Implications of recent epidemiologic studies for the linear nonthreshold model and radiation protection

Abstract

The recently published NCRP Commentary No. 27 evaluated the new information from epidemiologic studies as to their degree of support for applying the linear nonthreshold (LNT) model of carcinogenic effects for radiation protection purposes (NCRP 2018 Implications of Recent Epidemiologic Studies for the Linear Nonthreshold Model and Radiation Protection, Commentary No. 27 (Bethesda, MD: National Council on Radiation Protection and Measurements)). The aim was to determine whether recent epidemiologic studies of low-LET radiation, particularly those at low doses and/or low dose rates (LD/LDR), broadly support the LNT model of carcinogenic risk or, on the contrary, demonstrate sufficient evidence that the LNT model is inappropriate for the purposes of radiation protection. An updated review was needed because a considerable number of reports of radiation epidemiologic studies based on new or updated data have been published since other major reviews were conducted by national and international scientific committees. The Commentary provides a critical review of the LD/LDR studies that are most directly applicable to current occupational, environmental and medical radiation exposure circumstances. This Memorandum summarises several of the more important LD/LDR studies that incorporate radiation dose responses for solid cancer and leukemia that were reviewed in Commentary No. 27. In addition, an overview is provided of radiation studies of breast and thyroid cancers, and cancer after childhood exposures. Non-cancers are briefly touched upon such as ischemic heart disease, cataracts, and heritable genetic effects. To assess the applicability and utility of the LNT model for radiation protection, the Commentary evaluated 29 epidemiologic studies or groups of studies, primarily of total solid cancer, in terms of strengths and weaknesses in their epidemiologic methods, dosimetry approaches, and statistical modelling, and the degree to which they supported a LNT model for continued use in radiation protection. Recommendations for how to make epidemiologic radiation studies more informative are outlined. The NCRP Committee recognises that the risks from LD/LDR exposures are small and uncertain. The Committee judged that the available epidemiologic data were broadly supportive of the LNT model and that at this time no alternative dose-response relationship appears more pragmatic or prudent for radiation protection purposes.

Second follow-up of a German cohort on childhood cancer incidence after exposure to postnatal diagnostic x-ray

“…The analysis of Hammer and colleagues and this extended follow-up analysis suggest that exposure to very low doses of postnatal conventional diagnostic X-rays (median effective linear no-threshold (LNT) model for the purposes of radiological protection, which is in accordance with the older judgments by other national and international scientific committees [35]. In the light of this commentary of the NCRP on LNT and for reasons of radiation protection [36] an extension of this cohort with further follow-up of cancer incidence into adult age and additional dosimetry including CT could be a future goal, which would provide more insight into the risk of cancer in a cohort with very low doses of ionizing radiation due to postnatal diagnostic X-ray examination during childhood. A c c e p t e d M a n u s c A c c e p t e d M a n u s c…”

Individual Calculation of Effective Dose and Risk of Malignancy Based on Monte Carlo Simulations after Whole Body Computed Tomography

“…Forth, all tissue weighting factors that are provided in literature so far are averaged for sex and age, which may probably limit their applicability for patient specific risk estimation. Fifth, the linear Non-threshold Dose-Response Model itself is discussed controversially among experts for diagnostic dose levels 31,32 . Sixth, LAR and ERR calculations in this study are only related to low dose radiation exposure.…”

Patient-adapted organ absorbed dose and effective dose estimates in pediatric 18F-FDG positron emission tomography/computed tomography studies

“…Patients were divided into five groups according to age at the time of the examination: less than 1 year old (< 1), one to 5 years old (1-5), six to 10 years old (6-10), 11 to 15 years old (11)(12)(13)(14)(15), and 16 to 17 years old (16,17).…”

“…Tissue weighting factors, based on population-averaged values, as used in the calculation of E, make E no more a reliable indicator of individual detriment than population-based organ-specific factors [10]. In the current paradigm of radiation protection, the known relationship between dose and risk at higher radiation dose is assumed to extrapolate linearly to that at lower dose, and children are considered to be at greater risk of developing radiation-induced tumors due to their life expectancy and higher radiosensitivity of select tissues [7,[10][11][12]. The basis for the belief of relatively higher risk for children demonstrated in a report by the National Research Council is challenged by some in light of their view that the risks at low radiation doses such as those incurred during medical imaging procedures are not unequivocally supported by current epidemiological data [13,14].…”