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Deconstructing Linearity

Deconstructing Linearity. Kenneth L. Mossman Professor of Health Physics Director, Office of Radiation Safety Arizona State University Tempe, AZ. Deconstructing Linearity. Nature of the debate Dose extrapolation Uncertainties in risk estimates Other predictive theories

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Deconstructing Linearity

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  1. Deconstructing Linearity Kenneth L. Mossman Professor of Health Physics Director, Office of Radiation Safety Arizona State University Tempe, AZ

  2. Deconstructing Linearity • Nature of the debate • Dose extrapolation • Uncertainties in risk estimates • Other predictive theories • Problems / Solutions

  3. The LNT Debate • Economic costs • environmental clean up (>$100 billion) • regulatory compliance (>$10 billion/y) • Fear of radiation • abortions following Chernobyl • mammography

  4. Cost of Regulation • Viscusi, 1992 • 1990 US dollars • Nuclear regulations not cost effective Cost ($) Per Life Saved

  5. LNT Proponents Risk conservatism justified because of uncertainty in risk Precautionary principle LNT supported by LSS and other human data LNT is simple, easily explained to public LNT Opponents Regulatory compliance costs are excessive Fear of radiation at low doses LNT not supported by LSS and other human data Radiogenic risk is lower than predicted by LNT The LNT Debate

  6. Very large extrapolation factors Very large uncertainties in risk at low doses Uncoupling regulatory decision making from predictive theories What is a “safe dose” Precautionary principle The LNT Debate

  7. Extrapolating Health Risks

  8. Risk Uncertainty at Low Doses Lifetime cancer risk ~ 5%/Sv CL: ?? BEIR V: lower limit of risk includes zero at natural background levels Probability of Radiogenic Cancer Lifetime cancer risk ~ 5%/Sv 90% CL: 1.15-8.08%/Sv Dose Extrapolation Factor ~ 100 0 10 20 30 200 400 600 800 1000 Dose (mSv)

  9. Uncertainties in Risk(NCRP 126) • Population of all ages: 5%/Sv • Work population: 4%/Sv • 90% CL: 1.15% - 8.08%/Sv

  10. Sources of Uncertainty(NCRP 126) • DDREF (40%) • Population transfer (19.9%) • Statistical uncertainties (4.2%) • Dosimetric uncertainties (4.2%) • Misclassification of cancer deaths (0.6%) • Lifetime projection (0.5%) • Unspecified uncertainties (30.6%) • Uncertainty due to dose extrapolation (?)

  11. Extrapolating To Low Dose And Low Dose Rate • NCRP 126 • Tumor incidence in animals exposed at HDR and LDR • Curve A: Linear fit at HDR • Curve B: Curvilinear fit to experimental data • Curve C: Linear fit at LDR

  12. Evidence for LNT Uranium miner data Domestic radon exposure Total solid cancers in LSS Evidence against LNT Leukemia in A-bomb survivors Ecological studies of lung cancer from domestic radon exposure Total solid cancers in LSS LNT: To Be Or Not To Be?

  13. Hypotheses, Models and Theories Theory Conceptual Model Hypothesis Testing Data Observations

  14. Models Lead to Theories Model Theory Billiard balls collide and Kinetic theory of gases bounce off one another Bohr model of the atom Quantum theory Target model of radiation action Linear no-threshold theory

  15. LSS Data Supports Mutually Exclusive Theories Theory Source of Data Comment Linear no-threshold Pierce et al., 1996 The dose response for cancer mortality is linear down to 50 mSv Curvilinear or Little and Muirhead Upward curvature in dose response for threshold 1996 leukemia incidence and mortality; no curvature observed for solid cancers; evidence for threshold in non-melanoma skin cancer Curvilinear or Hoel and Li, 1998 A-bomb cancer incidence data agree more with threshold a threshold or nonlinear dose-response curve than a purely linear one although the linear dose-response is statistically equivalent Supralinearity Pierce et al., 1996 Excess relative risk per Sv increases with decreasing dose Hormesis Kondo, 1991 Cancer mortality is reduced in male survivors of the Nagasaki bomb below ~50 mGy

  16. LSS Data Supports Mutually Exclusive Theories • RERF - LSS data • Dose-response for pooled non-cancer disease mortality

  17. Radon-Induced Lung Cancer Mortality: Support for LNT? • Lubin and Boice, 1997 • Meta-analysis of 8 indoor radon studies • pooled analysis of uranium miner studies • Cohen’s ecological study

  18. Resilience of the LinearNo-Threshold Theory • External correction factors • e.g. DDREF • Anomolous results explained • e.g. Radon ecological studies

  19. Problems High cost of environmental cleanup (one radioactive atom might cause cancer?) Radon gas in homes causes about 16,000 deaths/year according to EPA (support from epidemiology?) Radiophobia: IAEA estimates 100,000-200,000 Chernobyl related induced abortions in Western Europe (insignificant risk from small doses? threshold?) Solutions Continue epidemiological studies (LSS) recognizing limitations Mechanistic studies to clarify shape of dose-response curve (eliminate competing theories) Wingspread and Airlie Conferences bridge policy and science coherent system of regulations use of best science available Sen. Domenici - $18M to DOE The LNT Debate

  20. If Not LNT, Then What? • No legal requirement to base regulations on predictive theories • Avoid use of predictive theories • Base exposure limits on annual average natural background levels in U.S. • Base exposure limits on lowest dose at which statistically significant risk is observed

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