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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：圆柱电离室中气腔尺寸对Co放射治疗束测量影响的实验研究。

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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1970年代后期，Johansson等人（1978年国际辐射剂量计量学国际对称标准（亚特兰大，1977年）第2卷（维也纳：国际原子能机构）第243-70页）报告了通过实验确定的位移校正因子（p（dis）） Co和高能光子束中的圆柱电离室剂量学。这些p（dis）因子已经实现，目前在许多剂量学协议中使用。但是，最近，Wang和Rogers（2009a Phys。Med。Biol。54 1609-20）质疑了这些因素的准确性，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）因子的不准确性源自Johansson等人使用的归一化程序。在他们的实验中，Johansson等人针对每个不同的电离室，在最大电离深度处对测得的深度电离曲线进行了归一化处理。在这项研究中，我们通过实验研究了PMMA幻像和Coγ射线中圆柱电离室的气孔尺寸的影响。使用了两对不同的充气圆柱电离室。每对中的腔室具有相同的结构和材料，但气腔体积（直径）不同。利用20 MeV电子束确定每对两个腔室中的空气质量比。然后使用每对空气中的空气质量比率来比较使用两对室在PMMA体模和Coγ射线束中在不同深度获得的电离比率。圆柱形电离室的空气腔直径不仅影响观察到最大电离的深度，还影响该深度下单位质量空气的电离。在50毫米和100毫米深度处确定的校正小于当前在许多剂量测定规程中使用的校正。此处给出的结果与Wang和Rogers的Monte Carlo模拟的发现相吻合，并表明Johansson等人使用的归一化程序是不正确的。

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