K
The Linear, No-Threshold Model

ADOPTION OF THE LINEAR, NO-THRESHOLD MODEL

A series of developments from 1954 through 1972 marked the transition to adoption of the linear, no-threshold model as a predictive model of radiation injury in exposed populations. In 1954, the National Council on Radiation Protection (NCRP) issued new guidance on radiation protection in which the tolerance dose was replaced by a new concept, the maximum permissible dose (MPD) (NCRP, 1954). Implicit in the MPD concept was rejection of the concept of tolerance dose and establishment of the idea of "acceptable risk" at low levels of exposure.

Divided Scientific Opinion, 1958–1966

In 1958 the United Nations Scientific Committee on the Effects of Atomic Radiation issued its first report on the effects of radiation exposures in humans (UNSCEAR, 1958). This report estimated the risk of adverse effects of low-level radiation exposure using both a no-threshold and a threshold model of radiation risk. The report included the following statement:

Present knowledge concerning long-term effects and their correlation with the amount of radiation received does not permit us to evaluate with any precision the possible consequence to man of exposure to low radiation levels. Many effects of radiation are delayed; often they cannot be distinguished from other agents; many will develop once a threshold dose has been exceeded; some may be cumulative and others not; and individuals in large populations or particular



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Radiation in Medicine: A Need for Regulatory Reform K The Linear, No-Threshold Model ADOPTION OF THE LINEAR, NO-THRESHOLD MODEL A series of developments from 1954 through 1972 marked the transition to adoption of the linear, no-threshold model as a predictive model of radiation injury in exposed populations. In 1954, the National Council on Radiation Protection (NCRP) issued new guidance on radiation protection in which the tolerance dose was replaced by a new concept, the maximum permissible dose (MPD) (NCRP, 1954). Implicit in the MPD concept was rejection of the concept of tolerance dose and establishment of the idea of "acceptable risk" at low levels of exposure. Divided Scientific Opinion, 1958–1966 In 1958 the United Nations Scientific Committee on the Effects of Atomic Radiation issued its first report on the effects of radiation exposures in humans (UNSCEAR, 1958). This report estimated the risk of adverse effects of low-level radiation exposure using both a no-threshold and a threshold model of radiation risk. The report included the following statement: Present knowledge concerning long-term effects and their correlation with the amount of radiation received does not permit us to evaluate with any precision the possible consequence to man of exposure to low radiation levels. Many effects of radiation are delayed; often they cannot be distinguished from other agents; many will develop once a threshold dose has been exceeded; some may be cumulative and others not; and individuals in large populations or particular

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Radiation in Medicine: A Need for Regulatory Reform groups such as children and fetuses may have special sensitivity. These facts render it very difficult to accumulate reliable information about the correlation between small doses and their effects either in individuals or in large populations. (UNSCEAR, 1958, p. 42) With respect to radiation-induced leukemia identified in the Japanese populations exposed to atomic radiation well above the low-dose limit, UNSCEAR concluded that the threshold and no-threshold models of radiation injury had equal validity. This conclusion was contested by the Committee on Pathologic Effects of Atomic Radiation of the National Academy of Sciences/National Research Council (NAS/NRC), which stated unequivocally that "a considerable body of experimental evidence" favored nonlinearity and hence presumably a threshold, and urged that nonlinear relationships between dose and effect should be given greater attention (NAS/NRC, 1959). The following year, the short-lived U.S. Federal Radiation Council (FRC, see Appendix G) observed that the linear, no-threshold model merely presented an extrapolated upper limit of radiation risk for low exposure levels (FRC, 1960). In UNSCEAR reports in the 1960s, the committee emphasized that extrapolation of the linear, no-threshold curve provided an upper limit to the risk of low-level exposures (UNSCEAR, 1962, 1964). This position was endorsed by the International Commission on Radiological Protection (ICRP, 1966). Joint Committee on Atomic Energy Hearings, 1957–1960s Meanwhile, in the late 1950s, the congressional Joint Committee on Atomic Energy (JCAE) conducted hearings that had a major influence on the thinking of both the scientific community and the public with regard to radiation hazards. The hearings began in 1957 with an inquiry into the nature of radioactive fallout from weapons testing and its possible effects on humans (JCAE, 1957). Testimony from scientific experts addressed but left unresolved the issue of the most appropriate model for estimating the degree of hazard at low exposure levels. The JCAE addressed this issue again in its 1959 hearings (JCAE, 1959) and again left it unresolved. However, the hearing report (p. 59) included testimony by K.Z. Morgan, Director of Health and Human Physics at the Oak Ridge National Laboratory, claiming that certain bioeffects, including genetic mutations, leukemia induction, and life shortening, occur without a threshold dose. Also influential was the testimony of E.B. Lewis, professor of biology at the University of California, San Francisco, who strongly supported the linear, no-threshold hypothesis as a model for radiation protection standards. Lewis proposed the concept of protection called "as low as reasonably achievable" (ALARA) (JCAE, 1960). In subsequent hearings over the course of the 1960s, the JCAE moved slowly to the endorsement of the linear, no-threshold model of radiation risk.

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Radiation in Medicine: A Need for Regulatory Reform The BEIR Report and the Code of Federal Regulations, 1972 In 1964 the NAS/NRC established an advisory committee on the biological effects of atomic radiation (BEAR) to examine issues related to radiation protection, including the shape of the dose-response curve at low doses. The BEAR committee introduced the concept of regulating doses to the population as a way of limiting the effects of radiation on future generations. The BEAR committee was renamed the Committee on the Biological Effects of Ionizing Radiation (BEIR), which issued its first report in 1972 (NAS/NRC, 1972). The 1972 report did not deal with the issue of the shape of the dose-response curve, but it did provide estimates of cancer risk at low doses based on a linear extrapolation from cancer mortality data at high doses in Japanese survivors and other exposed groups. These estimates implied that radiation carcinogenesis does not exhibit a threshold dose, in spite of the absence of confirmatory experimental data. Also in 1972, the U.S. Atomic Energy Commission (AEC) introduced the ALARA concept (also known as ALAP, as low as practicable) as Appendix I to Title 10, Part 50, of the Code of Federal Regulations. The implication of ALARA is that no threshold exists for adverse radiation effects and that any dose, no matter how small, is potentially injurious to exposed individuals. These actions of the NAS/NRC and the AEC completed a major transition in the conceptualization of radiation risk at low doses, and they provided a foundation for the evolution of health physics as a discipline devoted to the protection of workers and the public against small doses of ionizing radiation. WIDENING APPLICATIONS AND CONTINUING DEBATE Risk-Based Standards for Radiation Protection In 1977 the ICRP announced its risk-based approach to the establishment of standards for radiation protection (ICRP, 1977). This approach was a highly significant departure from traditional dose-based standards, and it defined the concept of acceptable risk from radiation exposure of workers in terms of the fatal accident rate in so-called safe industries. In taking this approach, the ICRP used extrapolation based on the linear, no-threshold model to estimate hypothetical death rates from radiation-induced cancers among workers exposed to low-dose radiation and compared these hypothetical deaths with real and measurable fatalities in other ("safe") industries. The ICRP also introduced a number of factors to express the risk of partial-body irradiation in terms of the equivalent risk of whole-body exposure. This risk-based approach to standards of radiation protection was refined and expanded not only by the ICRP (ICRP, 1978–1980, 1990, 1991), but also by the NCRP (NCRP, 1987) and by several U.S. regulatory agencies, including the Environmental Protection Agency (EPA,

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Radiation in Medicine: A Need for Regulatory Reform 1987), the Department of Energy (DOE, 1988), and the Nuclear Regulatory Commission (NRC, 1991). BEIR Reports, 1979–1990 In 1980 the NAS/NRC BEIR committee released a new report (the "BEIR III" report) on the risks of exposure to ionizing radiation. In the report a majority of the committee endorsed a linear-quadratic1 model of radiation-induced cancer. The report included two "minority opinions," in which one committee member supported a straightforward linear model of cancer induction and another member endorsed a purely quadratic model. This division among the committee members exemplified more general disagreement within the scientific community about the most appropriate way to characterize radiation risk at low doses. It also reflected concern over the growing practice of using dose-response models to estimate hypothetical cancer risks at doses substantially below levels where epidemiological studies have confirmed injury. 2 Two additional BEIR reports were issued after the 1980 report of the BEIR III committee. The BEIR IV report, which addressed the health risks of radon and other internally deposited radionuclides (NAS/NRC, 1988), offered several suggestions for further research that, collectively, called for intensified experimental efforts to characterize the shape of the dose-response curve for long-term health effects at low levels of exposure. The BEIR V report again considered the broad topic of adverse health effects from exposure to low levels of ionizing radiation (NAS/NRC, 1990). As in previous reports, the committee noted the failure of epidemiological studies to demonstrate hereditary effects in humans exposed to low radiation levels. Nevertheless, the committee confirmed previous estimates of radiation-induced genetic risk in humans, and computed a mutation-doubling dose of 1 Sv in agreement with the range of 0.2–2.0 Sv of BEIR I and 0.5–2.5 Sv of BEIR III. There was, however, a significant change in the BEIR V estimates of cancer risk from radiation compared with earlier BEIR reports. The new 1   The linear-quadratic model predicts that the risks of radiation injury at low-level exposures are less than those predicted from a linear extrapolation of risks associated with high dose exposure levels. 2   The BEIR III (NAS/NRC, 1980) report offered several important specific observations. The report noted that it was unknown and probably not determinable whether dose rates on the order of 1 mSv (millisievert) per year, on the order of dose rates from background radiation, were detrimental to people. The report concluded that data presented by Sternglass (1968) and others that purported to show an increased incidence of cancer in populations exposed to low doses were the result of flawed studies. The BEIR III committee recognized that different human genotypes may confer different degrees of cancer risk for a specific dose of radiation, and that developmental effects from radiation exposure in utero may exhibit a threshold dose. Finally, the report suggested that the linear, no-threshold model of radiation risk provided the best estimate of genetic risk.

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Radiation in Medicine: A Need for Regulatory Reform estimates were determined with the linear, no-threshold model, yielding a threefold increase in the risk of solid tumors and a fourfold increase for leukemia. Although the committee did not consider the rate of dose delivery in its estimates of cancer risk, it proposed a Dose Rate Effectiveness Factor (DREF) which, if applied, would reduce the lifetime cancer risk by a factor of two or more if the radiation were delivered over a protracted period. Scientific Studies, 1992–1994 Recently published articles addressing the linear, no-threshold model include those of Land (1993) and Peterson (1993). Land offers a critique of the model's foundation in epidemiological data. Peterson (1993) presents a tabular representation of the evolution of the liner, no-threshold extrapolation to establish an upper limit of radiation risk. He traces the evolution from the postulate that every dose, no matter how small, has an associated risk of ill health, through various steps until the final unequivocal statements are reached that radiation follows a linear, no-threshold dose-response relationship, and that all radiation exposure is unsafe. Other recent reports that bear on the issue of extrapolating risks from high dose to low-dose exposure are several reports that directly address mechanisms of response to low-dose exposures. For example, one-third of the most recent UNSCEAR report is devoted to adaptive responses to radiation in cells and organisms (UNSCEAR, 1994). Finally, a recent international meeting in Kyoto was devoted to examining evidence for biological defense mechanisms in response to low-dose exposures to ionizing radiation (Sugahara, et al., 1992). REFERENCES DOE (U.S. Department of Energy). Radiation Protection for Occupational Workers Order 5480.11. Washington, DC: U.S. Department of Energy, December 21, 1988. FRC (Federal Radiation Council). Background Material for the Development of Radiation Protection Standards: Report No. 1 . Washington, DC: Federal Radiation Council, 1960. ICRP (International Commission on Radiological Protection). The Evaluation of Risks from Radiation. ICRP Publication 8. Oxford, England: Pergamon Press, 1966. ICRP. Recommendations of the International Commission on Radiological Protection. Annals of the ICRP 1(3). Oxford, England: Pergamon Press, 1977. ICRP. Limits for Intakes of Radionuclides by Workers. ICRP Publication 30. Oxford, England: Pergamon Press, 1978–1980. ICRP. Individual Monitoring for Intakes of Radionuclides by Workers: Design and Interpretation. ICRP Publication 54. Oxford, England: Pergamon Press, 1990. ICRP. 1990 Recommendations of the International Commission on Radiological Protection. Annals of the ICRP 21(1–3):1–201, 1991. JCAE (Joint Committee on Atomic Energy) of the Congress of the United States. Hearings on the Nature of Radioactive Fallout and Its Effects on Man, May 27–29 and June 3–7 (2 vols.). Washington, DC: Government Printing Office, 1957.

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Radiation in Medicine: A Need for Regulatory Reform JCAE. Hearings on Fallout from Nuclear Weapons Tests, May 5–8, 1959 (2 vols. plus summary) . Washington, DC: Government Printing Office, 1959. JCAE. Selected Materials on Radiation Protection Criteria and Standards: Their Basis and Use. Washington, DC: Government Printing Office, 1960. NAS/NRC (National Academy of Sciences/National Research Council), Committee on Pathological Effects of Atomic Radiation. A Commentary on the Report of the United Nations Scientific Committee on the Effects of Atomic Radiation. NAS/NRC Publication 647. Washington, DC: National Academy of Sciences/National Research Council, 1959. NAS/NRC. The Effects on Populations of Exposure to Low Levels of Ionizing Radiation. Washington, DC: National Academy Press, 1972. NAS/NRC. The Effects on Populations of Exposure to Low Levels of Ionizing Radiation: BEIR III. Washington, DC: National Academy Press, 1980. NAS/NRC. Health Risks of Radon and Other Internally Deposited Alpha-Emitters: BEIR IV. Washington, DC: National Academy Press, 1988. NAS/NRC. Health Effects of Exposure to Low Levels of Ionizing Radiation: BEIR V. Washington, DC: National Academy Press, 1990. NCRP (National Council on Radiation Protection and Measurements). Permissible Dose from External Sources of Ionizing Radiation. NCRP Report No 17. Washington, DC: U.S. Department of Commerce, National Bureau of Standards Handbook 50, September 24, 1954. NCRP. Recommendations on Limits for Exposure to Ionizing Radiation. NCRP Report No. 93. Bethesda, MD: National Council on Radiation Protection and Measurements, 1987. NRC (U.S. Nuclear Regulatory Commission). Standards for Protection Against Radiation: Final Rule. In: Federal Register Vol. 56. Washington, DC: National Archives and Records Administration, pp. 23360–23474, 1991. Peterson, H.T., Jr. Public Aversion to Environmental Releases of Small Quantities of Radioactive Material. In: R.L. Kathren, D.H. Denham, and K. Salmon, eds. Environmental Health Physics. Proceedings of the Twenty-Sixth Midyear Topical Meeting of the Health Physics Society, January 24–28, 1993. Richland, WA: Research Enterprises Publishing Segment, pp. 255–262, 1993. Sugahara, T., Sagan, L. A., and Aoyama, T. (eds.). Low Dose Irradiation and Biological Defense Mechanisms. Proceedings of the International Conference on Low Dose Irradiation and Biological Defense Mechanisms. New York, NY: Excerpta Medica, 1992. UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation). Report of the United Nations Scientific Committee on the Effects of Atomic Radiation. General Assembly Official Records: Thirteenth Session Supplement No. 17 (A/3838). New York, NY: United Nations, 1958. UNSCEAR. Report of the United Nations Scientific Committee on the Effects of Atomic Radiation. General Assembly Official Records: Seventeenth Session Supplement No. 16 (A/5216). New York, NY: United Nations, 1962. UNSCEAR. Report of the United Nations Scientific Committee on the Effects of Atomic Radiation. General Assembly Official Records: Nineteenth Session Supplement No. 14 (A/5814). New York, NY: United Nations, 1964. UNSCEAR. Report of the United Nations Scientific Committee on the Effects of Atomic Radiation. General Assembly Official Records: Forty-Ninth Session Supplement No. 46 (A/49/46). New York, NY: United Nations, 1994.