Background: Several investigations reported the dosimetric properties of flattening filter free photon beams to enhance the entrance dose in the surface and build up region. This paper was aimed to investigate the effect of the flattening filter free to enhance the dose at the surface and buildup region. Materials and Methods: A 12 MV photon beam of a linear accelerator was modeled and developed in both flattening filter and flattening filter free modes using the Monte Carlo BEAMnrc code. For both modes, the beam dosimetric features, including central axis absorbed doses and photon energy spectra were investigated. Results: A remarkable increase in the dose rate on the surface and build up region were attained with the flattening filter free mode. At the depth of 0 mm on 2×2 cm2, 4×4 cm2, 5 × 5 cm2, and 10×10 cm2 field sizes, the surface doses between flattening filter mode and flattening filter free mode were augmented from 27.33% to 33.78%, from 28.89% to 35.75%, from 29.44% to 36.39%, and finally from 35.10% to 47.46%, respectively. At the depth of 25 mm for field size from 2×2 cm2 to 10×10 cm2, the buildup doses for flattening filter mode and flattening filter free mode were augmented from 124.89% to 136.72% and from 132.21% to 142.67%, respectively. Conclusion: A significant increase in the entrance and buildup dose rate was observed when using an unflattened photon beam, which can be a benefit for the treatment of some skin cancers.
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