engleski

Monte Carlo estimation of neutron dose equivalent at the inner side of the maze in the accelerator vault

Introduction. High energy medical linear accelerators of electrons (LINACS) that produce photons with energies higher than 10 MeV still have a wide use in radiotherapy (RT). Photon beams with such high energies could generate fast neutrons, in the giant dipole resonance reaction [1]⁠, which results in undesired contamination of the therapeutic beam. Within the accelerator vaults that have space limitations, such as those that are reconstructed after Co60 unit decommission, this effect is even more pronounced. Empirical formulas have been developed for estimation of neutron dose equivalent in accelerator vaults, and two of them are most common, one of them developed by Kersey [2] and its modification developed by Wu- McGinnley [3]⁠⁠. These formulas were obtained empirically in the series of already constructed vaults, so our goal was to estimate influence of various geometrical modifications on these formulas outcome, using Monte Carlo (MC) simulations. Methods and materials. A model of an average accelerator vault has been built using MCNP6.1.1. beta® code, and the geometrical changes within this vault have been introduced. Previously tested model of the Siemens Oncor Expreesion accelerator head has been put in this vault [4] with the energy of the electrons impinging the target of 18 MeV. The neutron dose equivalent has been estimated at the maze doors, from the inner side of the vault. Several modifications of the vault have been tested, such as elongation and dilatation of the vault outer walls and addition of steel to the one of vault walls. Results and discussion. Both formulas show good agreement with Monte Carlo calculations when size of the vault is increased. But their accuracy in not good when the size of the vault is decreased. Kersey method overestimates the neutron dose equivalent when reduced geometry is used, while in Wu-McGinnley method neutron dose equivalent is underestimated although this method shows much better agreement with results obtained form Monte Carlo simulations. The addition of the steel in the vault with reduced size causes reduction in the neutron dose equivalent in the maze. However, most of the neutrons, in all the simulations without steel barrier, originated from the accelerator head (more than 99%). But when the steel barrier was introduced, the number of neutrons coming to the maze rise to the amount form 5% up to 10% in all simulated cases. Conclusion. The discrepancies in assessment of the neutron dose equivalent between two most commonly used empirical formulas and Monte Carlo simulations are mostly pronounced within the vaults that are built with limited space. While Kersey formula overestimates the neutron dose equivalent in such cases, Wu-McGinnley formula systematically underestimates the neutron dose equivalent. Since the Wu-McGinnley formula follows the simulation results more consistently the correction should be introduced to this formula that will equalize result of this formula and MC simulations. [1] Poje M, Ivković A, Jurković S, Žauhar G, Vuković B, Radolić V, Miklavčić I, Kaliman Z, Planinić J, Brkić H and Faj D 2014 The neutron dose equivalent around high energy medical electron linear accelerators Nucl. Technol. Radiat. Prot. 29 171–8 [2] Kersey R W 1979 Estimation of neutron and gamma radiation doses in the entrance mazes of SL75-20 linear accelerator treatment rooms Med Mundi 24 151–5 [3] Wu R K and McGinley P H 2003 Neutron and capture gamma along the mazes of linear accelerator vaults J. Appl. Clin. Med. Phys. 4 162–71 [4] Brkić H, Ivković A, Kasabašić M, Poje Sovilj M, Jurković S, Štimac D, Rubin O, Faj D, Sovilj M P, Jurković S, Štimac D, Rubin O and Faj D 2016 The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam Radiat. Meas. 93 28–34

Monte Carlo ; Neutron dose equivalent ; vault

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