Thyroid volume’s influence on energy deposition from (131)I calculated by Monte Carlo (MC) simulation

BACKGROUND: It is well known that the success of the radiomethabolic (131)I treatment of hyperthyroidism could depend on the absorbed dose to the thyroid. It is, thus, very important to calculate the individual radiation dose as accurately as possible for different masses of thyroid lobes. The aim of this work is to evaluate the influence of thyroid volume on the energy deposition from beta and gamma rays of (131)I by Monte Carlo (MC) simulation. MATERIALS AND METHODS. We have considered thyroid lobes having an ellipsoidal shape, with a density of 1.05 g/ cm(3) and the material composition suggested by International Commission on Radiological Protection (ICRP). We have calculated the energy deposition of (131)I rays for different volumes of thyroid lobes by using the MCNPX code, with a full transport of beta and gamma rays. RESULTS AND CONCLUSIONS. The results show that the total energy deposition has a significant difference, till 11%, when the lobe’s volume varies from 1 ml to 25 ml, respect to the value presented in MIRDOSE for a 10 g sphere. The absorbed energy fraction increases by volume, because the increasing volume to surface ratio of ellipsoidal lobe causes the decrease of beta ray fraction escaping from the lobe.