Design of electron dual foil scattering systems for Elekta Infinity radiotherapy accelerators

Abstract

Purpose: To design a dual foil scattering system within Elekta Infinity radiotherapy accelerator constraints that results in clinical electron beams that meet flatness criteria of ±3% (±4%) along its principal axes (diagonal axes) for the 25x25cm2 applicator and most probable surface energies of 7-20 MeV (1 MeV increments). Methods: An analytical electron dual scattering foil system simulator was commissioned and verified using Monte Carlo (MC) simulations. Verification required comparing analytical simulator with MC-calculated electron fluence profiles for identical geometries: (1) only primary foil and (2) both primary and secondary foils in the beam. Also, simulator-calculated bremsstrahlung dose was validated by comparison to measured data. Measured dose profiles, with and without 25x25cm2 applicator, and simulator profiles were used to estimate objective profiles. Objective profiles (“ideal” profiles), which if achieved should produce uniform beams, were determined for current Elekta beam energies. Objective profiles were then interpolated for beam energies of 7-20 MeV in 1-MeV increments. Then, the simulator was used to design a new dual scattering foil system (5 primary and 3 secondary foils) such that the simulator’s design profiles closely matched the objective profiles. Design profiles were compared with MC-calculated dose profiles, after which the initial objective profiles were modified, and a second design optimization was performed. MC dose calculations were used to evaluate the modified dual scattering foil design. Results: For all design energies (7-20 MeV), the modified dual scattering foil design produced MC dose profiles that were within the flatness criteria of ±3% along the principal axes, except for 8 MeV (3.2% maximum deviation). Along the diagonal axes, the modified designs produced MC dose profiles within the flatness criteria of ±4% except at 7, 8, and 17 MeV (maximum deviation of 5.1% at 8 MeV). Conclusions: The dual scattering foil system simulator and present methodology should be capable of designing electron dual scattering foil systems provided a validated MC model of the accelerator produces accurate dose calculations (1% accuracy in beam’s umbra). Results of this study did not prove the hypothesis, believed due to the need for a greater number of iterations and more optimal primary foil thickness selections.