Neutron dose calculation at the maze entrance of medical linear accelerator rooms.

Currently, teletherapy machines of cobalt and caesium are being replaced by linear accelerators. The maximum photon energy in these machines can vary from 4 to 25 MeV, and one of the great advantages of these equipments is that they do not have a radioactive source incorporated. High-energy (E > 10 MV) medical linear accelerators offer several physical advantages over lower energy ones: the skin dose is lower, the beam is more penetrating, and the scattered dose to tissues outside the target volume is smaller. Nevertheless, the contamination of undesirable neutrons in the therapeutic beam, generated by the high-energy photons, has become an additional problem as long as patient protection and occupational doses are concerned. The treatment room walls are shielded to attenuate the primary and secondary X-ray fluence, and this shielding is generally adequate to attenuate the neutrons. However, these neutrons are scattered through the treatment room maze and may result in a radiological problem at the door entrance, a high occupancy area in a radiotherapy facility. In this article, we used MCNP Monte Carlo simulation to calculate neutron doses in the maze of radiotherapy rooms and we suggest an alternative method to the Kersey semi-empirical model of neutron dose calculation at the entrance of mazes. It was found that this new method fits better measured values found in literature, as well as our Monte Carlo simulated ones.