输出因子的两种测量法比较

Comparison between two methods for the measurement of the output factor

目的研究固定机器量和固定信号采集时间两种不同条件下测得的输出因子差别。材料与方法在ＶａｒｉａｎＣｌｉｎｉｃ－６和Ｍｅｖａｔｒｏｎ７７电子直线加速器下采用固定机器输出量和固定测量电离室信号采集时间两种方法测量了输出因子。结果两种结果间最大可有１．２～１．４％的差别，且随射野增大而减少。结论根据机器结构和射线与物质相互作用原理分析，产生这种差异的原因是在光栏上表面的反散射在机器电离室内产生的电离所致。

Purpose To evaluate the difference between the fixed monitor unit and fixed signal collecting time methods in measuring of output factors. Materials and Methods With a fixed monitor unit or a fixed signal collecting time,the output factors were measured under a Varian clinic-6 and a Mevatron 77 linear accelerators. Results The maximum difference between these two methods was 1.2～1.4%which had the decreasing tendency when the radiation field increased. Conclusion This phenomena is due to the back-scatter particles penetration into the beam monitor chamber from the upper surfaces of the collimator and causes its counting according to the structure of the machine and the principle of the interaction between photon and material.