Improved Monte Carlo clinical electron beam modelling

Abstract

Purpose: An EGSnrc based electron model was developed and validated for an Elekta Synergy® 160-leaf Agility™ linear accelerator. It was able to reproduce measured central axis (CAX) percentage depth dose (PDD) curves and off-axis profiles (OAPs) within 2%/2 mm, and relative output factors (ROFs) within 3%.

Methods: BEAMnrc component modules were used to model the accelerator accurately according to vendor supplied specifications. The electron beam focal spot size and input energy spectrum were determined through their effects on electron CAX PDDs and OAPs as benchmarked against water tank data. Phase space files were used as source input in DOSXYZnrc water phantom simulations. Dose distributions were calculated for six electron nominal energies, 11 field sizes and two source-to-surface distances.

Results: The full width at half maximum of the focal spot (assuming a Gaussian intensity distribution) was determined to be 1.50 mm. An asymmetrical input electron energy spectrum with a low-energy tail produced good agreement with measured data and solved the match in the build-up (BU) region for all electron energies used in this study.

Conclusions: The improved input electron spectra for the electron model could predict dose distributions within 2%/2 mm of measured data. The model's success is embedded in the asymmetrical energy spectrum which provided a valuable free parameter which, by fine adjustment, improved the match in the BU region of dose distributions. Furthermore, focal spot parameters could be determined by means of iterative simulations.

Keywords: BEAMnrc; DOSXYZnrc; EGSnrc; Electron modelling; Energy spectrum; Focal spot; Monte Carlo.