Author: Yunfeng Cui, Will Giles, Xinyi Li, Ke Lu, Jennifer C. O'Daniel, Anna E. Rodrigues, Chunhao Wang, Lana Wang, Yibo Xie, Sua Yoo, Jingtong Zhao 👨🔬
Affiliation: Duke University, Duke University Medical Center 🌍
Purpose:
To establish quantitative measurement of plan modulation for SRS/SBRT treated with Volumetric Modulated Arc Therapy (VMAT) on C-arm LINACs.
Methods:
A total of 461 plans were analyzed, including 106 SRS and 355 SBRT plans (liver, lung, spine, pancreas, prostate, bony oligometastasis). Plans were created using Eclipse™v16.0. Two quantitative evaluators of VMAT plan parameters were introduced: 1) Radiation Modulation Complexity Score (RMCS), which quantifies MLC aperture similarity and leaf travel distances of all control points, weighted by instantaneous dose rate. Higher values indicate stronger modulations; and 2) the ratio of target equivalent sphere surface area to MU-weighted average MLC aperture size (SA/MLC), with higher values suggesting stronger modulations due to beam design.
A script-based GUI was developed to calculate RMCS, SA/MLC, MU/cGy per fraction, and Eq.Sq. of MU-weighted MLC aperture size (FSMLC). Spearman’s rank correlation and Wilcoxon rank sum tests were used to analyze relationships between these metrics and compare treatment sites.
Results:
The average RMCS across sites were as follows: lung(0.91), pancreas(1.47), prostate(1.50), bony oligometastasis(1.60), liver(1.71), spine(1.78), and SRS(2.66). RMCS and MU/cGy rankings aligned partially, and positive correlation between RMCS and MU/cGy was observed for most sites (except pancreas). SA/MLC results showed the following ranking: SRS(17.14), lung(19.88), bone(27.10), liver(36.73), spine(37.99), prostate(42.49), and pancreas(94.80). RMCS did not show significant correlations with SA/MLC, except for lung, suggesting that these metrics assess different aspects of treatment complexity. Negative correlations between RMCS and FSMLC were found for several treatment sites, confirming that stronger beam modulation is associated with smaller MLC apertures.
Conclusion:
RMCS and SA/MLC metrics provide insights into SRS/SBRT planning and evaluation. RMCS assesses MLC dynamic complexity, while SA/MLC reflects beam design complexity relative to target shape(s). The script-based GUI will be made publicly available to support future multi-institutional studies in establishing standardized guidelines for radiation modulation evaluation in SRS/SBRT.