Linear no-threshold model

The linear no-threshold model (LNT) is a model used in radiation protection to quantify radiation exposure and set regulatory limits. It is most frequently used to calculate the probability of radiation-induced cancer at both high doses where epidemiology studies support its application but, controversially, it likewise finds applications in calculating the effects of low doses, a dose region that is fraught with much less statistical confidence in its predictive power but that nonetheless has resulted in major personal and policy decisions in regards to public health. The model assumes that the long-term, biological damage caused by ionizing radiation (essentially the cancer risk) is directly proportional to the dose. This allows the summation by dosimeters of all radiation exposure, without taking into consideration dose levels or dose rates. In other words, radiation is always considered harmful with no safety threshold, and the sum of several very small exposures are considered to have the same effect as one larger exposure (response linearity). This is contrary to numerous deterministic observations, primarily that of and those laying behind the rationale for Dose fractionation in radiotherapy and the repairs noted in the healthy Cell survival curve.The assumption that any stimulatory hormetic effects from low doses of ionizing radiation will have a significant health benefit to humans that exceeds potential detrimental effects from the radiation exposure is unwarranted at this time.The scientific research base shows that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial.Until the uncertainties on low-dose response are resolved, the Committee believes that an increase in the risk of tumour induction proportionate to the radiation dose is consistent with developing knowledge and that it remains, accordingly, the most scientifically defensible approximation of low-dose response. However, a strictly linear dose response should not be expected in all circumstances.Underlying the risk models is a large body of epidemiological and radiobiological data. In general, results from both lines of research are consistent with a linear, no-threshold dose (LNT) response model in which the risk of inducing a cancer in an irradiated tissue by low doses of radiation is proportional to the dose to that tissue.In conclusion, this report raises doubts on the validity of using LNT for evaluating the carcinogenic risk of low doses (< 100 mSv) and even more for very low doses (< 10 mSv). The LNT concept can be a useful pragmatic tool for assessing rules in radioprotection for doses above 10 mSv; however since it is not based on biological concepts of our current knowledge, it should not be used without precaution for assessing by extrapolation the risks associated with low and even more so, with very low doses (< 10 mSv), especially for benefit-risk assessments imposed on radiologists by the European directive 97-43.In accordance with current knowledge of radiation health risks, the Health Physics Society recommends against quantitative estimation of health risks below an individual dose of 5 rem (50 mSv) in one year or a lifetime dose of 10 rem (100 mSv) above that received from natural sources. Doses from natural background radiation in the United States average about 0.3 rem (3 mSv) per year. A dose of 5 rem (50 mSv) will be accumulated in the first 17 years of life and about 25 rem (250 mSv) in a lifetime of 80 years. Estimation of health risk associated with radiation doses that are of similar magnitude as those received from natural sources should be strictly qualitative and encompass a range of hypothetical health outcomes, including the possibility of no adverse health effects at such low levels.There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem or 100 mSv (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent. The linear no-threshold model (LNT) is a model used in radiation protection to quantify radiation exposure and set regulatory limits. It is most frequently used to calculate the probability of radiation-induced cancer at both high doses where epidemiology studies support its application but, controversially, it likewise finds applications in calculating the effects of low doses, a dose region that is fraught with much less statistical confidence in its predictive power but that nonetheless has resulted in major personal and policy decisions in regards to public health. The model assumes that the long-term, biological damage caused by ionizing radiation (essentially the cancer risk) is directly proportional to the dose. This allows the summation by dosimeters of all radiation exposure, without taking into consideration dose levels or dose rates. In other words, radiation is always considered harmful with no safety threshold, and the sum of several very small exposures are considered to have the same effect as one larger exposure (response linearity). This is contrary to numerous deterministic observations, primarily that of and those laying behind the rationale for Dose fractionation in radiotherapy and the repairs noted in the healthy Cell survival curve. One of the organizations for establishing recommendations on radiation protection guidelines internationally, the UNSCEAR, recommended in 2014 policies that do not agree with the Linear No-Threshold model at exposure levels below background levels of radiation to the UN General Assembly from the Fifty-Ninth Session of the Committee. Its recommendation states that 'the Scientific Committee does not recommend multiplying very low doses by large numbers of individuals to estimate numbers of radiation-induced health effects within a population exposed to incremental doses at levels equivalent to or lower than natural background levels.' This is a reversal from previous recommendations by the same organization. There are three active (2016) challenges to the LNT model currently being considered by the US Nuclear Regulatory Commission. One was filed by Nuclear Medicine Professor Carol Marcus of UCLA, who calls the LNT model scientific 'baloney'. Whether the model describes the reality for small-dose exposures is disputed. It opposes two competing schools of thought: the threshold model, which assumes that very small exposures are harmless, and the radiation hormesis model, which claims that radiation at very small doses can be beneficial. Because the current data are inconclusive, scientists disagree on which model should be used. Pending any definitive answer to these questions and the precautionary principle, the model is sometimes used to quantify the cancerous effect of collective doses of low-level radioactive contaminations, even though it estimates a positive number of excess deaths at levels that would have had zero deaths, or saved lives, in the two other models. Such practice has been condemned by the International Commission on Radiological Protection. The LNT model is sometimes applied to other cancer hazards such as polychlorinated biphenyls in drinking water. The association of exposure to radiation with cancer had been observed as early as 1902, six years after the discovery of X-ray by Wilhelm Röntgen and radioactivity by Henri Becquerel. In 1927, Hermann Muller demonstrated that radiation may cause genetic mutation. He also suggested mutation as a cause of cancer. Muller, who received a Nobel Prize for his work on the mutagenic effect of radiation in 1946, asserted in his Nobel Lecture, 'The Production of Mutation', that mutation frequency is 'directly and simply proportional to the dose of irradiation applied' and that there is 'no threshold dose'. The early studies were based on relatively high levels of radiation that made it hard to establish the safety of low level of radiation, and many scientists at that time believed that there may be a tolerance level, and that low doses of radiation may not be harmful. A later study in 1955 on mice exposed to low dose of radiation suggest that they may outlive control animals. The interest in the effect of radiation intensified after the dropping of atomic bombs on Hiroshima and Nagasaki, and studies were conducted on the survivors. Although compelling evidence on the effect of low dosage of radiation was hard to come by, by the late 1940s, the idea of LNT became more popular due to its mathematical simplicity. In 1954, the National Council on Radiation Protection and Measurements (NCRP) introduced the concept of maximum permissible dose. In 1958, United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) assessed the LNT model and a threshold model, but noted the difficulty in acquiring 'reliable information about the correlation between small doses and their effects either in individuals or in large populations'. The United States Congress Joint Committee on Atomic Energy (JCAE) similarly could not establish if there is a threshold or 'safe' level for exposure, nevertheless it introduced the concept of 'As Low As Reasonably Achievable' (ALARA). ALARA would become a fundamental principle in radiation protection policy that implicitly accepts the validity of LNT. In 1959, United States Federal Radiation Council (FRC) supported the concept of the LNT extrapolation down to the low dose region in its first report. By the 1970s, the LNT model had become accepted as the standard in radiation protection practice by a number of bodies. In 1972, the first report of National Academy of Sciences (NAS) Biological Effects of Ionizing Radiation (BEIR), an expert panel who reviewed available peer reviewed literature, supported the LNT model on pragmatic grounds, noting that while 'dose-effect relationship for x rays and gamma rays may not be a linear function', the 'use of linear extrapolation . . . may be justified on pragmatic grounds as a basis for risk estimation.' In its seventh report of 2006, NAS BEIR VII writes, 'the committee concludes that the preponderance of information indicates that there will be some risk, even at low doses'. Radiation precautions have led to sunlight being listed as a carcinogen at all sun exposure rates, due to the ultraviolet component of sunlight, with no safe level of sunlight exposure being suggested, following the precautionary LNT model. According to a 2007 study submitted by the University of Ottawa to the Department of Health and Human Services in Washington, D.C., there is not enough information to determine a safe level of sun exposure at this time.