevion S250 proton therapy unit (at the center of a spread-out Bragg peak, SOBP) and a Varian Clinac iX linear accelerator with the latter being a low LET irradiation. The photostimulated luminescence (PSL) emission spectra, excitation spectra, and luminescent lifetimes of the detector were measured after proton and photon irradiations. Dosimetric properties including dose linearity, dose rate dependence, radiation hardness, temporal, and readout stabilities were studied using a laboratory optical reader after proton irradiations. In addition, its proton energy dependence was analytically modeled and experimentally validated by irradiating the detectors at various depths within the SOBP (Range: 15.0 g/cm , Modulation: 10.0 g/cm ).
The active detector composition for the high Z storage phosphor detector was found to be BaFBr I :Eu . The BaFBr I :Eu material's excitation and emission spectra were in agreement under proton and photon irradiations, with peaks of 586 ± 1 nm and 400 ± 1 nm, respectively, with a full width at half maximum (FWHM) of 119 ± 3 nm and 30 ± 2 nm, respectively. As dosimeter response under photon irradiation is generally believed to be free from LET effect, these results suggest LET independence of charge storage center types resulted from ionizing radiations. There is sufficient spectral overlaps with KCl:Eu dosimeters allowing both dosimeters to be readout under equivalent readout conditions, that is, 594 nm stimulation and 420 nm detection wavelengths. Its PSL characteristic lifetime was found to be less than 5 microseconds which would make it suitable for fast 2D readout post irradiation. Its 420 nm emission band intensity was found to be linear up to 10 Gy absolute proton dose under the same irradiation conditions, dose rate independent, stable in time and under multiple readouts, and with high radiation hardness under cumulative proton dose histories up to 200 Gy as tested in this study. BaFBr I :Eu showed significant proton energy-dependent dose under-response in regions of high LET which could be modeled by stopping power ratio calculations with an accuracy of 3% in low LET regions and a distance-to-agreement (DTA) of 1 mm in high LET regions (>5 keV/μm).
We have proven the feasibility of dual-storage phosphor proton dosimetry for simultaneous proton dose and LET measurements. BaFBr I :Eu has shown equally excellent dosimetry performance as its low Z complement KCl:Eu with distinctive LET dependence merely as a result of its higher Z . These promising results pave the way for future studies involving simultaneous proton dose and LET measurements using this novel approach.
通过（a）测量 BaFBrI:Eu 的基本光学和剂量学特性，（b）评估其与 KCl:Eu 剂量计同时读出的兼容性，以及（c）在升高的质子 LET 辐照下对其 LET 依赖性进行建模和验证，来研究使用高 Z 存储荧光体材料 BaFBrI:Eu 与低 Z 存储荧光体材料 KCl:Eu 结合进行同时质子剂量和线性能量转移（LET）测量的可行性。
使用能量色散 X 射线光谱（EDS）分析对商业 BaFBrI:Eu 存储荧光体探测器（型号 ST-VI，富士）进行了表征，以获得其元素组成。使用 Mevion S250 质子治疗单元（在扩展布拉格峰的中心，SOBP）和瓦里安 Clinac iX 线性加速器对剂量计进行辐照，后者为低 LET 辐照。在质子和光子辐照后测量探测器的光激励发光（PSL）发射光谱、激发光谱和荧光寿命。在质子辐照后使用实验室光学读取器研究剂量线性度、剂量率依赖性、辐射硬度、时间和读出稳定性等剂量学特性。此外，通过在 SOBP 内的不同深度（范围：15.0 g/cm ，调制：10.0 g/cm ）辐照探测器，对其质子能量依赖性进行了分析建模和实验验证。
发现高 Z 存储荧光体探测器的有源探测器组成是 BaFBrI:Eu。在质子和光子辐照下，BaFBrI:Eu 材料的激发和发射光谱是一致的，分别具有 586 ± 1 nm 和 400 ± 1 nm 的峰值，半峰全宽（FWHM）分别为 119 ± 3 nm 和 30 ± 2 nm。由于一般认为光子辐照下的剂量计响应不受 LET 效应的影响，这些结果表明电离辐射导致的电荷存储中心类型的 LET 独立性。与 KCl:Eu 剂量计有足够的光谱重叠，允许在等效的读出条件下读出这两种剂量计，即 594nm 激发和 420nm 检测波长。它的 PSL 特征寿命小于 5 微秒，这使其适合在辐照后进行快速 2D 读出。在相同的辐照条件下，发现其 420nm 发射带强度在高达 10 Gy 绝对质子剂量下呈线性，剂量率独立，在时间和多次读出下稳定，在高达 200 Gy 的累积质子剂量历史下具有高辐射硬度，在本研究中进行了测试。BaFBrI:Eu 在高 LET 区域表现出显著的质子能量依赖性剂量响应不足，这可以通过阻止功率比计算进行建模，在低 LET 区域的精度为 3%，在高 LET 区域（>5 keV/μm）的距离一致性（DTA）为 1mm。
我们已经证明了使用双存储荧光体质子剂量计进行同时质子剂量和 LET 测量的可行性。BaFBrI:Eu 与低 Z 补充剂 KCl:Eu 一样具有出色的剂量学性能，其独特的 LET 依赖性仅仅是由于其较高的 Z 。这些有希望的结果为使用这种新方法进行同时质子剂量和 LET 测量的未来研究铺平了道路。
