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Nuclear Regulatory Commission (NRC)

Advisory Committee on the Medical Uses of Isotopes (ACMUI)

Report on the Hormesis/Linear No-Threshold Petitions

Subcommittee Members

Philip Alderson, M.D. (Chair), Susan Langhorst, Ph.D., Christopher Palestro, M.D., John Suh, 

M.D., Laura Weil, and Pat Zanzonico, Ph.D.

Charge

To consider the requests in the recent petitions to the NRC related to radiation hormesis and the 

linear no-threshold dose-response model of radiation carcinogenesis and to suggest 

recommendations the ACMUI should make to the Commission.

Recommendation

The “correct” dose-response model for radiation carcinogenesis remains an unsettled scientific 

question. There is a large, and growing, body of scientific literature as well as mechanistic 

considerations which suggest that 1) the LNT model may overstate the carcinogenic risk of 

radiation at diagnostic medical, occupational, and environmental doses and 2) such low doses

may, in fact, exert a hormetic (ie a beneficial or protective) effect. However, in the absence of 

definitive refutation of the LNT model and while strongly encouraging continued investigation 

critically comparing alternative models, regulatory authorities should exercise prudent (though 

not excessive) conservatism in formulating radiation protection standards. The ACMUI 

therefore recommends that, for the time being and subject to reconsideration as additional 

scientific evidence becomes available, the NRC continue to base the formulation of radiation 

protection standards on the LNT model.

Discussion

Linear no-threshold versus alternative dose-response models

The linear no-threshold (LNT) dose-response model posits that there is no “safe” dose of 

radiation below which there is no increased risk of cancer. Since the LNT model has not been 

scientifically validated in the low-dose range1, dose-effect data at high doses (eg the A-bomb 

survivor lifespan study) have been extrapolated linearly down to zero dose. Some have argued 

that the data supporting the LNT model are equivocal and are refuted by epidemiologic and 

experimental studies. The validity, applicability, and utility of the LNT model are questioned and 

subject to contentious debate (1-3). There is, in fact, mounting evidence that the risk of 

radiation carcinogenesis at doses below 100 mSv is overestimated by the LNT model and there 

is some evidence that such low doses actually exert a hormetic effect. The phenomenon of 

radiation hormesis posits that individuals exposed to such low radiation doses actually have a 

lower subsequent risk of cancer than unexposed individuals, presumably as a result of 

radiogenic upregulation of cellular repair mechanisms or other adaptive response(s) (1-3).

No prospective epidemiologic studies with appropriate non-irradiated controls have definitely 

demonstrated either the adverse or the beneficial effects of radiation doses less than 100 mSv

in man, and current estimates of the risks of low-dose radiation suggest that very large-scale 

 

1 While there is no rigorous distinction between “low”- and “high”-dose radiation, the former are generally 

consistent with radiation doses encountered in diagnostic medical, occupational, and environmental 

contexts – of the order of 100 mSv or less when delivered acutely and perhaps somewhat greater when 

delivered over a protracted period of time.
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epidemiological studies with long-term follow-up would be needed to actually quantify any such 

risks or benefits; such studies may be logistically and financially prohibitive. As a result, a 

mathematical extrapolation model remains the only practical approach to estimating the 

presumed excess cancer risk from low-dose radiation, and the dose-response data derived from 

epidemiological studies of human cohorts, such as the A-bomb survivors, exposed to high-dose 

radiation are largely consistent with an LNT model. The LNT model has thus been endorsed by

a number of authoritative scientific organizations including the National Council on Radiation 

Protection and Measurements (NCRP) and the International Commission on Radiological 

Protection (ICRP) as a prudently conservative model (4). Regulatory bodies such as the NRC 

formulate radiation protection standards based on the recommendations of organizations such 

as the NCRP and ICRP and thus have adopted, at least indirectly, the LNT model.

The literature on radiation carcinogenesis and on radiation hormesis is vast and continues to 

grow, and even a cursory review of this literature is beyond the scope of this Report. However, 

in order to provide some context for this ongoing controversy, several notable studies are 

summarized below2.

Atomic bomb survivors

The Radiation Effects Research Foundation (RERF) published its 14th report (1950-2003) on 

mortality in the Life Span Study (LSS) group. The LSS group is composed of ~120,000 subjects 

who were either atomic bomb survivors and residents of Hiroshima and Nagasaki or were not in 

either city at the time of the bombing (7). The last report provided six years of additional followup, and showed: 1) the risk of death from all causes, especially solid tumors, increases with 

radiation dose, with a linear dose-response relationship; 2) the sex-averaged excess relative

risk (ERR)3increased by ~29% (95% CI 17%, 41%) per decade decrease at exposure; and 3) 

the estimated lowest dose range with a significantly increased ERR for solid cancers was 0 to 

0.2 Gy with no non-zero dose threshold apparent. The study did show that a concave (linear 

quadratic) curve was the best fit for data restricted to doses less than 2 Gy, as the risk 

estimates for doses up to ~0.5 Gy were lower than those predicted by the linear model. Using a 

nonparametric statistical procedure, reanalysis of the LSS cohort of atomic bomb survivors has 

exhibited a low dose threshold (<0.2 Sv or 200 mSv) with negative ERR, suggesting a radiation 

hormesis model (8).

Low-dose protracted or intermittent radiation exposure

A study from the International Nuclear WORKers Study (INWORKS) followed 308,297 

monitored radiation workers from the United Kingdom, France and United States, who were 

employed for at least one year, to determine deaths caused by multiple myeloma, lymphoma, 

and leukemia (9). Although the estimated red bone marrow absorbed dose was very low (mean 

+ standard deviation (SD): 1.1 + 2.6 mGy/year), the ERR for leukemia excluding chronic 

lymphocytic leukemia (CLL) was 2.96 per Gy (90% confidence interval (CI): 1.17-5.21) with a 2-

year latency period). A simple linear function of cumulative dose described the trend in ERR of 

leukemia excluding CLL with dose. The ERR of leukemia excluding CLL persisted even for

doses less than 300 mGy and less than 100 mGy, although the 90% CI were much wider for 

these respective dose ranges.

Radiation exposure from CT scans in childhood

 

2 For additional references and critical, though reasonably succinct, reviews of the LNT model and 

alternative dose-response models (including hormesis), the reader is referred to references (5) and (6).

3 The excess relative risk is the fractional increase in the cancer incidence per unit dose.
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A retrospective cohort of over 176,000 children and young adults (younger than 22 years of age 

at exposure and without an antecedent history of cancer) was analyzed by the National Health 

Services Central Registry from 1985 to 2008 to assess excess risk for leukemia and brain 

tumors after CT scans (10). The absorbed dose to the brain and red bone marrow doses per CT 

scan in mGy were estimated using Poisson relative risk models. For example, a 10-year-old 

child had an estimated dose to the brain and red bone marrow of 35 and 6 mGy, respectively. 

The ERR per mGy was 0.036 (95% CI: 0.005-0.120; p=0.0097) for leukemia and 0.023 (95% 

CI: 0.010-0.049; p<0.0001) for brain tumors. The analysis showed little evidence for nonlinearity of the dose-response.

Biological Effects of Ionizing Radiation (BEIR) VII

For six decades, the US National Academy of Sciences has commissioned a series of reports,

the Biological Effects of Ionizing Radiation (BEIR) reports, to study the health effects from low 

levels of ionizing radiation. The most recent such report, BEIR VII, places the greatest emphasis 

on RERF data, which may not be applicable to the US and other Western populations. The 

Report concluded that, ‘‘...current scientific evidence is consistent with the hypothesis that there 

is a linear, no-threshold dose-response relationship between exposure to ionizing radiation and 

the development of radiation-induced solid cancers in humans” (11). The BEIR VII Report did 

acknowledge the significant limitations and uncertainties of its risk estimates. 

Mechanistic considerations and epidemiologic studies of background radiation and of nuclear 

accidents

Mechanistic studies and epidemiologic studies of background radiation and recent nuclear 

accidents (Fukushima and Chernobyl) suggest that LNT may not be the correct model. From a 

biologic standpoint, the frequency of foci of DNA damage low radiation doses is much lower 

than that associated with spontaneous mutation, suggesting that molecular repair mechanisms 

are very efficient at eliminating DNA damage and mitigating the impact of such damage at low 

radiation doses. There are some recent data that suggest an increased risk of cancer after CT 

scans. Opponents of LNT, however, contend that the dosimetry was not individualized and 

suggest that preexisting medical conditions may have led to higher cancer risks (12). Although 

areas such as Yangjiang, China and Kerala, India have high natural background levels of 

radiation (eg up to an order of magnitude higher than the mean natural background radiation 

doses per capita in the United States), the incidence of cancer in such areas does not appear to 

be increased (13,14). Finally, data from nuclear accidents suggest that low-dose radiation does

not increase the cancer risk among exposed residents.

References

1. Siegel JA, Stabin MG. RADAR commentary: use of linear no threshold hypothesis in 

radiation protection regulation in the United States. Health Phys. 2012;102(1):90-99.

2. Cuttler JM. Commentary of using LNT for radiation protection and risk assessment. Dose 

Response. 2010;8(3):378-383.

3. Calabrese EJ. How the U.S. National Academy of Sciences misled the world community on 

cancer risk assessment: new findings challenge historical foundations of the linear dose 

response. Arch Toxicol. 2013;87(12):2063-2081.

4. National Council on Radiation Protection and Measurement. Evaluation of the linearnonthreshold dose-response model for ionizing radiation, NCRP Report No 136, Bethesda, 

MD, 2001.

5. Little MP, Wakeford R, Tawn, EJ, et al. Risks associated with low doses and low dose rates 

of ionizing radiation: Why linearity may be (almost) the best we can do. Radiology 2009; 

251(1):6–12.
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6. Tubiana M, Feinendegen LE, Yang C, et al. The linear no-threshold relationship is 

inconsistent with radiation biologic and experimental data. Radiology 2009; 251(1):13-22.

7. Ozasa K, Shimizu Y, Suyama A, et al. Studies of the mortality of atomic bomb survivors, 

report 14, 1950-2003: an overview of cancer and noncancer diseases. Radiat Res. 

2012;177(3):229-243.

8. Sasaki MS, Tachibana A, Takeda S. Cancer risk at low doses of ionizing radiation: artificial 

neural networks inference from atomic bomb survivors. J Radiat Res. 2014;55(3):391-406.

9. Leuraud K, Richardson DB, Cardis E, et al. Ionising radiation and risk of death from 

leukaemia and lymphoma in radiation-monitored workers (INWORKS): an international 

cohort study. Lancet Haematol. 2015;2:e276-81.

10. Pearce MS, Salotti JA, Little MP, et al. Radiation exposure from CT scans in childhood and 

subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet. 

2012;380:499-505.

11. National Research Council of the National Academies. Health Risks from Exposure to Low 

Levels of Ionizing Radiation: BEIR VII Phase 2. Washington, DC: The National Academies 

Press; 2006.

12. UNSCEAR Report 2013. Scientific Annex B: Effects of Radiation Exposure of Children. Vol 

II. New York, NY: United Nations; 2013.

13. Nair MK, Nambi KS, Amma NS, et al. Population study in the high natural background 

radiation area in Kerala, India. Radiat Res 1999;152(6 suppl):S145–S148.

14. Tao Z, Zha Y, Akiba S, et al. Cancer mortality in the high background radiation areas of 

Yangjiang, China during the period between 1979 and 1995. J Radiat Res (Tokyo) 

2000;41(suppl):31–41.