Wave Propagation and Signal Processing, KU Leuven - KULAK, Kortrijk, 8500, Belgium.
Department of Radiation Oncology, University Hospitals Leuven, Leuven, 3000, Belgium.
Med Phys. 2017 Jun;44(6):2532-2543. doi: 10.1002/mp.12246. Epub 2017 May 12.
The aim of this work was to model the dose dependence of the darkening of GafChromic™ EBT3 films by combining the optical properties of the polydiacetylene polymer phases, and a modified version of the single-hit model, which will take the stick-like shape of the monomer microcrystals into account. Second, a comparison is made between the quantification of the film darkening by flatbed scanning and by UV-vis absorption spectroscopy.
GafChromic EBT3 films were irradiated with a 6 MV photon beam at dose levels between 0 and 50 Gy. The radiation-induced darkening of the films is quantified by a flatbed scanner, and by UV-vis absorption spectroscopy in the wavelength range of 220-750 nm. From the UV-vis absorption spectra, the contribution of each polymer phase to the absorbance was deduced. Next, the dose dependence of the polymer content is described by a modified single-hit model where the size distribution of polymerizable centers is approximated by way of the size distribution of the monomer microcrystals in the film.
The absorption properties of the film can be accurately quantified by UV-vis spectroscopy for dose levels between 0 and 10 Gy. Over 10 Gy, the absorption spectrum saturates due to the limited sensitivity of the spectrometer. The modified single-hit model was successful in describing the increasing polymer concentration with radiation dose, using a log-normal distribution for the length of the stick-like monomer microcrystals. The dose dependence of the polymer content, deduced from the UV-vis absorption spectrum, differs from that of the flatbed scanning method and is more sensitive to changes in dose.
The dose dependence of the polymer concentration can be modeled by taking into account the distribution of active centers using the microstructure of the active layer for dose levels between 0 and 10 Gy. The dissimilar dose dependence of the polymer concentration and the absorbance must be accounted for when modeling darkening from the kinetics of the photopolymerization reaction.
本工作的目的是通过组合聚二乙炔聚合物相的光学性质和改进的单击模型来模拟 GafChromic™ EBT3 膜变暗的剂量依赖性，该模型将考虑单体微晶的棒状形状。其次，比较平板扫描和 UV-vis 吸收光谱法对膜变暗的定量。
将 GafChromic EBT3 膜用 6 MV 光子束照射，剂量范围为 0 至 50 Gy。通过平板扫描仪和 220-750nm 波长范围内的 UV-vis 吸收光谱法来量化膜的辐射致暗化。从 UV-vis 吸收光谱中，推导出每个聚合物相的吸光度贡献。接下来，通过改进的单击模型来描述聚合物含量随剂量的变化，其中聚合中心的尺寸分布通过膜中单体微晶的尺寸分布来近似。
对于 0 至 10 Gy 的剂量水平，UV-vis 光谱法可以准确地定量膜的吸收特性。超过 10 Gy 时，由于光谱仪的灵敏度有限，吸收光谱会饱和。改进的单击模型成功地描述了聚合物浓度随辐射剂量的增加，使用对数正态分布来描述棒状单体微晶的长度。从 UV-vis 吸收光谱推断的聚合物含量随剂量的变化与平板扫描方法不同，对剂量变化更敏感。
在 0 至 10 Gy 的剂量范围内，通过考虑活性层的微观结构，使用活性中心的分布来建模聚合物浓度的剂量依赖性。在从光聚合反应动力学模拟变暗时，必须考虑聚合物浓度和吸光度的不同剂量依赖性。
