In this study, using the FLUKA code (version 2011.2c.6), a C/D2300 linear accelerator head manufactured by Varian (18 MeV) were simulated within a radiation therapy room. Electron source and transporting particles using the FLUKA code from the source in each calculation replaced by an alternative photon which is an important aspect of the work. After verifying this alternative source, a composite shielding layer was considered as the interior covering of the treatment room, and the effect of six polymer-based shielding materials with different compositions on the photon and neutron doses at different locations was calculated. As a general conclusion from this research, using a polymer shielding layer of material including 32.5 wt% elastomer + 60 wt% tungsten + 7.5 wt% boron carbide on the interior of the roof, floor, and concrete walls of the treatment room is recommended, except for the second wall at the maze entrance. Additionally, we found that a 2 cm thickness of this shield is almost equivalent to 3 mm of pure lead. The optimized thickness depends on the specifications of the LINAC, its energy, the dimensions of the treatment room, and the thickness of the concrete walls which can be calculated based on these specifications.

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