Transcript Uncertainties Major Risk (NCRP 126)

Deconstructing Linearity
Kenneth L. Mossman
Professor of Health Physics
Director, Office of Radiation Safety
Arizona State University
Tempe, AZ
Deconstructing Linearity
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Nature of the debate
Dose extrapolation
Uncertainties in risk estimates
Other predictive theories
Problems / Solutions
The LNT Debate
• Economic costs
– environmental clean up (>$100 billion)
– regulatory compliance (>$10 billion/y)
• Fear of radiation
– abortions following Chernobyl
– mammography
Cost of Regulation
100,000,000
• Viscusi, 1992
• 1990 US dollars
• Nuclear
regulations not
cost effective
10,000,000
1,000,000
100,000
10,000
1,000
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Cost ($) Per Life Saved
1,000,000,000
The LNT Debate
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
• 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
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Extrapolation Factor
Extrapolating Health Risks
1000
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10
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Probability of Radiogenic Cancer
Risk Uncertainty at Low Doses
Lifetime cancer risk ~ 5%/Sv
CL: ??
BEIR V: lower limit of risk
includes zero at natural
background levels
Lifetime cancer risk ~ 5%/Sv
90% CL: 1.15-8.08%/Sv
Dose Extrapolation Factor ~ 100
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Dose (mSv)
400
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1000
Uncertainties in Risk
(NCRP 126)
• Population of all ages:
• Work population:
• 90% CL:
5%/Sv
4%/Sv
1.15% - 8.08%/Sv
Sources of Uncertainty
(NCRP 126)
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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 (?)
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
LNT: To Be Or Not To Be?
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
Hypotheses, Models and Theories
Theory
Conceptual
Model
Hypothesis
Testing
Data
Observations
Models Lead to Theories
Model
Theory
Billiard balls collide and
bounce off one another
Kinetic theory of gases
Bohr model of the atom
Quantum theory
Target model of radiation action
Linear no-threshold theory
LSS Data Supports Mutually
Exclusive Theories
Theory
Source of Data
Comment
Linear no-threshold
Pierce et al., 1996
Curvilinear or
threshold
Little and Muirhead
1996
Curvilinear or
threshold
Hoel and Li, 1998
Supralinearity
Pierce et al., 1996
Hormesis
Kondo, 1991
The dose response for cancer mortality is linear
down to 50 mSv
Upward curvature in dose response for
leukemia incidence and mortality; no curvature
observed for solid cancers; evidence for
threshold in non-melanoma skin cancer
A-bomb cancer incidence data agree more with
a threshold or nonlinear dose-response curve
than a purely linear one although the linear
dose-response is statistically equivalent
Excess relative risk per Sv increases with decreasing
dose
Cancer mortality is reduced in male survivors of the
Nagasaki bomb below ~50 mGy
LSS Data Supports Mutually
Exclusive Theories
• RERF - LSS
data
• Dose-response
for pooled noncancer disease
mortality
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
Resilience of the Linear
No-Threshold Theory
• External correction factors
– e.g. DDREF
• Anomolous results explained
– e.g. Radon ecological studies
The LNT Debate
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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?)
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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
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