Improved Monte Carlo clinical electron beam modelling. (October 2019)

• Record Type: Journal Article
• Authors: Sachse, K.N.; du Plessis, F.C.P.
• Is Part Of: Physica medica. Volume 66(2019)
• Page Start: 36
• Page End: 44
• Publication Date: 2019-10
• Subjects: Electron modelling -- Monte Carlo -- EGSnrc -- BEAMnrc -- DOSXYZnrc -- Energy spectrum -- Focal spot; Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2019.09.073
• Languages: English
• ISSNs: 1120-1797

Abstract

Highlights: A benchmarked EGSnrc electron model was developed to within 2%/2 mm. Monte Carlo simulations can predict the shape and size of the electron focal spot. The build-up dose is sensitive to focal spot changes. An asymmetrical poly-energetic electron spectrum solves build-up dose discrepancies. 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.