Neutrons are produced in high-energy linear accelerator (linac) operating above 7 MV voltages. These secondary particles have high RBE and stored undesired dose in patients' body during the radiotherapy. In this study, it was tried to design the proper neutron shield to reduce the effect of neutrons on patients. For this propose, the MCNPX code was used and the 15 MV Siemens Primus linac, treatment room and patient's body were simulated. To have detailed model of patient, the ICRP male and female reference voxel phantoms were employed. Results showed that the shield made of borated polyethylene could decrease the neutron dose received by patient, especially in the healthy organs/tissues outside the treatment area up to 70%. Thus, the risk of secondary cancers in the normal tissues reduces and the survival rates increase.

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