Dosimetric Parameters Estimation of I-125 Brachytherapy Source in fat phantom using GATE8.0 code

Introduction:
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 or inside the patient's body. In this technique, it is important to determine dose distribution around the brachytherapy capsule to create optimal treatment plant. In this way, dosimetric parameters are estimated according to TG-43U1 protocol in the water phantom as reference medium while the attenuation coefficient of the sources in the water phantom is different from that of different tissues. The purpose of this study is to investigate the effects of the fat phantom materials on the TG-43U1 dosimetery parameters of the I-125 brachytherapy source using GATE 8.0 Monte Carlo code.
Materials and Methods:
In this work, we used GATE_v8.0 (Geant4 Application for Emission Tomography) to calculate of dosimetric parameters of the I-125 brachytherapy source model 2301. At first, validation of the GATE platform was performed by some criteria such as dose rate constant, radial dose function, 2D anisotropy function inside liquid water according to the AAPM TG-43. Then, the Monte Carlo simulated dosimetric parameters of the 2301 I-125 were calculated in fat phantom. The TG-43 dosimetry parameters of the brachytherapy source were compared with those of the fat phantom.
Results:
Dosimetric parameters of simulated I-125 brachytherapy capsule show good consistency compared with the other study of the Monte Carlo simulation of I-125 source. The maximum deviation was about 9%. The maximum relative deviation of radial dose function of the fat tissue compared with water medium were about 13% at distances below 1cm. These differences increased, when the distance from the source increased. At the distance of 5cm from the source, it approximately reached to 55.67%. The maximum relative deviation of the anisotropy parameter of the fat phantom was observed about 6.7% compared with the water phantom.
Conclusion:
There was a good agreement between the results of this work and other study in calculation of dosimetric parameters of brachytherapy I-125 (model 2301) brachytherapy base on the recommendations of TG-43U1 protocole, using GATE Monte Carlo simulation method. Acquired results show that the obtained data can be used to develop GATE code for treatment plan of brachytherapy I- 125 brachytherapy source. Furthermore, In the clinical application of the I-125 brachytherapy source, which is contracted in the treatment of the adjacent tumors to the fat tissues, the correction factors of the fat tissues must be applied after dosimetric parameters calculation in water phantoms in the treatment planning to estimate accurate results for brachytherapy treatment planning.