en Radiation Biology Radiation Biology تحقيق بديع Original Research <span style="font-weight: bold">Background:</span> Iodine brachytherapy sources with low photon energies have been widely used in treating cancerous tumors. Dosimetric parameters of brachytherapy sources should be determined according to AAPM TG-43U1 recommendations before clinical use. Monte Carlo codes are reliable tools in calculation of these parameters for brachytherapy sources. <span style="font-weight: bold">Materials and Methods:</span> Dosimetric parameters (dose rate constant, radial dose function, and anisotropy function) of two I-125 brachytherapy sources (models LS-1 and Intersource) were calculated with MCNP4C Monte Carlo code following task group #43 (TG-43U1) recommendations of American Assossiation of Physicists in Medicine. The simulations were done inside a spherical water phantom because of its tissue equivalent properties. The Monte Carlo simulations for radial dose function were performed at distances ranging from 0.25 to 10 cm from the source center. The anisotropy functions F(r, θ), for both sources, were calculated at distances of 1, 2, 3, 5 and 7 cm from the source center for angles ranging from 15 to 90 degree. <span style="font-weight: bold">Results:</span> The results of the Monte Carlo simulation indicated a dose rate constant of 0.952 cGyh -1U-1 and 0.986 cGyh -1U-1 for models LS-1 and Intersource, respectively. The tabulated data and fifth order polynomial coefficients for radial dose functions along the source are described in this paper. The results indicated that the anisotropy in dose distribution increased along the source axis. <span style="font-weight: bold">Conclusion: </span>The obtained results were in good agreement with measurements and calculations of other investigators, using other Monte Carlo codes. Iran. J. Radiat. Res., 2010 7 (4): 217-222 Brachytherapy, radial dose function, Monte Carlo, TG-43. 217 222 http://ijrr.com/browse.php?a_code=A-10-1-340&slc_lang=en&sid=1 M. Zehtabian mehdizehtabian@yahoo.com 790031947532846001809 790031947532846001809 Yes R. Faghihi 790031947532846001916 790031947532846001916 No S. Sina 790031947532846001917 790031947532846001917 No