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首页> 外文期刊>Physics in medicine and biology. >Experimental investigation of the effect of air cavity size in cylindrical ionization chambers on the measurements in Co radiotherapy beams.
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Experimental investigation of the effect of air cavity size in cylindrical ionization chambers on the measurements in Co radiotherapy beams.

机译：气腔尺寸在圆柱电离室中对CO放射疗法测量的实验研究。

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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 Co 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 Co gamma-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 Co gamma-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.
机译：在20世纪70年代后期，约翰逊等人（1978 int。Symp。国家和国际辐射剂量标准化（Atlanta 1977）Vol 2（维也纳：原子能机构）PP 243-70）报道了实验确定的位移校正因子（P（DIS）） CO和高能光子梁中的圆柱电离室剂量法。这些p（DIS）因素已经实施，目前正在使用多种剂量协议。然而，这些因素的准确性最近被Wang和罗杰斯质疑（2009a的物理学。医学生物学杂志54 1609至20年），谁进行的实验的Monte Carlo模拟约翰松等人进行的。他们报告说，P（DIS）因子的不准确起源于约翰逊等人使用的归一化程序。在实验中，Johansson等人在每个不同的电离室中的每个不同电离室的深度处归一化测量的深度电离曲线。在这项研究中，我们通过实验研究了圆柱电离室在PMMA幻像和COγ梁中的空气腔尺寸的影响。使用两对不同的充气圆柱电离室。每对的腔室具有相同的结构和材料，而是不同的空腔体积（直径）。使用20MeV电子束来确定每对中两个腔室中的空腔中的空气质量的比率。然后使用每对空气质量的这种比率比较使用两对腔室在PMMA幻像和COγ梁中在不同深度处获得的电离的比率。圆柱形电离室的空气腔的直径影响了观察到最大电离的深度，并且在该深度下每单位质量的空气电离。处于50mm和100mm的深度确定的校正小于许多剂量协议中当前使用的校正。此处提出的结果同意王和罗杰斯蒙特卡罗模拟的结果，并表明约翰逊等人采用的正常化程序不正确。

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  • 来源
    《Physics in medicine and biology.》 |2011年第22期|共15页
  • 作者

    Swanpalmer J; Johansson KA;

  • 作者单位

    Department of Medical Physics and Biomedical Engineering Sahlgrenska University Hospital Gothenburg Sweden.;

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