Evaluation of electron Monte Carlo algorithm accuracy for dose calculations in extended source-to-surface distances electron beam therapy.

BACKGROUND

Extending the source-to-surface distance (SSD) is an effective approach to cover a large irradiation area in electron beam therapy for large planning target volumes (PTVs). However, the accuracy of dose calculations at extended SSDs has not been fully validated.

PURPOSE: This study evaluated the dose calculation accuracy of the electron Monte Carlo (eMC) algorithm implemented in Varian's Eclipse radiation therapy planning system (RTPS) under extended SSD conditions.

METHODS

Simulations were conducted using Eclipse version 13.6 for 6, 12, and 18 MeV electron beams, with SSDs ranging from 100 to 140 cm. A 25 cm × 25 cm applicator and a virtual water phantom were utilized to compute percent depth dose (PDD), off-axis ratio (OAR), and output factor (OPF). The calculated values were compared with measured data and independent Monte Carlo (MC) simulations performed using Particle and Heavy Ion Transport System (PHITS), referred to as PHITS-MC in this study.

RESULTS

The eMC algorithm achieved high accuracy along the central axis, with PDD deviations within 2 mm and OPF differences within 3% across all SSDs, including 140 cm. However, eMC exhibited increasing deviations in OAR field size (>3 mm) at SSD ≥ 120 cm. A detailed parameter-based analysis further revealed underperformance in OAR calculations at field peripheries for low-energy beams (6 MeV), compared with PHITS-MC.

CONCLUSION

The findings delineate the performance and limitations of the eMC algorithm under extended SSD conditions. These limitations should be considered in algorithm evaluation and quality assurance processes. The results provide guidance for algorithm assessment and may serve as a foundation for future studies to explore its clinical relevance in large-field electron beam therapy.