MI_VMAT: A New Comprehensive Modulation Index for Volumetric Modulated Arc Therapy Based on Three Key Variables

This study introduces a novel modulation index for volumetric modulated arc therapy (MI_VMAT), which simultaneously accounts for the combined effects of three key delivery variables. The index is validated for its ability to predict gamma pass rates (GPRs) across two different linear accelerator platforms.

MI_VMAT is derived from beam modulation at the control points (CPs) of VMAT plans. Modulation due to variations in gantry speed and dose rate is represented by changes in meter set weight across CPs and combined with the corresponding aperture area at each CP. In addition, the Field Weight (FW=MU/cGy), obtained from plan optimization and inherently dependent on these variables, is incorporated into the MI_VMAT formulation. The predictive performance of MI_VMAT was evaluated by analyzing its correlation with measured GPRs for VMAT plans delivered on two linear accelerators with different characteristics. Its performance was also compared with previously published and commonly used plan complexity metrics (PCMs). Patient plans were prospectively analyzed on both machines.

MI_VMAT demonstrated strong predictive capability. For the Unique linac, deviations were within 3% for GPR criteria of 3%/3 mm and 2%/3 mm. For the TrueBeam linac, deviations were within 1% for GPR criteria of 3%/3 mm, 3%/2 mm, and 3%/1 mm. Compared with a multivariate prediction model incorporating multiple PCMs, the univariate MI_VMAT model showed comparable or slightly superior performance, with lower mean deviations and reduced variability. MI_VMAT also exhibited strong to moderate correlations with most established metrics.

The univariate MI_VMAT model shows substantial predictive power for VMAT delivery accuracy and performs comparably or better than multivariate PCM-based models, supporting its clinical utility.