Background and purpose: High-energy photon beams above 8-10 MV used in external radiation therapy produce neutron in dealing with the linear accelerator components and other parts in radiation field. The weakening of neutrons due to the closing jaws collimator angle, in smaller field sizes can also be a contributing factor in changing the neutron spectrum. Since a direct measurement of the neutron flux and spectrum require complex measurements in treatment room, the Monte Carlo is an alternative method for determining the true environmental neutron spectrum.

Materials and methods: Geometry of linear accelerator Elekta SL75/25 was designed in FLUKA simulation code and its validation was tested. Spectrum of neutrons produced in the accelerator was determined in the center of four field sizes of 20 × 20,   15 × 15, 10 × 10, and 5 × 5 cm². Data analysis was done applying Mann-Whitney test.

Results: In the condition with flattening filter, significant differences was found in the neutron spectrum only in 5 × 5 to 15 × 15 cm² and 5 × 5 to 20 × 20 cm² field sizes (P<0.05).

Conclusion: Our results of the neutron spectrum using FLUKA simulation code are in a good conformity with the results of practical dosimetry of other researchers; therefore, checking the neutron flux in radiotherapy by simulation can be an appropriate solution to improve patient treatment and protection issues. Since neutron dose increases with an increase in field size, so, we can conclude that field size influences secondary effects of treatment.