Photon doses at the entrance of 60Co and low-energy medical accelerator rooms under unusual irradiation conditions.

In this paper, the general-purpose Monte Carlo code MCNP5 was used to study the dose variance due to the position of medical linear accelerators, under unusual conditions, for shielding design of radiotherapy facilities. It was found that the computational methods generally used to estimate the scattered photon doses at the entrance of radiotherapy unit vaults provide conservative results when compared with the MCNP results, considering the standard condition. On the other hand, for the situations where the axis of gantry rotation is redirected at, for example, 45 degrees with respect to the walls of the room, the photon doses at the entrance can reach values up to seven times higher than those obtained under the standard condition, depending on the energy of the primary beam.