Modulation indices and plan delivery accuracy of volumetric modulated arc therapy

Abstract

Abstract 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 ST x, Spearman's rank correlation coefficients ( r ) were calculated between the MI s and measures of conventional methods. Results For the Trilogy system, MI c showed the highest r values with gamma passing rates ( GPR s) ( r = −0.624 with P < 0.001 for Map CHECK 2 and r = −0.655 with P < 0.001 for Arc CHECK ). For TrueBeam ST x, MI c also showed the highest r values with GPR s ( r = −0.625 with P < 0.001 for the Map CHECK 2 and r = −0.561 with P < 0.001 for the Arc CHECK ). The MI t and MI c showed the highest r values to the MLC position errors for the Trilogy and TrueBeam ST x 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 ST x systems (31.8%). Conclusion To comprehensively review the results, the MI c showed the best performance to predict the VMAT delivery accuracy.