Abstract ID: 116 Comparison of beam output factors in MCNP6 and Geant4 based IAEA phase-space files

Abstract

Monte Carlo (MC) dose calculation algorithms demand an accurate characterization of the radiation beam. At present, three MC-based beam models are commonly used for dose calculation; namely, full MC simulation, virtual source model, and phase space (phsp) files. The first two require detailed information of the LINAC head: information that is not always available from vendors. Therefore, properly validated phase-space data for external beam radiotherapy, available from the IAEA Nuclear Data Services section, remains a valuable alternative for MC beam simulation. The aim of this work is to compare the beam output factors obtained by MC-based phsp simulations in MCNP6 and Geant4. The simulations were performed with the photon mode energies of 6, 10, and 25 MV on the ELEKTA Precise. The simulations are divided into two steps: (1) the development of a method for directly reading the phase-space files provided by the IAEA in MCNP6, and (2) the determination of cut-off values, variation reduction techniques, and field size. Patient-specific beam line devices (blocks, jaws, wedges, etc.) were not simulated. The comparison of the output factors as percentage depth dose (PDD), lateral dose profiles, and dose distributions for different field sizes were computed using Geant4 and MCNP6. Both calculations generate matching PDD for a range of open-field sizes within the statistical uncertainties. Variation in lateral dose profiles varies up to 5% in the 25 MV energy mode. Comparison of dose distributions for an open 10 × 10 field by the gamma evaluation test returns a value of 0.93. Despite the lack of a user-friendly MCNP6 interface, this work shows that MCNP6 is suitable for beam simulation based IAEA-phsp.