Dosimetric effect of MLC speed obtained with machine log files of VMAT delivery

Abstract

This paper aims to investigate the stability of volumetric modulated arc therapy (VMAT) delivery during treatment using the multi-leaf collimator (MLC) speed recorded in the log files. The mechanical error, such as the position error and speed of the MLC recorded in the log file, affects the dose accuracy of the VMAT delivery. We evaluated the stability of VAMT delivery by determining the relationship between the gamma pass rate (GPR) and MLC speed. The 15 VMAT plans were used to investigate the stability of the beam delivery. The Mobius software calculated the dose distribution by the log files from the planning CT image set. The GPR was evaluated by comparing the reconstructed dose with the planned dose. To validate the gamma analysis calculated from the log file, we compared the results with the GPR measured by the ArcCHECK phantom. A comparison of the GPRs between ArcCHECK and Mobius was performed based on different criteria: 3%/3 mm and 2%/2 mm. Based on the results, we represent that (1) the 2%/2 mm criterion of the Mobius system is comparable to perform the quality assurance (QA) with ArcCHECK using the criterion of 3%/3 mm, and (2) the interfractional variation of the VMAT delivery depends on the distribution of the MLC speed. In this study, we determined that the variation of the GPRs during the whole treatment is less than 2% and the average leaves speed is less than 10 mm/s. These results reveal that the log files are one of the indicators to monitor the variation of VMAT delivery.