en Medical Physics Medical Physics تحقيق بديع Original Research <span style="font-weight: bold">Background:</span> The presence of a wedge filter in the beam trajectory can modify the beam quality and cause some changes in the dosimetry parameters which are usually difficult to be measured directly and accurately. <span style="font-weight: bold">Material and Methods: </span>In this study the MCNP-4C Monte Carlo code was used to simulate the 9 MV photon beam generated by a linear accelerator. Upon getting a good agreement between the Monte Carlo simulated and measured dose distribution in open fields, the model was used to simulate the physical wedges. The steel wedges with angles from 15º-60º were modeled and the primary and the secondary photon beams were calculated. The beam profiles and wedges factors were calculated for each wedge. The output factors were determined for 45 wedge. The calculated data were compared with the measured values of the same parameters. <span style="font-weight: bold">Results:</span> The results showed that the use of wedges reduced the fluencies of the primary and scattered photons and also increased the average energy of the primary and the scattered photons. The agreement between the calculated and the measured data was better than 2% for all wedges. The results also showed that as the wedge angle increased, the electron contamination of photon beam decreased. <span style="font-weight: bold">Conclusion: </span>The presence of a wedge in a 9 MV photon beam alters the primary and the scattered components generated by a linear accelerator. The simulated linac machine and its associated data can be used to predict the dose distribution in other complex fields. Iran. J. Radiat. Res., 2010 7 (4): 223-227 Monte Carlo, photon spectra, physical wedge, MCNP4c code, simulation. 223 227 http://ijrr.com/browse.php?a_code=A-10-1-341&slc_lang=en&sid=1 M. Momennezhad 790031947532846001810 790031947532846001810 No M.T. Bahreyni Toosi 790031947532846001918 790031947532846001918 No R. Sadeghi 790031947532846001919 790031947532846001919 No H. Gholamhoseinian 790031947532846001920 790031947532846001920 No Sh. Nasseri NaseriSh801@mums.ac.ir 790031947532846001921 790031947532846001921 Yes