Ding G X, Rogers D W, Cygler J E, Mackie T R
Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Canada.
Med Phys. 1997 Feb;24(2):161-76. doi: 10.1118/1.597930.
In radiation dosimetry protocols, plastic is allowed as a phantom material for the determination of absorbed dose to water in electron beams. The electron fluence correction factor is needed in conversion of dose measured in plastic to dose in water. There are large discrepancies among recommended values as well as measured values of electron fluence correction factors when polystyrene is used as a phantom material. Using the Monte Carlo technique, we have calculated electron fluence correction factors for incident clinical beam energies between 5 and 50 MeV as a function of depth for clear polystyrene, white polystyrene and PMMA phantom materials and compared the results with those recommended in protocols as well as experimental values from published data. In the Monte Carlo calculations, clinical beams are simulated using the EGS4 user-code BEAM for a variety of medical accelerators. The study shows that our calculated fluence correction factor, phi pw, is a function of depth and incident beam energy Eo with little dependence on other aspects of beam quality. However the phi pw values at dmax are indirectly influenced by the beam quality since they vary with depth and dmax also varies with the beam quality. Calculated phi pw values at dmax are in a range of 1.005-1.045 for a clear polystyrene phantom, 1.005-1.038 for a white polystyrene phantom and 0.996-1.016 for a PMMA phantom. Our values of phi pw are about 1-2% higher than those determined according to the AAPM TG-25 protocol at dmax for clear or white polystyrene. Our calculated values of phi pw also explain some of the variations of measured data because of its depth dependence. A simple formula is derived which gives the electron fluence correction factor phi pw as a function of R50 at dmax or at the depth of 0.6R50-0.1 for any clinical electron beam with energy between 5 and 25 MeV for three plastics: clear polystyrene, white polystyrene and PMMA. The study also makes a careful distinction between phi pw and the corresponding IAEA Code of Practice quantity, hm.
在辐射剂量测定规程中，塑料被允许作为一种模体材料，用于确定电子束中水体的吸收剂量。在将塑料中测量的剂量转换为水中剂量时，需要电子注量修正因子。当使用聚苯乙烯作为模体材料时，电子注量修正因子的推荐值和测量值之间存在很大差异。我们使用蒙特卡罗技术，计算了5至50 MeV的临床入射束能量下，透明聚苯乙烯、白色聚苯乙烯和聚甲基丙烯酸甲酯（PMMA）模体材料的电子注量修正因子随深度的变化，并将结果与规程中推荐的值以及已发表数据中的实验值进行了比较。在蒙特卡罗计算中，使用EGS4用户代码BEAM对各种医用加速器的临床束进行模拟。研究表明，我们计算的注量修正因子φpw是深度和入射束能量Eo的函数，对束流质量的其他方面依赖性很小。然而，dmax处的φpw值会受到束流质量的间接影响，因为它们随深度变化，而dmax也随束流质量变化。对于透明聚苯乙烯模体，dmax处计算的φpw值在1.005 - 1.045范围内；对于白色聚苯乙烯模体，在1.005 - 1.038范围内；对于PMMA模体，在0.996 - 1.016范围内。在dmax处，我们的φpw值比根据AAPM TG - 25规程确定的透明或白色聚苯乙烯的值高约1 - 2%。我们计算的φpw值也解释了一些测量数据的变化，因为它与深度有关。推导了一个简单公式，对于能量在５至２５ＭｅＶ之间的任何临床电子束，给出了透明聚苯乙烯、白色聚苯乙烯和PMMA三种塑料在dmax或0.6R50 - 0.1深度处，电子注量修正因子φpw作为R50的函数。该研究还仔细区分了φpw和相应的国际原子能机构操作规范量hm。
