Commissioning and verification of a 3D Monte Carlo independent calculation software for O-ring linac systems.

BACKGROUND

O-ring linac systems improve radiotherapy efficiency but require rigorous pretreatment verification due to increased delivery uncertainty in IMRT/VMAT. Existing methods face limitations: measurement-based approaches incur setup errors, while calculation-based methods (e.g., Monte Carlo) need machine-specific validation per AAPM TG-219.

PURPOSE: To commission the O-ring linac model in RadCalc Monte Carlo software and establish its clinical dosimetric accuracy.

METHODS

The additional radiation to light field offset (ARLFO) parameter in RadCalc was adjusted from -0.08 cm (-0.8 mm) to +0.08 cm (+0.8 mm) (nine sets). TG-119 and clinical benchmark cases were used to design IMRT/VMAT plans. Verification plans were measured experimentally, and all plans were imported into RadCalc for secondary dose calculation. Triangular gamma analysis (3%/2 mm) compared Monte Carlo-simulated, measured, and TPS-calculated doses. The optimal ARLFO was determined by weighted averaging of gamma pass rates. The model was validated on 10 clinical cases per site (head, thorax, abdomen, pelvis).

RESULTS

Commissioning identified the optimal ARLFO parameter as -0.02 cm (-0.02 mm). Under the gamma analysis criterion of 3%/2 mm, the comparison between TPS-calculated doses and Monte Carlo-calculated doses for 10 clinical plans across four anatomical sites yielded: 98.9% ± 0.8% (head and neck), 98.5% ± 0.8% (thorax), 99.6% ± 0.3% (abdomen), and 99.1% ± 0.5% (pelvis). For verification plans, the gamma pass rates between Monte Carlo-calculated doses and measured doses were 97.5% ± 2.8% (head and neck), 95.6% ± 2.8% (thorax), 96.3% ± 2.9% (abdomen), and 99.4% ± 0.5% (pelvis), while comparisons of Monte Carlo-calculated doses versus TPS-calculated doses reached 99.5% ± 0.8% (head and neck), 99.2% ± 1.0% (thorax), 99.5% ± 0.9% (abdomen), and 99.9% ± 0.2% (pelvis), demonstrating consistent dosimetric accuracy of the optimized model across all clinical sites.

CONCLUSION

This study establishes a commissioning methodology to determine the optimal ARLFO value for RadCalc, enabling clinics to achieve reliable independent plan verification for O-ring linac.