Olivares M, DeBlois F, Podgorsak E B, Seuntjens J P
Department of Medical Physics, McGill University Health Centre, Montreal General Hospital, Québec, Canada.
Med Phys. 2001 Aug;28(8):1727-34. doi: 10.1118/1.1388536.
Relative to solid water, electron fluence correction factors at the depth of dose maximum in bone, lung, aluminum, and copper for nominal electron beam energies of 9 MeV and 15 MeV of the Clinac 18 accelerator have been determined experimentally and by Monte Carlo calculation. Thermoluminescent dosimeters were used to measure depth doses in these materials. The measured relative dose at dmax in the various materials versus that of solid water, when irradiated with the same number of monitor units, has been used to calculate the ratio of electron fluence for the various materials to that of solid water. The beams of the Clinac 18 were fully characterized using the EGS4/BEAM system. EGSnrc with the relativistic spin option turned on was used to optimize the primary electron energy at the exit window, and to calculate depth doses in the five phantom materials using the optimized phase-space data. Normalizing all depth doses to the dose maximum in solid water stopping power ratio corrected, measured depth doses and calculated depth doses differ by less than +/- 1% at the depth of dose maximum and by less than 4% elsewhere. Monte Carlo calculated ratios of doses in each material to dose in LiF were used to convert the TLD measurements at the dose maximum into dose at the center of the TLD in the phantom material. Fluence perturbation correction factors for a LiF TLD at the depth of dose maximum deduced from these calculations amount to less than 1% for 0.15 mm thick TLDs in low Z materials and are between 1% and 3% for TLDs in Al and Cu phantoms. Electron fluence ratios of the studied materials relative to solid water vary between 0.83+/-0.01 and 1.55+/-0.02 for materials varying in density from 0.27 g/cm3 (lung) to 8.96 g/cm3 (Cu). The difference in electron fluence ratios derived from measurements and calculations ranges from -1.6% to +0.2% at 9 MeV and from -1.9% to +0.2% at 15 MeV and is not significant at the 1sigma level. Excluding the data for Cu, electron fluence correction factors for open electron beams are approximately proportional to the electron density of the phantom material and only weakly dependent on electron beam energy.
相对于固态水，已通过实验和蒙特卡罗计算确定了Clinac 18加速器标称电子束能量为9 MeV和15 MeV时，骨、肺、铝和铜中剂量最大值深度处的电子注量校正因子。使用热释光剂量计测量这些材料中的深度剂量。当用相同数量的监测单位照射时，测量得到的各种材料中dmax处的相对剂量与固态水的相对剂量，已用于计算各种材料与固态水的电子注量之比。Clinac 18的射束使用EGS4/BEAM系统进行了全面表征。开启相对论自旋选项的EGSnrc用于优化出口窗口处的初级电子能量，并使用优化后的相空间数据计算五种体模材料中的深度剂量。将所有深度剂量归一化为固态水中经阻止本领比校正后的剂量最大值，测量得到的深度剂量与计算得到的深度剂量在剂量最大值深度处相差小于±1%，在其他位置相差小于4%。蒙特卡罗计算得到的每种材料与LiF中剂量的比值用于将剂量最大值处的热释光剂量计测量值转换为体模材料中热释光剂量计中心处的剂量。从这些计算中推导出的LiF热释光剂量计在剂量最大值深度处的注量微扰校正因子，对于低Z材料中0.15 mm厚的热释光剂量计小于1%，对于铝和铜体模中的热释光剂量计在1%至3%之间。对于密度从0.27 g/cm³（肺）到8.96 g/cm³（铜）变化的材料，所研究材料相对于固态水的电子注量比在0.83±0.01和1.55±0.02之间变化。测量值和计算值得出的电子注量比差异在9 MeV时为-1.6%至+0.2%，在15 MeV时为-1.9%至+0.2%，在1σ水平上不显著。不包括铜的数据，开放电子束的电子注量校正因子近似与体模材料的电子密度成正比，且仅微弱依赖于电子束能量。
