Modulation indices and plan delivery accuracy of volumetric modulated arc therapy

PURPOSE: We evaluated the performance of various modulation indices (MI) for volumetric modulated arc therapy (VMAT) to predict plan delivery accuracy. METHODS: The specific indices evaluated were MI quantifying the mechanical uncertainty (MI (t)), MI quantifying the mechanical and dose calculation uncertainties (MI (c)), MI for station parameter optimized radiation therapy (MI(SPORT)), modulation complexity score for VMAT (MCS (v)), leaf travel modulation complexity score (LTMCS), plan averaged beam area (PA), plan averaged beam irregularity (PI), plan averaged beam modulation (PM), and plan normalized monitor unit (PMU) to predict VMAT delivery accuracy. By utilizing 240 VMAT plans generated with the Trilogy and TrueBeam STx, Spearman's rank correlation coefficients (r) were calculated between the MIs and measures of conventional methods. RESULTS: For the Trilogy system, MI (c) showed the highest r values with gamma passing rates (GPRs) (r = −0.624 with P < 0.001 for MapCHECK2 and r = −0.655 with P < 0.001 for ArcCHECK). For TrueBeam STx, MI (c) also showed the highest r values with GPRs (r = −0.625 with P < 0.001 for the MapCHECK2 and r = −0.561 with P < 0.001 for the ArcCHECK). The MI (t) and MI (c) showed the highest r values to the MLC position errors for the Trilogy and TrueBeam STx systems (r = 0.770 with P < 0.001 and r = 0.712 with P < 0.001, respectively). The PA showed the highest percent of r values (P < 0.05) to differences in the dose‐volume parameters between original VMAT plans and actual deliveries for the Trilogy systems (30.9%). Both the MI (t) and MI (c) showed the highest percent of r values (P < 0.05) to differences in the dose‐volume parameters between original VMAT plans and actual deliveries for the TrueBeam STx systems (31.8%). CONCLUSION: To comprehensively review the results, the MI (c) showed the best performance to predict the VMAT delivery accuracy.