Comparison of Geant4 and MCNPX simulations of a clinical proton therapy facility

IPurpose Full proton beamline Monte Carlo simulation provides detailed information suitable for deriving the input parameters for Monte Carlo driven calculation of treatment plans. The usability of Geant4 and MCNPX for this purposes is studied. Methods and Materials The beamline at Hahn Meitner Institute HMI in Berlin used for proton therapy of ocular tumours is simulated with Monte Carlo codes Geant4 and MCNPX. Phase space files containing information about particle properties are compared. Simulated dose distributions in water phantom are compared with measured dose distributions. Different patient specific geometry setups type of range shifter, modulator wheel, collimator are simulated. Results Both codes give satisfactory, but not exactly the same results for the same initial conditions. Particle properties like spectra are compared on different positions and show large influence of multiple scattering on collimator tube on the low energy part of proton spectra. Differences between simulated and measured dose distributions for different range shifters and modulator wheels can also be observed. Beam model parameters suitable to describe particle properties below the vacuum beamline is derived from the simulation. Discussion and Conclusion Both codes have proved their usability to describe this type of application. Different results between both codes are caused especially by different multiple scattering models. Therefore, the initial beam conditions have to differ slightly for both codes to achieve comparable result