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Accuracy limits of the equivalent field method for irregular photon fields

机译:不规则光子场的等效场法的精度极限

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A mathematical approach is developed to evaluate the accuracy of the equivalent field method using basic clinical photon beam data. This paper presents an analytical calculation of dose errors arising when field equivalencies, calculated at a certain reference depth, are translated to other depths. The phantom scatter summation is expressed as a Riemann-Stieltjes integral and two categories of irregular fields are introduced: uniform and multiform. It is shown that multiform fields produce errors whose magnitudes are nearly twice those corresponding to uniform fields in extreme situations. For uniform field shapes, the maximum, local, relative dose situations. For uniform field shapes, the maximum, local, relative dose errors, when the equivalencies are calculated at 10 cm depth on the central axis and translated to a depth of 30 cm, are 3.8% and 8.8% for 6 MV and cobalt-60 photon beams, respectively. In terms of maximum dose those errors are within 1-2%. This supports the conclusion that the equivalencies between rectangular fields, which are examples of uniform fields, are applicable to dose ratio functions irrespective of beam energy. However, the magnitude of such errors could be of importance when assessing the exist dose for in vivo monitoring. This work provides a better understanding of the influence of the irregular field shapes on the accuracy of the equivalent field method.
机译:开发了一种数学方法来评估使用基本临床光子束数据的等效场法的准确性。本文介绍了将在一定参考深度处计算出的等效场换算为其他深度时产生的剂量误差的分析计算方法。幻像散布总和表示为黎曼-斯蒂格斯积分,并引入了两类不规则场:均匀场和多重场。结果表明,在极端情况下,多形式场产生的误差几乎是对应于均匀场的误差的两倍。对于均匀的场形状,最大,局部,相对剂量情况。对于均匀的场形状,当在中心轴上10 cm深度处计算等效量并将其转换为30 cm深度时,最大,局部,相对剂量误差对于6 MV和钴60光子而言分别为3.8%和8.8%梁。就最大剂量而言,这些误差在1-2%之内。这支持了这样的结论,即矩形场之间的等价度是均匀场的示例,无论光束能量如何,都适用于剂量比函数。但是,当评估体内监测的现有剂量时,此类误差的大小可能很重要。这项工作可以更好地理解不规则场形状对等效场法精度的影响。

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