The aim of this study is to evaluate absorbed dose from right lung for brachytherapy and to estimate the effects of tissue heterogeneities on dose distribution for Iridium-192 source using Monte Carlo simulation. It is whether Monte carlo simulation can investigate dose distribution for brachytherapy due to their material differences to verify the reliability of the obtained Geant4 code results.
The study employed Geant4 code as Monte Carlo simulation to calculate the dosimetry parameters. For each simluation, 10^8 photon histories were simulated. The dose distribution of Iridium-192 source in solid water equivalent phantom was calculated and compared with the measured dose by the ion chamber. The volume of the experiment in solid water equivalent phantom was simulated as 30 × 21 × 30 cm^3, with 21 cm in the Y-axis direction, including aluminium plate or steel plate inserted. Dose was normalized to 9 mm at D(X)=0 mm, D(Y)=5 mm point. The dosimeter measured the dose in solid water equivalent phantom at various distances from the center of Iridium-192 source.
The simulation was used to evaluate the energy of gamma radiation absorbed in the lung organ and other organs around it. The dose distribution embedded in right lung was calculated due to the presence of heart, thymus, spine, stomach as well as left lung. Tissue compositions were taken from ICRU Report 46. The geometry of the human body was made up of adult male MIRD type of the computational human phantom. Right lung geometry made as target organ with density 0.26 g·cm^-3 and located at a gps position that is 9.5 0.0 58.5 cm.
The dosimetric characteristics obtained for aluminium plate inserted were in good agreement with experimental results within 4%. The simulation results of steel plate inserted agreed well with a maximum difference 2.75%. The dose decreased with increasing the distance from the source. Target organ considered to receive a dose of 100%, the surrounding organs were left the left lung of 3.93%, heart of 10.04%, thymus of 11.19%, spine of 12.64% and stomach of 0.95%. The source delivered the minimum dose to stomach.
Based on dosimetric parameters, the accuracy of simulation of Iridium-192 source was verified. The relative doses in solid water equivalent phantom were observed to depend on both compositions and density. When the statistical error is performed for the computational human phantom, the statistical error of value is under 1%. This simulation results could represent valid clinical reference data. This study demonstrates a useful approach using Geant4 code in dose calculation that can be applied in the brachytherapy for the internal dosimetry calculations.