Estimation of Patient Effective Dose from I-131 Using Monte Carlo Calculation

During the oral application of radionuclide therapy it is from the vital importance to measure effective dose in stomach in which the (NaI)-I-131 therapeutic capsule stays during the 15 minutes which is long enough to make risky exposure. As it is not possible to determine it by direct measurements there is a strong recommendation to estimate the dose by calculation. The main goal is to calculate effective dose and risk as a result of I-131 capsules remaining in stomach before the absorption starts. Monte Carlo code MCNP4b was used to model the transport of gamma and beta particles emitted by radionuclide I-131 treated as a point source at the bottom of the stomach. Absorbed energy per unit transformation in stomach and surrounding organs has been calculated. The dose equivalents in these organs have been calculated in aim to determine the effective doses using appropriate tissue weighting factor values. The local doses in stomach wall reached the values in order of several hundreds of grays in a very short time. In such case the traditional concept of risk is not applicable, so it becomes necessary to create the very new concept which is able to cover higher risks under presented circumstances.