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Abstract
Background
As part of the Million Person Study (MPS), dose reconstructions for internal emitters have been performed for several U.S. facilities where large quantities of radionuclides were handled. The main challenges and dominant sources of potential error in retrospective dose estimates for internally exposed workers have been found to vary from site to site. This article discusses some important issues encountered in dose reconstructions performed for selected MPS sites and the approaches used to address those issues. The focus is on some foundational components of retrospective dose assessments that have received little attention in the literature.
Methods
The discussion is built around illustrative exposure data and dose reconstructions for workers at selected facilities addressed in the MPS. Related findings at some non-MPS sites are also discussed.
Results
Each of the following items has been found to be a major source of potential error in reconstructed tissue doses for some MPS sites: identification of all dosimetrically important internal emitters; the time pattern of intake; the mode(s) of intake; reliability of bioassay measurements; application of surrogate (coworker) information in lieu of, or in conjunction with, worker-specific monitoring data; the chemical and physical forms of inhaled radionuclides; and the relation of air monitoring data to actual intake.
Conclusions
(1) Much of the dose reconstruction effort for internal emitters should be devoted to development of best feasible exposure scenarios. (2) Coworker data should be used to assign exposure scenarios or dose estimates to workers with missing exposure data only if there is compelling evidence of similar coworker exposure. (3) Bioassay data for some radionuclides and periods of operation at MPS sites are of questionable reliability due to sizable uncertainties associated with contamination, recovery, or background issues. (4) Dose estimates derived solely from air monitoring data should be treated as highly uncertain values in the absence of site-specific information demonstrating that the data are reasonably predictive of intake. (5) For intakes known or assumed to be via inhalation, the uncertainty in lung dose typically is much greater than the uncertainty in dose to systemic tissues, when dose estimates are based on urinary excretion data. (6) The lung dose estimate often can be improved through development of site-specific respiratory absorption parameter values. (7) There is generally insufficient site-specific information to justify development of site-specific systemic models.
KEYWORDS::
Acknowledgements
The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The U.S. Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The authors also are grateful for the financial support received by the NCRP from the U.S. Department of Energy.
Disclosure statement
The authors declare no conflict of interest. The authors alone are responsible for the content and writing of the article.
Additional information
Funding
Notes on contributors
Richard W. Leggett
Richard W. Leggett is a research scientist in the Environmental Sciences Division at Oak Ridge National Laboratory (ORNL). His main research interest is in physiological systems modeling, with primary applications to the biokinetics and dosimetry of radionuclides and radiation risk analysis. He is a member of Committee 2 of the International Commission on Radiological Protection (ICRP) and the ICRP Task Group on Internal Dose Coefficients (IDC). His physiological systems models of the human circulation, skeleton, and gastrointestinal transfer and his systemic biokinetic models for many elements are used by the ICRP as dosimetry and bioassay models. He is the author of ICRP Publication 70, Basic Anatomical and Physiological Data for Use in Radiological Protection: The Skeleton, and co-author of several other ICRP reports.
Keith F. Eckerman
Keith F. Eckerman retired in 2013 from Oak Ridge National Laboratory and is an emeritus member of the National Council on Radiation Protection (NCRP) and Committee 2 of the International Commission on Radiological Protection (ICRP). He has over 40 years’ experience in radiation protection at Argonne National Laboratory, the Nuclear Regulatory Commission, and Oak Ridge National Laboratory. He served on ICRP Committee 2 for over 20 years and chaired their task group on dose calculations. He received the Health Physics Society Distinguished Scientific Achievements Award in 1995, the Loevinger-Berman Award of the Society of Nuclear Medicine in 2001, the Radiological Protection Gold Medal from Royal Swedish Academy of Sciences in 2012, and Lauriston S. Taylor award of the NCRP in 2015. He has a PhD in radiological physics from Northwestern University.
Michael Bellamy
Dr. Michael Bellamy is a nuclear engineer and health physics researcher at the Oak Ridge National Laboratory in the United States. He has served for over 10 years at the Center for Radiation Protection Knowledge, specializing in Monte Carlo radiation transport research and internal dosimetry modeling. He is currently focused on internal dose reconstruction for the MPS. Dr. Bellamy’s research supports several federal entities including the Centers for Disease Control and Prevention, the U.S. Department of Energy, the U.S. Nuclear Regulatory Commission, and the U.S. Environmental Protection Agency.