Calculating Error Percentage in Using Water Phantom Instead of Soft Tissue Concerning 103Pd Brachytherapy Source Distribution via Monte Carlo Method

103Pd is a low energy source, which is used in brachytherapy. According to the standards of American Association of Physicists in Medicine, dosimetric parameters determination of brachytherapy sources before the clinical application was considered significantly important. Therfore, the present study aimed to compare the dosimetric parameters of the target source using the water phantom and soft tissue. According to the TG-43U1 protocol, the dosimetric parameters were compared around the 103Pd source in regard with water phantom with the density of 0.998 gr/cm3 and the soft tissue with the density of 1.04 gr/cm3 on the longitudinal and transverse axes using the MCNP4C code and the relative differences were compared between the both conditions. The simulation results indicated that the dosimetric parameters depended on the radial dose function and the anisotropy function in the application of the water phantom instead of soft tissue up to a distance of 1.5 cm, between which a good consistency was observed. With increasing the distance, the difference increased, so as within 6 cm from the source, this difference increased to 4%. The results of the soft tissue phantom compared with those of the water phantom indicated 4% relative difference at a distance of 6 cm from the source. Therefore, the results of the water phantom with a maximum error of 4% can be used in practical applications instead of soft tissue. Moreover, the amount of differences obtained in each distance regarding using the soft tissue phantom could be corrected.