Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA-INMRI, I-00123 Roma, Italy.
Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA-INMRI, I-00123 Roma, Italy.
Phys Med. 2019 Jan;57:221-230. doi: 10.1016/j.ejmp.2018.12.006. Epub 2019 Jan 17.
To calculate by Monte Carlo simulations k factors for Farmer-type ionization chambers in megavoltage photon beams using the new key dosimetry data recommended by the International Commission on Radiation Units and Measurements (ICRU) Report 90.
Monte Carlo calculations were performed with the EGSnrc code system using both the ICRU 90 and the ICRU 37 data. Farmer-type ionization chambers with graphite and plastic walls and with graphite wall and a plastic waterproofing sleeve were considered (Nuclear Enterprise NE 2571, IBA FC65-G and FC65-P). k factors were calculated for photon beams in the range 6-25 MV using phase-space files as input radiation sources. The photon beam qualities in terms of TPR and %dd(10) were established by simulating the depth-dose curves in water. Absorbed doses to the air cavity and to water were calculated using the egs_chamber user code with a target statistical uncertainty below 0.1%.
The update of key dosimetry data according to the ICRU report 90 had an impact of -0.2% in the absorbed dose to water and up to 0.5% in the absorbed dose to the air cavity. Nevertheless, changes partially offset each other when entering in k as ratio, and the final impact on the k values was below 0.3%.
The calculated values of k tend to be lower than the current values in the IAEA TRS-398 protocol with differences up to about 0.5%. A slightly better agreement (within 0.3%) is observed with the Monte-Carlo calculated values provided by the addendum to the AAPM's TG-51 protocol.
使用国际辐射单位与测量委员会（ICRU）报告 90 中推荐的新关键剂量学数据，通过蒙特卡罗模拟计算适用于兆伏光子射束的 Farmer 型电离室的 k 因子。
使用 EGSnrc 代码系统进行蒙特卡罗计算，同时使用 ICRU 90 和 ICRU 37 数据。考虑了具有石墨和塑料壁以及石墨壁和塑料防水套的 Farmer 型电离室（Nuclear Enterprise NE 2571、IBA FC65-G 和 FC65-P）。使用相空间文件作为输入辐射源，为 6-25MV 范围内的光子射束计算 k 因子。通过模拟水中的深度剂量曲线来建立光子射束质量，以 TPR 和 %dd(10)表示。使用 egs_chamber 用户代码计算空气腔和水中的吸收剂量，目标统计不确定性低于 0.1%。
根据 ICRU 报告 90 更新关键剂量学数据，对水的吸收剂量有-0.2%的影响，对空气腔的吸收剂量有高达 0.5%的影响。然而，当进入 k 作为比值时，变化部分相互抵消，对 k 值的最终影响低于 0.3%。
计算出的 k 值往往低于 IAEA TRS-398 协议中的当前值，差异高达约 0.5%。与 AAPM 的 TG-51 协议附录中提供的蒙特卡罗计算值相比，观察到稍好的一致性（在 0.3%以内）。
