Radiotherapy is one of the successful methods of treatment of diseases such as cancer .Radiotherapy can treat a lot of diseases by means of radiation. Depending to the distance between source and body, radiotherapy is divided into Teletherapy and Brachytherapy. A medical accelerator may be used in teletherapy, which works in either electron or photon mode. Photons are used for treatment of deep tumors, because of their high penetration depth. In the photon mode, they are produced by incident electrons on a target. Then photons are collimated by other elements and are used for treatment. Neutrons are produced by high energy photons and electrons incident on various materials of the accelerator. The cross section for (e,n) reactions are smaller by a factor of about 10 than those for (?,n) reactions. Because of this, the neutron production during photon beam therapy mode is important. These contaminated neutrons, cause a patient to absorb extra dose. Neutrons have higher quality factor (Q) than gamma rays, beside they have Oxygen Extortionate Ratio (OER) smaller than normal amount; so a low dose of neutrons can destroy patient tissues. So it is necessary to search for a suitable shield which can reduce neutron dose. In this work, the Monte Carlo code MCNP was used to simulate the medical accelerator SATURN-20 and water phantom. Simulation results compared with those of experiments, showed consistency, which approved our program code. We used shields with various materials and different thicknesses at different distances from collimator to reduce neutrons dose. In each case, simulation was done by the Monte Carlo code. Dose of neutrons were calculated at different cases with and without shields. We used three materials ( plexiglass, polyethylene and graphite) as shields, among which polyethylene showed the highest reduction and graphite the lowest reduction of dose.
