Tatcher M, Bjärngard B E
Department of Radiation Oncology, Roger Williams Cancer Center, Brown University, Providence, Rhode Island 02908.
Med Phys. 1993 Jul-Aug;20(4):1229-32. doi: 10.1118/1.596973.
The tables of equivalent fields published by the British Journal of Radiology (BJR) are intended for calculation of depth-dose functions in rectangular photon fields. We have investigated the validity of the equivalent-field concept for fields of arbitrary shape over a range of photon energies, field sizes and depths. We show that the empirical scatter-radius function (Day function) used to generate the equivalent-field tables is a good approximation to the average over energy of normalized scatter-air ratios extracted from BJR beam data for depths up to 10 cm. However, this function tends to diverge from the data as depth increases. Accuracy can be improved by making the Day function depend on depth. Equivalent squares, determined by sector integration of the original or modified Day functions, are suitable for megavoltage photon-beam dose calculations at central-axis and off-axis points in irregular as well as rectangular fields.
《英国放射学杂志》（BJR）发布的等效射野表旨在用于计算矩形光子射野中的深度剂量函数。我们研究了等效射野概念在一系列光子能量、射野大小和深度条件下对于任意形状射野的有效性。我们表明，用于生成等效射野表的经验散射半径函数（戴氏函数），对于从BJR射束数据中提取的、深度达10 cm的归一化散射空气比的能量平均值而言，是一个很好的近似。然而，随着深度增加，该函数往往会偏离数据。通过使戴氏函数依赖于深度，可以提高准确性。通过对原始或修正后的戴氏函数进行扇形积分确定的等效方野，适用于不规则和矩形射野中中心轴和离轴点的兆伏级光子束剂量计算。
