摘要目的 针对无均整器(FFF)光子束,建立一种描述其射野离轴比(OAR)曲线特性的新方法.方法 用扫描水箱测量得到Varian Edge、Elekta VersaHD、Tomotherapy三类治疗机在FFF模式下1.5、5.0、10.0、20.0 em深度处的OAR曲线,然后分别由野内向野外、由野外向野内求离轴比的二次导数,前者的最大值和后者的最小值所在位置连线的中点定义为射野边界,左右两侧射野边界之间的距离定义为剂量学射野大小.用高斯函数对80％射野范围内的OAR曲线进行拟合,拟合所得参数用于描述OAR形状特点.结果 用新方法计算得到的射野大小误差＜0.11 cm,射野中心误差＜0.05cm.用高斯函数拟合OAR曲线时,拟合相关系数＞0.998.OAR曲线拟合误差随深度的变化不明显,但随射野增大略有增加,10、20、30、40 cm射野时拟合最大误差分别为0.49％、0.67％、1.25％、2.52％.结论 建立一种无需借助有均整器光子束即可独立描述FFF光子束射野离轴比特性的方法.该方法能计算FFF光子束射野大小,可用高斯函数拟合FFF光子束OAR曲线形状.

abstractsObjective To establish a novel method of describing the off-axis ratio (OAR) characteristics of the flattening filter-free (FFF) beam.Methods The OAR curves at a depth of 1.5,5,10 and 20 cm were measured for Varian Edge,Elekta VersaHD and Tomotherapy using the water tank.The second derivatives of the OAR in the positive and negative directions were calculated.The center of the line connecting the maximum and minimum second derivatives was defined as the field edge.The distance between the left and right field edges was defined as the dosimetric field size.The OAR curve within the 80％ of dosimetric field size was fitted using the gaussian function and the fitting parameters were adopted to describe the shape of OAR.Results The calculated field size error was less than 0.11 cm and the central axis position error was less than 0.05 cm.The fitting correlation coefficient was greater than O.998.The fitting maximum error of OAR curve did not significantly alter with the depth,whereas slightly increased over the increased field size.The maximum error for a field size of 10,20,30 and 40 cm was 0.49％,0.67％,1.25％ and 2.52％,respectively.Conclusions A method which can independently and accurately describe the OAR characteristics of FFF photon beam is established for the first time,which can calculate the field size of FFF beam and fit the OAR curve of FFF beam using the gaussian function.