A method for assessing subject-specific counting efficiency of whole-body monitoring systems for radioiodine measurements

This study aims to numerically perform individualized calibration of a whole-body monitoring system for measuring the activity of I-131 internal contamination. A set of ten computational phantoms inside the wholebody counter (WBC) unit were simulated using Monte Carlo code MCNPX 2.6 to investigate the effect of various morphometric parameters on the calibration factors. The counting efficiencies for separated source organs (epsilon(s)) were calculated to convert the response of the detectors to the activity of I-131 in the thyroid and other organs. The biokinetic model of iodine was combined with human respiratory tract model (HRTM), and distribution of iodine in body organs was calculated for three hypothetical scenarios (acute) and a typical real scenario (chronic). The organ-specific counting efficiencies (CEOrgan) and the total counting efficiencies (CETotal) were obtained as a function of time after the end of I-131 radionuclide intake. The results showed that the thyroid was the major contributor to the CETotal (>90%) 2 days after the end of intake. Therefore, the individualized calibration process were performed using CEThyroid which is strongly dependent on the detectors solid angle seen by the thyroid more than other influencing factors. As a summary, a practical and sufficiently fast method was introduced to estimate the individual-related CEThyroid (within +/- 13%) for measurement times of >2d after the end of intake, based on estimation of the detectors solid angle seen by a point in front of the neck.