Estimation of dosimetric parameters of I-125 brachytherapy source model 6711 using GATE8.1 code

Brachytherapy is one type of internal radiation therapy where radiation sources, which are usually encapsulated are placed as close as possible to the tumor site inside the patient's body. In this technique, it is important to determine dose distribution around the brachytherapy capsule. Hereby, in this paper, dosimetric parameters of I-125 brachytherapy source model 6711 are estimated according to TG-43U1 protocol using GATE 8.1 Monte Carlo code. In this work, we used GATE_v8.1 to calculate dosimetric parameters of the I-125 brachytherapy source model 6711. At first, validation of the GATE platform were performed by some criteria such as radial dose function, 2D anisotropy function inside liquid water according to the AAPM TG-43U1. On the other hand, since the attenuation coefficient of the sources in the water phantom is different from that of various tissues, the effects of the various tissues on the radial dose function parameter of the I-125 brachytherapy source were investigated using GATE 8.1 code. Dosimetric parameters of simulated I-125 brachytherapy capsule show good consistency compared with the other study. The maximum average deviation was about 3.61% and 7.29% at radial dose function and anisotropy function, respectively. The relative deviation of radial dose function in the fat, muscle, breast and lung tissue compared with water phantom in  radial distance of 5cm were about 68.73%, 10.98%, 25.83% and 12.23%, respectively. There was a good agreement between the results of this work and other study in calculation of dosimetric parameters of brachytherapy I-125 source base on the recommendations of TG-43U1 protocol. The results show that the dosimetric parameters of I-125 brachytherapy can be calculated using the GATE code and appropriate physic list in spite of low energy of radiation and high variation in dose rate with increasing distance from the center of the source. The results of the dose calculation in different phantom could be used in clinical treatment planning systems.