Estimation of dose absorbed fraction for I-131-beta rays in rat thyroid

The dose absorbed fraction of rat thyroid by internal deposit of I-131 has been calculated as a function of effective diameter of thyroid. The calcula tions were done using two types of Monte Carlo simulations: one was by a si mple energy-loss calculation in spherical volume according to the electron stopping power, and another by a more realistic simulation using Monte Carl o N-Particle Transport code system Version 4A (MCNP). These two calculation s were consistent with each other within a deviation of 5%. The absorbed fr actions in spherical thyroid were drastically changed up to 5 mm diameter, and then almost all energy was deposited within 10 mm diameter. For the practical application to the animal experiment, the absorbed fracti ons of ellipsoid-shaped thyroids were also calculated for 1-, 4- and 9-week -old rats, where the fractions were estimated to be 0.61,0.67 and 0.68, res pectively. It was also found that the absorbed fraction of the ellipsoid wi th various dimensions can be simulated by a calculation for spherical volum e with a comparable effective diameter.