deconstructing linearity kenneth l. mossman professor of health physics director, office of...

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

• Problems / Solutions

The LNT Debate

• Economic costs– environmental clean up (>$100 billion)– regulatory compliance (>$10 billion/y)

• Fear of radiation– abortions following Chernobyl– mammography

Cos

t ($)

Per

Lif

e S

aved

Cost of Regulation

• Viscusi, 1992

• 1990 US dollars

• Nuclear regulations not cost effective

1

10

100

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

1,000,000,000

Passiv

e Res

traint

s & B

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Alcoho

l/Dru

g Con

trol

Radion

uclid

es/U

Mine

s

U mill

tailin

gsRad

ionuc

lies

DOE Fac

ilities

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

Extrapolating Health Risks

1

10

100

1000

Ex

tra

po

lati

on

Fa

cto

r

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 ~ 100Pro

babi

lity

of

Rad

ioge

nic

Can

cer

Dose (mSv)

0 10 20 30 200 400 600 800 1000

Uncertainties in Risk(NCRP 126)

• Population of all ages: 5%/Sv

• Work population: 4%/Sv

• 90% CL: 1.15% - 8.08%/Sv

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 (?)

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

Data Observations

Theory

Hypothesis Testing

ConceptualModel

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

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

LSS Data Supports Mutually Exclusive Theories

• RERF - LSS data

• Dose-response for pooled non-cancer 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 LinearNo-Threshold Theory

• External correction factors– e.g. DDREF

• Anomolous results explained– e.g. Radon ecological studies

The LNT Debate

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

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