Room: AAPM ePoster Library
Purpose: To study if the leaf position errors recorded in the MLC leaf trajectory log files can predict gamma-passing rates (GPRs) of ArcCheck-based QA for VMAT patients treated on a Varian Truebeam.
Methods: Varian Truebeam GPR data and leaf trajectory log files were obtained after performing patient-specific QA for 15 head-and-neck and pelvis VMAT treatment plans. Using in-house software with the trajectory-log files, we calculated the root-mean-square (RMS) errors of leaf-positions and the combined errors of monitor units (MUs) and the leaf-positions, which was named as the zeta, for the entire treatment delivery. The patient QA was done using an ionization chamber for point dose measurement and Sun Nuclear ArcCheck for fluence quantification with GPRs of 3%/3mm, 3%/0mm, and 2%/2mm as evaluation parameters. The data were analyzed by using the R-Studio program to find the correlations among the measured physical quantities.
Results: At first glance, we did not observe clear evidence to show large errors of leaf positions and MUs at control points. When the data were analyzed with the statistical software, however, we found a moderate correlation between the GPRs and the errors of both MLC leaves and MUs. Particularly, there were noticeable negative correlations between the maximum zeta value and the 3%/3mm, 2%/2mm GPRs and between the maximum leaf position errors and the point dose errors. Furthermore, the maximum RMS errors were also correlated with the point dose and 3%/0mm GPR.
Conclusion: The zeta is a more sensitive quantity to predict the dosimetric errors observed in the ArcCheck data than other standard error definitions. But even with this, the trajectory log file data are not sufficient to predict the gamma passing rates of treatment delivery against the plans.
Treatment Verification, MLC, Intensity Modulation