Impact of plan parameters and modulation indices on patient-specific QA results for standard and stereotactic VMAT

•Plan parameters in VMAT delivery: mean aperture size, monitor unit, jaw tracking.•Quantification for the degree of modulation: MLC, gantry rotation speed, dose rate.•Main cause of uncertainties in standard VMAT delivery is degree of MLC modulation.•More importance on MU and the use of jaw tracking in stereotactic VMAT. To demonstrate the impact of modulation indices and plan parameters on the gamma passing rates (GPR) of patient-specific quality assurance of standard and stereotactic volumetric modulated arc therapy (VMAT) plans. A total of 758 patients’ QA plans were utilized, including standard VMAT plans with Trilogy (n = 87, group A) and TreuBeam STx (n = 332, group B), and 339 stereotactic VMAT plans with TrueBeam STx (group C). Modulation indices were obtained considering the speed and acceleration of the multileaf collimator (MLC) (MIs, MIa), and MLC, gantry speed, and dose rate changes (MIt). The mean aperture size (MA), monitor unit (MU), and amount of jaw tracking (%JT) were acquired. Gamma analysis was performed with 2 mm/2% and 1 mm/2% for the standard and stereotactic VMAT plans, respectively. Statistical analyses were performed to investigate the correlation between modulation index/plan parameters and GPR. Spearman’s rank correlation to GPRs with MIs, MIa, and MIt, were −0.44, −0.45, and −0.46 for group A; −0.39, −0.37, and −0.38 for group B; and −0.04, −0.11, and −0.10 for group C, respectively. While MU and MA showed significant correlations in all groups, %JT showed a significant correlation only with stereotactic VMAT plans. The most influential parameter combinations were MU-MA (rs = 0.50), MIs-%JT (rs = 0.43), and MU-%JT (rs = 0.38) for groups A, B, and C, respectively. MLC modulation mostly affected the GPR in the delivery of standard VMAT plans, while MU and %JT showed more importance in stereotactic VMAT plans.