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

Abstract

Purpose: 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. Methods: 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) (MI s , MI a ), and MLC, gantry speed, and dose rate changes (MI t ). 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. Results: Spearman's rank correlation to GPRs with MI s , MI a , and MI t , 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 (r s = 0.50), MI s -%JT (r s = 0.43), and MU-%JT (r s = 0.38) for groups A, B, and C, respectively. Conclusions: MLC modulation mostly affected the GPR in the delivery of standard VMAT plans, while MU and %JT showed more importance in stereotactic VMAT plans. © 2019 Associazione Italiana di Fisica Medica