Huq M S, Nath R
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut 06510.
Med Phys. 1991 Jan-Feb;18(1):26-35. doi: 10.1118/1.596720.
The IAEA 1987 protocol is an international protocol which has made a number of improvements over the AAPM 1983 protocol for calibration of high-energy photon and electron beams. We present a detailed numerical comparison between the two protocols by calculating (i) Ngas and ND for PTW (PMMA wall), Capintec (air-equivalent plastic wall) and NEL (graphite wall) Farmer type ionization chambers for 60Co gamma rays; (ii) dose-to-water with chamber in water irradiated by 4- or 25-MV x rays; (iii) dose-to-water with chamber in water, PMMA, and polystyrene phantoms irradiated by 5- and 10-MeV electrons; and (iv) dose-to-water with chamber in water irradiated by 20-MeV electrons. For photons, the IAEA protocol gives results which are in good agreement with the AAPM protocol; on average the IAEA results are 0.6% smaller than the AAPM results while discrepancies between the two are in the range of -0.4% to -1.2%. For 10-MeV electrons also, the IAEA protocol gives results which are in excellent agreement with the AAPM protocol; on average the IAEA results are 0.3% smaller than the AAPM results while discrepancies between the two are in the range of -1.0% to +0.5%. In contrast to the above, for 5-MeV electrons, the IAEA protocols give results smaller than the AAPM protocol by 2.0% on average with discrepancies between protocols ranging from -4.1% to -0.7% depending upon the ionization chamber and phantom material used. For 5-MeV electrons, the discrepancies are particularly large for polystyrene phantom; the average discrepancies being -1.4%, -1.1%, and -3.6% for water, PMMA, and polystyrene, respectively. If data for 5-MeV electrons with polystyrene phantom are excluded, then the overall agreement between the two protocols for photons and electrons is within the range of -1.9% to +0.5%. Principal reasons for the observed discrepancies are (i) IAEA uses the correct expression for ND resulting in up to +0.8% correction; (ii) IAEA uses the most recent stopping power ratio for graphite-to-air resulting in up to +0.5% correction; (iii) IAEA uses a correction of up to +0.8% for the central electrode which AAPM ignores; (iv) the present estimates of the percent depth doses which arise from the differences in measurement depths in the two protocols; and (v) IAEA uses measured values of the fluence correction factor while AAPM uses a theoretical estimate resulting in corrections of up to -2.2%.
国际原子能机构（IAEA）1987年协议是一项国际协议，它在1983年美国医学物理师协会（AAPM）的高能光子和电子束校准协议基础上进行了多项改进。我们通过计算（i）60Co伽马射线在PTW（聚甲基丙烯酸甲酯壁）、Capintec（空气等效塑料壁）和NEL（石墨壁）Farmer型电离室中的Ngas和ND；（ii）4兆伏或25兆伏X射线在水中照射时电离室在水中的水吸收剂量；（iii）5兆电子伏和10兆电子伏电子在水中、聚甲基丙烯酸甲酯和聚苯乙烯模体中照射时电离室在水中的水吸收剂量；以及（iv）20兆电子伏电子在水中照射时电离室在水中的水吸收剂量，对这两个协议进行了详细的数值比较。对于光子，IAEA协议给出的结果与AAPM协议高度一致；平均而言，IAEA的结果比AAPM的结果小0.6%，而两者之间的差异在-0.4%至-1.2%范围内。对于10兆电子伏电子，IAEA协议给出的结果也与AAPM协议非常一致；平均而言，IAEA的结果比AAPM的结果小0.3%，而两者之间的差异在-1.0%至+0.5%范围内。与上述情况相反，对于5兆电子伏电子，IAEA协议给出的结果平均比AAPM协议小2.0%，协议之间的差异根据所使用的电离室和模体材料在-4.1%至-0.7%范围内。对于5兆电子伏电子，聚苯乙烯模体的差异特别大；水、聚甲基丙烯酸甲酯和聚苯乙烯的平均差异分别为-1.4%、-1.1%和-3.6%。如果排除聚苯乙烯模体的5兆电子伏电子数据，那么这两个协议在光子和电子方面的总体一致性在-1.9%至+0.5%范围内。观察到的差异的主要原因是：（i）IAEA使用了正确的ND表达式，导致高达+0.8%的修正；（ii）IAEA使用了最新的石墨与空气的阻止本领比，导致高达+0.5%的修正；（iii）IAEA对中心电极使用了高达+0.8%的修正，而AAPM忽略了这一点；（iv）由于两个协议中测量深度的差异而产生的当前百分深度剂量估计值；以及（v）IAEA使用注量修正因子的测量值，而AAPM使用理论估计值，导致高达-2.2%的修正。
