Dose accuracy evaluation of multi-leaf collimator modeling in single-isocenter multi-target stereotactic radiosurgery

Accurate modeling of multi-leaf collimators is critical for dose calculation in stereotactic radiosurgery (SRS), where steep dose gradients and small target volumes amplify modeling uncertainties. This study compared enhanced leaf modeling and dosimetric leaf gap approaches using two clinically implemented dose calculation algorithms in single-isocenter SRS plans.

Sixty clinical volumetric modulated arc therapy (VMAT) single-isocenter SRS plans (30 single-target, SIST; 30 multi-target, SIMT) were selected and measured with a two-dimensional diode array, normalized to a point measurement. Dose was calculated using both enhanced leaf model (ELM) and dosimetric leaf gap (DLG) models and the anisotropic analytical algorithm (AAA) and Acuros XB (AXB) algorithms across several spot-size configurations. Plan quality assurance was assessed using gamma analysis for several dose-distance criteria. Dose differences were further analyzed as functions of target size and distance from isocenter.

Under 2%/1 mm gamma criteria, AXB-DLG having spot size 0.5 × 0.5 mm² showed the highest median and mean gamma pass rate (fraction of points having gamma >1), 98.7% and 97.4% respectively, while AXB-ELM having spot-size 0.5 × 0.7 mm² had the lowest, 98% and 96.3%, respectively. SIST and SIMT plans showed no clinically significant differences. AAA-ELM (spot size: 0 × 0 mm²) achieved the highest median and mean gamma values, while AXB-DLG remained robust across plan types. Dose calculations were accurate for both configurations, with minimal mean differences (0.0%, range - 2.7% to +1.8%) reported.

DLG showed slightly better VMAT SRS agreement than ELM, but differences were clinically negligible. ELM's model-based MLC configuration removes user-specific DLG variability, potentially improving dose-calculation precision and consistency across institutions.