Comparison of MLC Positioning Deviations Using Log Files and Establishment of Specific Assessment Parameters for Different Accelerators With IMRT and VMAT

Abstract Background and purpose: The study evaluated the differences in leaf positioning deviations by the log files of three advanced accelerators with two delivery techniques, and established specific assessment parameters of leaf positioning deviations for different types of accelerators. Methods: A total of 300 treatment plans with 5 consecutive treatment log files were collected from the Trilogy, TrueBeam and Halcyon accelerators. 50 IMRT and 50 VMAT plans were selected randomly on each accelerator. The log files information was parsed by SunCheck software from Sun Nuclear Corporation. The maximum leaf RMS errors, 95th percentile errors and percentages of different leaf positioning errors were statistically analyzed. The correlations between these evaluation parameters and accelerator performance parameters (maximum leaf speed, mean leaf speed, gantry and arc angle) were analyzed. Results: The average maximum leaf RMS errors of the Trilogy in the IMRT and VMAT plans were 0.45±0.1mm and 0.80±0.07mm, respectively, which were higher than the TrueBeam's 0.03±0.01mm, 0.03±0.01 mm and the Halcyon's 0.06±0.01 mm, 0.07±0.01mm. Similar data results were shown in the 95th percentile error. The maximum leaf RMS errors were strongly correlated with the 95th percentile errors. The leaf positioning deviations in VMAT were higher than those in IMRT for all accelerators. In TrueBeam and Halcyon, leaf position errors above 1 mm were not found in IMRT and VMAT plans. The main influencing factor of leaf positioning deviation was the leaf speed, which has no correlation with gantry and arc angles. Conclusions: Compared with the quality assurance guidelines, the MLC positioning deviations tolerances of the three accelerators should be tightened. For both IMRT and VMAT techniques, the 95th percentile error and the maximum RMS error are suggested to be tightened to 1.5 mm and 1 mm for the Trilogy accelerator respectively. In TrueBeam and Halcyon accelerators, the 95th percentile error and maximum RMS error of 1 mm and 0.5 mm, respectively, are considered appropriate.