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Introduction: 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.

Methods: 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.

Results: 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%.

Conclusions: 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.

Introduction: The dosimetry of the brachytherapy sources is performed in the water source medium according to protocol TG-43U1. To achieve a resonable results on treatment, the use of the water material for all body tissues can be one of the faulty sources in delivering the correct dose to the tumor. Thus, we have focused on the dosimetry parameters in fat density 0.95 gr/cm³ and the muscle with density 1.05 gr/cm³ in the present study. The results are compared with the results of the water phantom to show the accurate accessment via the differences of the results.

Methods: Dissymmetry simulations for determining the parameters of the radial dose function and the anisotropy function in fat tissues, muscle and water phantom distances and different angles has been using MCNP4C code.

Results: The greatest relative differences of the phantom radial dose function of the fat tissue at distances below 1cm approximately reached 13%; and this rate increased, when the distance from the source increased; whereas at the distance of 5cm from the source, it approximately reached 167%. In the muscle tissue, the relative difference of these parameters was about 3% at the distance of 0.5cm, while at the distance of 5cm from the source, it approximately increased 16%. The maximum relative difference of the anisotropy parameter of the fat and muscle tissue phantom compared with the water were observed more than 2% and 3%, respectively.

Conclusion: In the clinical application of the ¹⁰³Pd brachytherapy source, which is contracted in the treatment of the adjacent malignant tumors to the fat and muscle tissues, the correct decisions must be applied on the tissue dosimetry parameters in the treatment planings according to the tables  of 3, 4, 5 and 6 in this study.