Plan Complexity Reduction In Lung SBRT Using the Aperture Shape Controller and MU Objective
Abstract
Purpose
Lung stereotactic body radiation therapy (SBRT) delivers high doses with tight margins and is often associated with excessive modulation characterized by irregular apertures, high number of monitor units (MU), and significant multi-leaf collimator (MLC) motion. This may prolong treatment time and increase sensitivity to delivery uncertainties (e.g., MLC interplay effect with respiratory motion). In this study we investigate the feasibility of reducing lung SBRT plan complexity while maintaining clinically acceptable dosimetric metrics using two Eclipse (Varian) treatment planning system tools: the Aperture Shape Controller (ASC) and the MU Objective.
Methods
Twenty-seven lung SBRT treatment cases were retrospectively studied. For each case, three MU Objective re-optimized plans were generated using the following parameters: (1) Maximum 2000 MU (Strength 75), (2) Maximum 2000 MU (Strength 100), (3) Maximum 1500 MU (Strength 100), to compare with the original plan. Independently, five ASC plans were generated at Very Low, Low, Moderate, High, and Very High levels. Complexity metrics included MU/cGy, small aperture size ratio (SAS10mm), 1 minus modulation complexity score (1-MCS), and MLC irregularity score.
Results
MU Objective reduced MU/cGy by approximately 20-40% and SAS10mm from 0.45 to 0.23-0.33 on average across settings. MU Objective also decreased 1-MCS from 0.83±0.03 to 0.67-0.78. In contrast, ASC primarily improved MLC smoothness, reducing the MLC irregularity score from 2.14±0.30 cm (No ASC) to 1.72±0.21 cm (Very High ASC), with only a modest reduction in 1-MCS from 0.83±0.03 to 0.80±0.04 and no significant changes in MU/cGy or SAS10mm.
Conclusion
MU Objective achieved the largest reductions in MU/cGy, SAS10mm, and 1-MCS, indicating a substantial decrease in modulation complexity, while ASC mainly reduced MLC irregularity to smooth MLC movement. Using these optimization tools can reduce complexity, improve delivery efficiency and robustness in lung SBRT plans while achieving clinically acceptable plan goals.