This work studies the design of the treatment room for an 18-MV linac to ensure radiation protection and safety of hospital staff and patients. The walls' thickness, the door, and the maze were designed according to the National Council on Radiation Protection and Measurements Report 151 recommendations. The results of this work are contrasted with the Monte Carlo calculations performed with the MCNP5 code where dose equivalents due to neutrons and neutron spectra estimated at different points inside and outside the radiotherapy room verify that the shielding thicknesses obtained are enough to reduce the dose level permitted by Mexican regulation.