OAK RIDGE ORNL/TM-2006/583 NATIONAL LABORATORY MANAGED BY UT-BATTELLE FOR THE DEPARTMENT OF ENERGY Uncertainties in Cancer Risk Coefficients for Environmental Exposure to Radionuclides An Uncertainty Analysis for Risk Coefficients Reported in Federal Guidance Report No. 13 January 2007 Prepared by D. J. Pawela R. W. Leggettb K. F. Eckermanb C. B. Nelsona aOffice of Radiation and Indoor Air U.S. Environmental Protection Agency Washington, DC 20460 bOak Ridge National Laboratory Oak Ridge, Tennessee 37831 DOCUMENT AVAILABILITY Reports produced after January 1, 1996, are generally available free via the U.S. Department of Energy (DOE) Information Bridge: Web site: http://www.osti.gov/bridge Reports produced before January 1, 1996, may be purchased by members of the public from the following source: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-605-6000 (1-800-553-6847) TDD: 703-487-4639 Fax: 703-605-6900 E-mail: [email protected] Web site: http://www.ntis.gov/support/ordernowabout.htm Reports are available to DOE employees, DOE contractors, Energy Technology Data Exchange (ETDE) representatives, and International Nuclear Information System (INIS) representatives from the following source: Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831 Telephone: 865-576-8401 Fax: 865-576-5728 E-mail: [email protected] Web site: http://www.osti.gov/contact.html This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. This work was sponsored by the Office of Radiation and Indoor Air, U. S. Environmental Protection Agency, under Interagency Agreement DOE No. 1824-S581-A1, under contract DE-AC05-84OR21400 with UT-Battelle. ORNL/TM-2006/583 Uncertainties in Cancer Risk Coefficients for Environmental Exposure to Radionuclides An Uncertainty Analysis for Risk Coefficients Reported in Federal Guidance Report No. 13 Authors: D. J. Pawel R. W. Leggett K. F. Eckerman C. B. Nelson Date Published: January 2007 Published by OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37831 Managed by UT-Battelle, LLC for the U.S. DEPARTMENT OF ENERGY under contract DE-AC05-00OR227. ii PREFACE Federal Guidance Report No.13 (FGR 13) provides cancer risk coefficients for various modes of environmental exposure to each of more than 800 radionuclides (EPA 1999), including inhalation of airborne activity and ingestion of activity in food or drinking water. A risk coefficient for inhalation or ingestion of a radionuclide is an estimate of the probability of radiogenic cancer mortality or morbidity per unit activity taken into the body. A risk coefficient may be interpreted either as the average risk per unit intake for persons exposed throughout life to a constant activity concentration of a radionuclide, or as the average risk per unit intake for persons exposed briefly to the radionuclide in an environmental medium. The risk coefficients in FGR 13 apply to an average member of the public in the sense that estimates of risk are averaged over the age and gender distributions of a hypothetical population whose survival functions and cancer mortality rates are based on recent data for the U.S. Uncertainties in cancer mortality risk coefficients were assessed in FGR 13 for inhalation or ingestion of each of 11 radionuclides: 3H, 60Co, 90Sr, 106Ru, 125Sb, 131I, 137Cs, 226Ra, 232Th, 234U, and 239Pu. Assigned levels of uncertainty were based on sensitivity analyses in which various combinations of plausible biokinetic and dosimetric models and radiogenic cancer risk models were used to generate alternative risk coefficients. Uncertainties relating to the validity of the linear-no-threshold hypothesis were not addressed in the analysis because this is not feasible. Also, the analysis did not address uncertainties associated with absorbed dose as a measure of cancer risk. The present report updates the uncertainty analysis in FGR 13 for the cases of inhalation and ingestion of radionuclides and expands the analysis to all radionuclides addressed in that report. This analysis is in response to suggestions that the uncertainty analysis in FGR 13 should be expanded as an aid in determining the kinds of additional information that are most needed for improving confidence in risk assessments for exposure to environmental radionuclides. As in FGR 13, results of the uncertainty analysis are given in terms of semi-quantitative “uncertainty categories” identified by letters A-E, with Category A representing the most narrowly determined risk coefficients, Category E representing the least well characterized coefficients, and Categories B, C, and D representing intermediate, increasing levels of uncertainty. Assignment of a risk coefficient to an uncertainty category was based on propagation of uncertainties in components of a simplified version of the complex computational model used in FGR 13 to calculate risk coefficients. The uncertainty in a risk coefficient was viewed as the net result of uncertainties in the following main components of the derivation: biokinetic models describing the biological behavior of ingested or inhaled radionuclides; specific energies that relate emissions from source organs to energy deposition in target organs; risk model coefficients representing the risk of cancer per unit absorbed dose to sensitive tissues from low-LET radiation at high dose and high dose rate; tissue-specific dose and dose rate effectiveness factor (DDREF); and tissue-specific high-dose relative biological effectiveness (RBE). The biokinetic models and specific energies together make up the dose model, and the iii risk model coefficients, DDREF, and RBEs together make up the risk model. This formulation allows a convenient allocation of uncertainties associated with the models used to derive the risk coefficients. Peer reviewers of this report were Dr. Andre Bouville of the U.S. National Institutes of Health, Dr. Lawrence Miller of the Department of Nuclear Engineering, University of Tennessee, and Dr. Wilson McGinn of the Environmental Sciences Division, Oak Ridge National Laboratory (ORNL). Dr. Bouville has chaired or served as a member of a number of committees of the U.S. National Council on Radiation Protection and Measurements (NCRP) and the International Commission on Radiological Protection (ICRP) concerned with uncertainties in radiation dose or risk estimates. Dr. Miller has been involved in investigations of uncertainties in dose estimators for environmentally important radionuclides including 90Sr, 131I, and 137Cs. Dr. McGinn leads the Risk and Regulatory Analysis Group at ORNL and has experience in the area of uncertainties in risk estimators for chemical hazards. The main comments received from the peer reviewers are paraphrased below. Each comment is followed by a summary of related changes made in the report or reasons for not making changes. 1) The section titled “Results” is too general. Presentation of results for a few radionuclides would be helpful. Fifteen selected risk coefficients are now examined in Section 3 (Results), six for inhaled radionuclides and nine for ingested radionuclides. The main sources of uncertainty for each of these cases are summarized in a table. 2) The section titled “Discussion” should be expanded. This section was expanded to provide a fuller discussion of the strengths and weaknesses of the analytical approach. Issues related to characterization of uncertainties in radiogenic cancer risk models were addressed at length because these models are indicated by the analysis to be the dominant source of uncertainty in most of the risk coefficients for inhalation or ingestion given in FGR 13. 3) Explain why relatively large uncertainties were assigned to risk coefficients for the extensively studied radionuclide 131I while relatively small uncertainties were assigned to coefficients for some radionuclides such as 95Zr for which little information is available. Factors determining uncertainty categories for risk coefficients for 131I, 95Zr, and a number of other radionuclides representing different levels of information were examined and are now addressed in Section 3 (Results). Limitations of the methodology specific to the important radionuclide 131I are further addressed in Section 4 (Discussion). In response to this review comment and the subsequent critical reexamination of the analytical methods, the methodology was revised slightly and uncertainty categories were recalculated for all cases addressed in the report. Based on the revised methods, the uncertainty categories for ingestion of 131I and inhalation of soluble forms of 131I were both revised from Category D (accurate within a factor iv of 10) to Category C (accurate within a factor