Quality Assurance Procedure for Linear Accelerators Measuring Direct Tissue Maximum Ratio with a Calibration Water Phantom

Abstract

In this paper, we proposed an efficient quality assurance method which can measure direct tissue maximum ratio (TMR_Dir), total scatter factor (S_{cp, Dir}), wedge factor (WF_Dir), tissue phantom ratio 20/10 (TPR20/10_Dir) by using a calibration water phantom and a Farmer chamber. The TMR_Dir was compared with the calculated TMR (TMR_Cal) that was calculated from the percentage depth dose at the time of the linear accelerator installation. S_{cp, Dir}, WF_Dir and TPR20/10_Dir calculated from TMR_Dir were compared with S_{cp, BD}, WF_BD, and TPR20/10_BD measured at the time of the linear accelerator installation. The difference between TMR_Dir and TMR_Cal was approximately within 1% except for using 60° wedge filter. The difference between S_{cp, Dir} and S_{cp, BD} was within 1%, between WF_Dir and WF_BD was within 2%, between TPR20/10_Dir and TPR20/10_BD was within 1%, these differences were acceptable levels of AAPM TG-142 report. Also, coefficient of variation (CV) of TMR_Dir, S_cpDir, WF_Dir and TPR20/10_Dir when changing days and measuring multiple times were approximately within 1%, these CVs were reference levels of AAPM TG-106 report. We validated that was an efficient quality assurance method by measuring direct tissue maximum ratio, but the propose method has limited in measurable field size.