Division of Radiation Oncology, Chiba Cancer Center, Chiba, Chiba, 260-8717, Japan.
Graduate School of Human Health Sciences, Tokyo Metropolitan University, Arakawa, Tokyo, 116-8551, Japan.
Med Phys. 2019 Nov;46(11):5185-5194. doi: 10.1002/mp.13743. Epub 2019 Aug 31.
The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (s and s ) and the individual perturbation correction factors P was calculated. The effect on the beam quality conversion factors k for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol.
The s , s , individual P and k were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the P and k were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one Co beam and ten photon beams with nominal energy between 4 MV and 25 MV. Then, the change in k for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of Co beam and 24 MV photon beam were used.
The significant changes (p < 0.05) were observed for s , s , the wall correction factor P , and the waterproofing sleeve correction factor P . The decrease in s varied from -0.57% for a Co beam to -0.36% for the highest beam quality. The decrease in s varied from -0.72% to -1.12% in the same range. The changes in P and P were up to 0.41% and 0.14% and those maximum changes were observed for the Co beam. All changes in the central electrode correction factor P , the stem correction factor P , and the replacement correction factor P were from -0.02% to 0.12%. Those changes were statistically insignificant (p = 0.07 or more) and were independent of photon energy. The change in k was mainly characterized by the change in s , P , and P . The relationship between the change in k and the beam quality index was linear approximately. The changes in k of the simple-models were agreed with those of the precise-models within 0.08%.
The effects of ICRU-90 recommendations on k for the ionization chambers listed in the Addendum to AAPM's TG-51 protocol were from -0.15% to 0.30%. To remove the known systematic effect on the clinical reference dosimetry, the k based on ICRU-37 should be updated to the k based on ICRU-90.
ICRU 发布了新的电离辐射剂量学建议。在这项工作中，计算了这些建议对水-空气和石墨-空气限制质量电子阻止本领比（s 和 s ）以及个体微扰修正因子 P 的影响。还估计了这些建议对列出在 AAPM 的 TG-51 协议附录中的所有电离室的高能光子束参考剂量转换系数 k 的影响。
使用 EGSnrc 蒙特卡罗代码系统计算了 s 、 s 、个体 P 和 k ，并使用了 ICRU 报告 37 和 ICRU 报告 90 的关键数据。首先，使用 8 个电离室的精确模型计算了 P 和 k ：NE2571（核企业）、30013、31010、31021（PTW）、Exradin A12、A12S、A1SL（标准成像）和 FC-65P（IBA）。在这种模拟中，辐射源是一个 60Co 束和十个标称能量在 4MV 到 25MV 之间的光子束。然后，通过改变 AAPM 的 TG-51 协议附录中列出的电离室的简单模型的规格来计算 k 的变化。简单模型仅由圆柱形组件模块制成。在这种模拟中，使用 60Co 束和 24MV 光子束作为辐射源。
s 、 s 、壁修正因子 P 和防水套修正因子 P 都发生了显著变化（p<0.05）。对于 60Co 束，s 的降低幅度从-0.57%到最高束质的-0.36%。在相同范围内，s 的降低幅度从-0.72%到-1.12%不等。P 和 P 的变化可达 0.41%和 0.14%，最大变化发生在 60Co 束中。中央电极修正因子 P 、杆修正因子 P 和替代修正因子 P 的变化均在-0.02%至 0.12%之间。这些变化在统计学上无显著性差异（p≥0.07），且与光子能量无关。k 的变化主要由 s 、 P 和 P 的变化决定。k 的变化与束质指数之间的关系大致呈线性。简单模型的 k 变化与精确模型的 k 变化一致，相差在 0.08%以内。
ICRU-90 建议对 AAPM 的 TG-51 协议附录中列出的电离室的 k 的影响在-0.15%到 0.30%之间。为了消除临床参考剂量学上已知的系统效应，应该用基于 ICRU-37 的 k 更新基于 ICRU-90 的 k。
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