Department of Radiation Oncology, Radiation Physics, and Princess Margaret Hospital, University of Toronto, Toronto, Ontario M5G 2M9, Canada.
Med Phys. 2010 Feb;37(2):571-6. doi: 10.1118/1.3291622.
The purpose of this study was to quantify the extent of energy dependence of Gafchromic film to x-ray energies ranging in quality from 105 kVp to 6 MV, and relate this dependency to the film's chemical composition and date of production.
Lots of Gafchromic EBT film manufactured in 2004 and 2005 together with more recent batches produced in 2007 were evaluated for energy dependence. Multiple batches of EBT-2 film were also evaluated. Energy dependence was quantified as Rx-the ratio of net optical density (netOD) measured at a given energy x relative to the netOD measured at 6 MV, as measured on a linear accelerator. Rx was evaluated for beam qualities of 105 and 220 kVp on a clinical orthovoltage unit using two separate techniques-a flatbed scanner (Epson) and a real-time fiber-optic readout system. Neutron activation analysis for chlorine and bromine content was performed on all the films to determine whether the composition of the film had changed between batches of film exhibiting different energy dependence responses.
For batches of EBT manufactured in 2007, R105 kVp was 0.75 and R220 kVp was 0.85, indicating an under-response at orthovoltage energies. These results were confirmed using both the Epson flatbed scanner as well as the real-time readout system. For batches of EBT film manufactured before 2006, Rl05 kVp ranged from 0.9 to 1.0. The results from the neutron activation analysis confirmed a direct relationship between the concentration of chlorine and the magnitude of under-response at orthovoltage energies. EBT-2 film exhibited R105 kVp values ranging from 0.79 (under-response) to 1.20 (over-response) among batches containing varying concentrations of bromine, chlorine, and potassium.
The results of this study indicated that differences in energy response of EBT and EBT-2 films were due to differences in the chemical composition and therefore the effective atomic number of the film, which have changed over time. To achieve an energy independent dosimeter over a range of kilovoltage energies, the effective atomic number of the dosimeter must be closely matched to that of water. Small deviations in chemical composition can lead to large deviations in response as a function of energy.
本研究的目的是量化 Gafchromic 胶片对从 105 kVp 到 6 MV 的 X 射线能量的依赖程度，并将这种依赖性与胶片的化学成分和生产日期联系起来。
评估了 2004 年和 2005 年制造的大量 Gafchromic EBT 胶片以及 2007 年生产的较新批次的胶片，以评估其能量依赖性。还评估了多个批次的 EBT-2 胶片。能量依赖性被量化为 Rx——给定能量 x 下测量的净光学密度（netOD）与在直线加速器上测量的 6 MV 下测量的 netOD 的比值。在临床正交电压装置上使用两种不同的技术——平板扫描仪（Epson）和实时光纤读出系统，评估了 105 和 220 kVp 的束质。对所有胶片进行氯和溴含量的中子活化分析，以确定在表现出不同能量响应的胶片批次之间，胶片的成分是否发生了变化。
对于 2007 年制造的 EBT 批次，R105 kVp 为 0.75，R220 kVp 为 0.85，表明在正交电压能量下响应不足。这些结果使用 Epson 平板扫描仪和实时读出系统得到了证实。对于 2006 年之前制造的 EBT 胶片批次，R105 kVp 范围为 0.9 至 1.0。中子活化分析的结果证实了氯浓度与正交电压能量下响应不足的程度之间存在直接关系。EBT-2 胶片在含有不同浓度的溴、氯和钾的批次中，R105 kVp 值的范围从 0.79（响应不足）到 1.20（响应过度）。
本研究的结果表明，EBT 和 EBT-2 胶片的能量响应差异是由于胶片的化学成分（因此有效原子数）随时间的变化而导致的。为了在千伏电压范围内实现能量独立的剂量计，剂量计的有效原子数必须与水的有效原子数紧密匹配。化学成分的微小偏差会导致能量响应的大偏差。
