Analysis of the uncertainties associated with the age-dependent thyroid doses and risk of thyroid cancer due to exposure to 131I

Effects on the thyroid gland due to exposure to 131I are currently of interest for ongoing retrospective studies of historical releases in Oak Ridge, Tennessee, and Hanford, Washington. Most of the work to date has been limited to dose estimation. This work focuses on estimating both dose and risk of thyroid cancer to an exposed individual. The age-dependent thyroid dose is calculated using a standard metabolic model for iodine. Updated information on thyroid mass from measurements using modem ultrasound techniques was used. The age-dependent risk is calculated using a linear excess relative risk model. An analysis of uncertainties in dose and risk estimates was performed for an individual in a population characterized by the mass of thyroid, by the iodine metabolic parameters, by the background incidence of thyroid cancer and by the excess relative risk per Gy of absorbed dose. The uncertainty analysis was performed using Monte-Carlo simulation, by considering the age-dependent parameters as random functions. The correlation between the metabolic age-dependent parameters was considered explicitly. Special attention is given to a modifying factor that accounts for the effectiveness of 131I in inducing thyroid cancer as compared to gamma irradiation, for which most of the excess risk factors are derived. This factor is based on review of recent literature and on informal interviews with outside experts, and thus, the expressed uncertainty is subjective in nature. The paper summarizes the age-dependent dose conversion factors (Sv Bq-1) and slope factors (risk Bq-1) as well as the uncertainty associated with them. An analysis that identifies the parameters of dominant importance by their contributions to the overall uncertainty is also included