Assessment of the point-source method for estimating dose rates to members of the public from exposure to patients with 131I thyroid treatment

The U.S. Nuclear Regulatory Commission (USNRC) initiated a contract with Oak Ridge National Laboratory (ORNL) to calculate radiation dose rates to members of the public that may result from exposure to patients recently administered iodine-131 (¹³¹I) as part of medical therapy. The main purpose was to compare dose rate estimates based on a point source and target with values derived from more realistic simulations of a human source and target. The latter simulations considered the time-dependent distribution of ¹³¹I in the patient and attenuation of emitted photons by the patient's tissues. The external dose rate estimates were derived using Monte Carlo methods and two representations of the Phantom with Movable Arms and Legs (PIMAL), previously developed by ORNL and the USNRC, to model the patient and a nearby member of the public. Dose rates to tissues and effective dose rates were calculated for distances ranging from 10 cm to 300 cm between the phantoms. Dose rates estimated from these simulations are compared to estimates based on the point-source method, as well as to results of previous studies that estimated exposure from ¹³¹I patients. The point-source method overestimates dose rates to members of the public in very close proximity to an ¹³¹I patient but is a broadly accurate method of dose rate estimation at separation distances of 300 cm or more at times closer to administration.