Department of Medical Physics and Biomedical Engineering, Sahlgrenska University Hospital, Gothenburg, Sweden.
Phys Med Biol. 2011 Nov 21;56(22):7093-107. doi: 10.1088/0031-9155/56/22/007. Epub 2011 Oct 21.
In the late 1970s, Johansson et al (1978 Int. Symp. National and International Standardization of Radiation Dosimetry (Atlanta 1977) vol 2 (Vienna: IAEA) pp 243-70) reported experimentally determined displacement correction factors (p(dis)) for cylindrical ionization chamber dosimetry in ⁶⁰Co and high-energy photon beams. These p(dis) factors have been implemented and are currently in use in a number of dosimetry protocols. However, the accuracy of these factors has recently been questioned by Wang and Rogers (2009a Phys. Med. Biol. 54 1609-20), who performed Monte Carlo simulations of the experiments performed by Johansson et al. They reported that the inaccuracy of the p(dis) factors originated from the normalization procedure used by Johansson et al. In their experiments, Johansson et al normalized the measured depth-ionization curves at the depth of maximum ionization for each of the different ionization chambers. In this study, we experimentally investigated the effect of air cavity size of cylindrical ionization chambers in a PMMA phantom and ⁶⁰Co γ-beam. Two different pairs of air-filled cylindrical ionization chambers were used. The chambers in each pair had identical construction and materials but different air cavity volume (diameter). A 20 MeV electron beam was utilized to determine the ratio of the mass of air in the cavity of the two chambers in each pair. This ratio of the mass of air in each pair was then used to compare the ratios of the ionizations obtained at different depths in the PMMA phantom and ⁶⁰Co γ-beam using the two pairs of chambers. The diameter of the air cavity of cylindrical ionization chambers influences both the depth at which the maximum ionization is observed and the ionization per unit mass of air at this depth. The correction determined at depths of 50 mm and 100 mm is smaller than the correction currently used in many dosimetry protocols. The results presented here agree with the findings of Wang and Rogers' Monte Carlo simulations and show that the normalization procedure employed by Johansson et al is not correct.
在 20 世纪 70 年代末，Johansson 等人（1978 年国际辐射剂量学与放射性测量标准化专题讨论会（亚特兰大 1977 年）第 2 卷（维也纳：IAEA），第 243-270 页）报道了用于 60Co 和高能光子束的圆柱形电离室剂量测定的实验确定的位移校正因子（p(dis)）。这些 p(dis)因子已经被实施，并且目前在许多剂量学协议中使用。然而，Wang 和 Rogers（2009a Phys. Med. Biol. 54 1609-20）最近对这些因子的准确性提出了质疑，他们对 Johansson 等人进行的实验进行了蒙特卡罗模拟。他们报告说，p(dis)因子的不准确性源于 Johansson 等人使用的归一化程序。在他们的实验中，Johansson 等人将每个不同电离室在最大电离深度处的测量深度电离曲线归一化。在本研究中，我们在 PMMA 体模和 60Co γ 射束中实验研究了圆柱形电离室的空气腔大小的影响。使用了两对不同的充空气圆柱形电离室。每对中的腔室具有相同的结构和材料，但空气腔体积（直径）不同。利用 20 MeV 电子束确定两对腔室中每个腔室空气腔的空气质量比。然后，使用这两对腔室，将每对的空气质量比用于比较在 PMMA 体模和 60Co γ 射束中不同深度处获得的离子化的比值。圆柱形电离室的空气腔直径会影响观察到最大电离的深度以及在此深度下每单位空气质量的电离。在 50mm 和 100mm 深度处确定的校正值小于许多剂量学协议中当前使用的校正值。这里呈现的结果与 Wang 和 Rogers 的蒙特卡罗模拟的结果一致，并表明 Johansson 等人使用的归一化程序是不正确的。
