直线加速器注入电流与照射野对称性的关系研究

目的：电子直线加速器的注入电流在使用中会发生改变。从而对剂量学参数照射野对称性产生影响。作者对注入电流的变化规律及其造成照射野对称性改变的程度进行实验研究。探讨加速器照射野质量控制的要求。方法：手动模拟加速器注入电流的改变。在照射野对称性良好的注入电流工作点附近设置正负模拟值，以胶片法测最和分析对应的照射野对称性改变。结果：注入电流的数值在加速器的枪靶方向不会影响照射野对称性的质量。在左右方向的对称性偏差值受注入电流数值的影响近似为线性关系，当注入电流偏离中值0.2A时，相应的对称性误差接近3％。作者工作部门的正常工作量下注入电流每月约下降（0．1～0．2）A。结论：检测加速器注入电流的改变可以提示照射野对称性的误差。照射野剂量特性和注入电流应纳入每周的质控检验内容。

The Relationship between Field Symmetry and Injecting Current Fluctuation of Linear Accelerator

Objective: To Study on the relationship between the change of injection current,field symmetry of linear accelerator,frequency and efficiency of calibration on gun filament,and to establish the necessary quality control(QC) procedure to ensure a better field symmetry for clinical use.Methods: In the service mode of a clinically used linear accelerator,the gun filament current was changed manually with a step of 0.1 A in both positive and minus way from the symmetric point.The corresponding field symmetry and flatness were measured with films.Results: When the injection value was changed 0.2 A away from the best field symmetry point,the symmetry and flatness became worse 3%.Approximately,the actual injection current became(0.1~0.2) A smaller every month for our normal workload.Conclusions: It is possible to evaluate the change on symmetry and flatness caused from the fluctuation of injection current,by monitoring the actual value of the injection current.Periodical checking and calibrating on injection current or gun filament current should be taken to have a good field symmetry and flatness.