Our objectives were twofold: to derive a set of occupational organ-specific doses for all study participants using individual monitoring data, work history and the evolution of radiation protection policies over time; and to characterize uncertainty in individual dose estimates by creating multiple sets of organ-specific dose estimates for the entire cohort. In this schema, each set reflects multiple sources of uncertainty including possible biases and our interpretations of the uncertainties of data, and our assumptions. We refer to each set of doses derived for the entire group as a single “realization”.
To create the realizations of the cohort dose distribution, we developed a year-by-year probabilistic record of badge dose6 and organ-specific radiation absorbed doses for each participant that accounts for uncertainties shared among individuals or subgroups. Information about each technologist's annual badge dose can be represented by a sample from the probability density function (PDF) that was designed to capture the range and the likelihood of plausible values for the technologist's true annual badge dose given what is known about the population dose distribution and shielding practices in that year, together with individual information on work practices, type of facility where work was performed, and when available, film badge readings. Multiple values of individual annual doses were simulated from the PDFs using Monte Carlo simulation techniques that allowed for within-individual inter-year correlations. These individual annual badge dose values were used to generate estimates of annual organ doses that account for uncertainties in individual apron usage patterns, apron thickness and dose conversion factors. The realizations of cohort member doses derived in this study reflect both uncertainties in reconstructing individual doses and uncertainties that are shared between individuals.
Table 1 summarizes changes in the USRT dosimetry since the 2006 publication (13 (link)) and highlights important attributes of the current dosimetry system. These improvements include newly acquired film badge dose readings, and additional information on work history and practices collected in a 2003–2005 survey and methods and models that allow greater individualization of annual dose estimates. The updated badge dose data and work history information were used to estimate annual population badge dose distributions from 1916–1997. These distributions are dependent on the type of facility in which the technologist worked, e.g., a clinic or physician office and the nature of the employer (civilian or military). The badge dose and work history data were used to develop individual exposure scores, a metric that captures the dependence of the geometric mean (GM) of the annual population badge dose distribution on the frequency of performing specific types of radiologic procedures and the number of hours worked per week. Methods were developed to use exposure scores to individualize badge dose sampling densities for years in which a cohort member was believed to have worked but for which a badge dose reading was not available.
Doses to specific organs are dependent on use of apron protection and shielding. Since publication of the original dosimetry system (13 (link)), we have developed methods that use a combination of literature-based and questionnaire-derived information on individual protection practices and use of shielding according to time period. Conversion from an estimated film badge dose to organ-specific doses utilizes newly developed energy- and period-specific dose conversion coefficients and apron transmission factors (TFs) (14 (link)).