Comparisons of volumetric modulated arc therapy (VMAT) quality assurance (QA) systems: sensitivity analysis to machine errors

Abstract

BackgroundIn volumetric modulated arc therapy (VMAT), gantry angles, dose rate and the MLC positions vary with the radiation delivery. The quality assurance (QA) system should be able to catch the planning and machine errors. The aim of this study was to investigate the sensitivity of three VMAT QA systems to machine errors.MethodsSeveral types of potential linac machine errors unique to VMAT delivery were simulated in sinusoidal function of gantry angle, including gantry angle itself, MLC position and linac output. Two commercial QA systems, ArcCheck and Delta4, and an in-house developed EPID technique were compared in this study. Fifteen full arcs from head and neck plans were selected and modified to include five magnitudes of each type of error, resulting in measurements and γ analyses of 240 arcs on each system. Both qualitative and quantitative comparisons were performed using receiver operating characteristic (ROC), γ pass rate gradient, and overlap histogram methods.ResultsIn ROC analysis, the area under curve (AUC) represents the sensitivity and increases with the error magnitude. Using the criteria of 2 %/2 mm/2° (angle to agreement, ATA, only for EPID) and keeping AUC > 0.95, the minimum error detectable of ArcCheck, Delta4 and EPID are (2, 3, 3)° in gantry angle and (4, 2, 3) mm in MLC positions for the head and neck plans. No system is sensitive to the simulated output error, the AUC values were all below 0.70 even with 5 % output error. The γ gradient for gantry angle, MLC position and output errors are (−5.1, −2.6, −3.6)%/°, (−2.6, −7.1, −3.3)%/mm and (−0.2, −0.2, −0.3)%/% for ArcCheck, Delta4 and EPID, respectively. Therefore, these two analyses are consistent and support the same conclusion. The ATA parameter in EPID technique can be adjusted to tune its sensitivity.ConclusionsWe found that ArcCheck is more sensitive to gantry angle error and Delta4 is more sensitive to MLC position error. All three systems are not sensitive to the simulated output error. With additional analysis parameter, the EPID technique can be tuned to have optimal sensitivity and is able to perform QA for full field size with highest resolution. In addition, ROC analysis avoids the choice of γ pass rate threshold and is more robust compared with other analysis methods.

Online Adaptive Radiation Therapy

“…During the quality assurance process, the plan is reviewed by several staff and often is measured by a QA phantom such as a diode array, ion chamber array, or film. 47,48,49 The…”

Practical application of Octavius®‐4D: Characteristics and criticalities for IMRT and VMAT verification

“…Other assemblies furnish dosimetric data corresponding to the gantry position using phantoms characterized by opportune geometries or rotation capability, as SunNuclear ArcCHECK ® , Scandidos Delta4 ® phantom, the IBA Compass, and PTW Octavius ® ‐4D. Many studies report a comparison of these devices, assessing their different responses as well as their ability in detecting intentional errors,4, 5, 6 or they were used in the validation process of novel QA strategies 7, 8. The present study is focused on PTW Octavius ® ‐4D.…”

A comparison of planned radiotherapy dose distributions calculated by Monte Carlo with variation of technique, statistical uncertainty and voxel size on resulting gamma pass rates

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