Monte Carlo modeling of a proton fixed beam line featuring non-isocentric PBS treatment capabilities
Résumé
In non-isocentric proton scanned pencil beam treatments, it is of paramount importance to precisely describe beam optics for different air-gaps. Indeed, nozzle components and air-gaps drastically affect the lateral dose profile due to the multiple coulomb scattering. This work shows an extended Monte Carlo proton beam modeling based on final medical commissioning data, with special emphasis at non-isocentric conditions.A full description of the nozzle design was implemented. Physics-builder QBBC_EMZ was selected in Gate 7.2/Geant4 10.02. Beam optics parameters (beam size, divergence and emittance) were adjusted to match the measured spot sizes in air at 7 different air gaps for 20 representative energies. The energy parameters (mean energy and energy spread) were optimized in order to match the measured beam range (R80) and Bragg peak width evaluated in water.Absolute mean deviation from measured data used for beam modeling was 0.2 mm for FWHM at ISO and non-ISO positions. Higher deviations were observed at non-relevant clinical conditions: 62.4 MeV at position ISD20cm (6.6% relative agreement). Maximum deviation up to 10% was observed for 97.4 MeV at isocenter using range shifter (figure 1). Residuals in terms of physical range were always within 0.1 mm (figure 2). Discrepancies up to 2.5% at worst were found in depth dose profile while Bragg peak width mean deviation at 80% of maximum dose was 0.1 mm, corresponding to a relative deviation of 3%.Presented results in 1D are very promising and further validation in 2D, 3D and absolute dose is ongoing.