Reft C S, Kuchnir F T
Department of Radiation and Cellular Oncology, University of Chicago, Illinois 60637.
Med Phys. 1991 Nov-Dec;18(6):1237-43. doi: 10.1118/1.596740.
When parallel-plate chambers are used for dosimetry in electron fields, the AAPM dosimetry protocol recommends a value of 1.0 for the replacement correction factor, P(repl),pp,E, until further data become available. Here, P(repl),pp,E for five commercially available parallel-plate chambers was measured as a function of electron energy from a nominal value of 5.5 to 22 MeV by comparison with a cylindrical chamber whose P(repl),cyl,E was obtained from data in the protocol. Since this method is based on the concept of a constant value for Ngas,pp, the energy and modality dependence of Ngap,pp is also investigated for these chambers for Co-60, 4-, 6-, 24-MV photons and for 22-MeV electrons. It is found that for three of the chambers P(repl),pp,E is independent of energy, consistent with unity within one or two standard deviations (s.d.). For the fourth chamber P(repl),pp,E is similarly consistent with one above 10 MeV, but decreases at lower energies, while for the fifth one it shows a systematic drop with decreasing energy.
当平行板电离室用于电子束剂量测定时，在有更多数据可用之前，美国医学物理师协会（AAPM）剂量测定协议推荐替代修正因子P(repl),pp,E的值为1.0。在此，通过与一个圆柱形电离室进行比较，测量了五种市售平行板电离室的P(repl),pp,E随电子能量的变化情况，电子能量范围从标称值5.5 MeV到22 MeV，圆柱形电离室的P(repl),cyl,E是根据协议中的数据获得的。由于该方法基于Ngas,pp为恒定值的概念，还研究了这些电离室对于Co-60、4-MV、6-MV、24-MV光子以及22-MeV电子的Ngap,pp的能量和模态依赖性。结果发现，对于其中三个电离室，P(repl),pp,E与能量无关，在一两个标准差（s.d.）范围内与1一致。对于第四个电离室，P(repl),pp,E在10 MeV以上同样与1一致，但在较低能量时下降，而对于第五个电离室，它随能量降低呈现系统性下降。
