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

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Volumetric Modulated Arc Therapy (VMAT) delivery accuracy is crucial for effective radiation therapy.
• Existing plan complexity metrics (PCMs) may not fully capture the interplay of key delivery variables.
• Predicting gamma pass rates (GPRs) is essential for quality assurance in VMAT.

Purpose of the Study:

• To introduce and validate a novel modulation index for VMAT (MI_VMAT).
• To assess the ability of MI_VMAT to predict GPRs across different linear accelerator platforms.
• To compare the performance of MI_VMAT with existing PCMs.

Main Methods:

• MI_VMAT was derived from beam modulation at control points, gantry speed, dose rate, and Field Weight (FW).
• The index's predictive performance was evaluated by correlating it with measured GPRs for VMAT plans.
• Prospective analysis of patient plans was conducted on two distinct linear accelerator platforms.
• MI_VMAT performance was compared against multivariate prediction models using multiple PCMs.

Main Results:

• MI_VMAT demonstrated strong predictive capability for GPRs on both Unique and TrueBeam linacs, with low deviations across various GPR criteria.
• The univariate MI_VMAT model showed comparable or superior performance to multivariate PCM-based models.
• MI_VMAT exhibited strong to moderate correlations with most established plan complexity metrics.

Conclusions:

• The MI_VMAT model possesses substantial predictive power for VMAT delivery accuracy.
• MI_VMAT offers a robust and potentially more efficient tool for predicting VMAT quality assurance outcomes.
• The findings support the clinical utility of MI_VMAT in optimizing and verifying VMAT plans.