Monte Carlo calculated absorbed-dose energy dependence of EBT and EBT2 film
Monte Carlo calculated absorbed-dose energy dependence of EBT and EBT2 film
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DOI：
10.1118/1.3301574
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发表时间：
2010-03-01
期刊：
影响因子：
3.8
通讯作者：
Rogers, D. W. O.
中科院分区：
文献类型：
--
作者：
Sutherland, J. G. H.;Rogers, D. W. O.
方法：利用基于egsnrc的EGS_chamber和DOSRZnrc代码，通过计算光子能量在3 keV ~ 18 MeV范围内的剂量与水的比值，计算吸收剂量的能量依赖关系。将这些数据与水对活性膜材料的质量能量吸收系数比和限制停止功率比以及以往的实验结果进行了比较。结果：在100 keV ~ 18 MeV的光子能量范围内，吸收剂量的能量依赖性在+/- 0.6%以内与能量无关。然而，在100 keV以下，由于水与薄膜材料的质能吸收系数比的变化，以及在薄膜中央表面层产生和散射的电子数量的增加，EBT的吸收剂量能量依赖变化了约10%。结果发现与先前的实验研究不一致，表明在较低的光子能量下存在内在能量依赖的可能性。对于EBT2薄膜，由于活性乳化液层与水的质能吸收系数之比的变化，在低光子能量下的吸收剂量能量依赖随制造批次的不同而变化50%或10%。结论：建议在光子能量低于100 keV时使用GAFCHROMIC EBT/EBT2薄膜。建议未来薄膜的有效原子序数应接近水的原子序数，而活性层越厚越有利。
Methods: The absorbed-dose energy dependence is calculated using the EGSnrc-based EGS_chamber and DOSRZnrc codes by calculating the ratio of dose to water to dose to active film layers at photon energies ranging from 3 keV to 18 MeV. These data are compared to the mass energy absorption coefficient ratios and the restricted stopping power ratios of water to active film materials as well as to previous experimental results.Results: In the photon energy range of 100 keV to 18 MeV the absorbed-dose energy dependence is found to be energy independent within +/- 0.6%. However, below 100 keV, the absorbed-dose energy dependence of EBT varies by approximately 10% due to changes in mass energy absorption coefficient ratios of water to film materials, as well as an increase in the number of electrons being created and scattered in the central surface layer of the film. Results are found to disagree with previous experimental studies suggesting the possibility of an intrinsic energy dependence at lower photon energies. For EBT2 film the absorbed-dose energy dependence at low photon energies varies by 50% or 10% depending on the manufacturing lot due to changes in the ratio of mass energy absorption coefficients of the active emulsion layers to water.Conclusions: Caution is recommended when using GAFCHROMIC EBT/EBT2 films at photon energies below 100 keV. It is recommended that the effective atomic number of future films be produced as close to that of water and that thicker active layers are advantageous.