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Background: During intensity modulated radiation therapy (IMRT) technique, theoretically, presence of flattening filter (FF) across the beamline of clinical linear accelerator (linac) is not essential to obtain uniform lateral profiles due to intensity modulation of photon beams by multileaf collimators (MLCs). The aim of this study was to investigate the dosimetrical properties of 18 MV photon beam-Varian linac with and without FF. Materials and Methods: All dose measurements were performed on 18 MV, FF mode-Varian 2100C/D linac. The FF and flattening filter free (FFF) modes of linac were modeled by MCNPX 2.4. code. The photon and contaminant electrons spectra, dose rate, present depth doses (PDD), lateral dose profiles, total and collimator scatter factors and out of field doses were calculated and compared with and without FF. Results: Removing the FF increased the photon and contaminant electron fluences by factors of 5.48 and 3.94 for a 5 × 5 cm2 field size, respectively. The surface dose increased up to 155%. The flatness of dose profile was disturbed and deteriorated with increase of field size. Despite the dependence of the total scattering factor on field size, the variation of collimator scattering factors was neglected. The out-of-field dose decreased about 81.5 % for a 5 × 5 cm2 field size. Conclusion: Removing FF from the linac head increases the dose rate and decreases the out-of-field dose, but the increased skin dose and deteriorated flatness of lateral dose profile are the main disadvantages of the FFF mode.

Keywords: Dosimetry, fattening filter, linear accelerator, Monte Carlo simulation, radiotherapy.

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