Purpose: To develop and validate a Monte Carlo (TOPAS) model for electrons on a linear accelerator (LINAC), and to investigate the feasibility of using MLC tertiary collimation to shape electron fields in lieu of conventional electron applicators.
Methods: A TOPAS (version3.1.p3) model of a LINAC was developed. Treatment head was simulated with 3x10â?¸ primary particles. Percent depth dose curves (PDDs) and in-plane/cross-plane profiles at depths of Râ‚?â‚€â‚€ and Râ‚…â‚€ were calculated in a water phantom geometry at 80 cm source-to-surface distance with a voxel size of (0.2 cm)Â³. Electron energies of 6, 9, 12, 16, and 20 MeV with jaw- and MLC-defined field sizes of 2x2 (for modulated segments), 6x6, 10x10, and 20x20 cmÂ², were evaluated. An MLC-defined field (irregular-polygon shape) was used to validate the electron scattering from leaf ends and sides. Measurements acquired with a model CC13 ionization chamber in a water phantom (IBA Blue Phantom2) were compared with simulations. The evaluation metrics included gamma index criteria of 2%/2mm and 3%/2mm for PDDs and profiles respectively.
Results: Calculated PDDs agreed with measurements with passing rate of >93% for jaw-defined fields and >90% for MLC-defined fields. The statistical uncertainty of dose profiles were <1% in the central region and <2% outside the central region. Measured profiles agreed with calculations at all depths and all energies considered, satisfying the 3%/2mm gamma criteria with passing rates >95%. An excellent agreement was observed (>96% gamma pass) between measured and simulated profiles with the irregular MLC-shaped field.
Conclusion: The developed TOPAS model of a LINAC was successfully validated against measurements. The model was able to predict electron scattering from MLC-leaf sides and round ends with reasonable accuracy. This model can thus be used to study and plan electron beam delivery with MLCs for field shaping and eventually beam modulation.
Not Applicable / None Entered.