Comparative study of MIRD,experimental and GEANT4 simulations for uniformly distributed I-131 in cylindrical and spherical thyroid models

Due to worldwide use of I-131 for the treatment of benign and malignant thyroid diseases, reliable dose estimation for various thyroid mass values is necessary. In this study computation of the radiation absorbed dose due to β- and γ-activity of I-131 to thyroid has been carried out using the GEANT4 code for water, ICRP and ICRU soft tissues simulating thyroid tissue. The results are validated by comparison with the MIRD calculations. The effect of size and shape of thyroid on absorbed dose has also been studied by varying the volume of thyroid in 10–150 cm³ range and by using spherical and cylindrical models for thyroid. Experimental measurements have also been performed using a group of 20 patients, who were treated for thyro-toxicosis with I-131. Dose equivalent was measured with a portable ionization chamber after the administration of 405–851 MBq of I-131. Comparison of peak energy and effective energy per transformation with the MIRD has been performed yielding only 0.5% relative differences between these two energies. For spherical model, the relative difference of peak and effective energies with the MIRD is 5.2% and 4.7% respectively and very similar for cylindrical models. Spherical model shows 0.45% increase as compared to that of cylindrical model. The use of water, ICRP or ICRU materials only implies negligible difference. Total energy deposition per decay of I-131 increases with volume for both models. The estimated results of spherical and cylindrical thyroid models show an excellent agreement with the corresponding the MIRD results. Simulated results for ambient dose equivalent rate show a good agreement with our experiment and corresponding errors remain less than 13% in the case of dose at 1 m, and are less than 8% for doses at the neck surface.