THE DOSE AND DOSE-RATE EFFECTIVENESS FACTOR (DDREF)

Interplay of immune modulation, adaptive response and hormesis: Suggestive of threshold for clinical manifestation of effects of ionizing radiation at low doses?

The health impacts of low-dose ionizing radiation exposures have been a subject of debate over the last three to four decades. While there has been enough evidence of "no adverse observable" health effects at low doses and low dose rates, the hypothesis of "Linear No Threshold" continues to rule and govern the principles of radiation protection and the formulation of regulations and public policies. In adopting this conservative approach, the role of the biological processes underway in the human body is kept at abeyance. This review consolidates the available studies that discuss all related biological pathways and repair mechanisms that inhibit the progression of deleterious effects at low doses and low dose rates of ionizing radiation. It is pertinent that, taking cognizance of these processes, there is a need to have a relook at policies of radiation protection, which as of now are too stringent, leading to undue economic losses and negative public perception about radiation.

Noncancer Effects of Ionizing Radiation Exposure on the Eye, the Circulatory System and beyond: Developments made since the 2011 ICRP Statement on Tissue Reactions

For radiation protection purposes, noncancer effects with a threshold-type dose-response relationship have been classified as tissue reactions (formerly called nonstochastic or deterministic effects), and equivalent dose limits aim to prevent occurrence of such tissue reactions. Accumulating evidence demonstrates increased risks for several late occurring noncancer effects at doses and dose rates much lower than previously considered. In 2011, the International Commission on Radiological Protection (ICRP) issued a statement on tissue reactions to recommend a threshold of 0.5 Gy to the lens of the eye for cataracts and to the heart and brain for diseases of the circulatory system (DCS), independent of dose rate. Literature published thereafter continues to provide updated knowledge. Increased risks for cataracts below 0.5 Gy have been reported in several cohorts (e.g., including in those receiving protracted or chronic exposures). A dose threshold for cataracts is less evident with longer follow-up, with limited evidence available for risk of cataract removal surgery. There is emerging evidence for risk of normal-tension glaucoma and diabetic retinopathy, but the long-held tenet that the lens represents among the most radiosensitive tissues in the eye and in the body seems to remain unchanged. For DCS, increased risks have been reported in various cohorts, but the existence or otherwise of a dose threshold is unclear. The level of risk is less uncertain at lower dose and lower dose rate, with the possibility that risk per unit dose is greater at lower doses and dose rates. Target organs and tissues for DCS are also unknown, but may include heart, large blood vessels and kidneys. Identification of potential factors (e.g., sex, age, lifestyle factors, coexposures, comorbidities, genetics and epigenetics) that may modify radiation risk of cataracts and DCS would be important. Other noncancer effects on the radar include neurological effects (e.g., Parkinson's disease, Alzheimer's disease and dementia) of which elevated risk has increasingly been reported. These late occurring noncancer effects tend to deviate from the definition of tissue reactions, necessitating more scientific developments to reconsider the radiation effect classification system and risk management. This paper gives an overview of historical developments made in ICRP prior to the 2011 statement and an update on relevant developments made since the 2011 ICRP statement.

Radiation dose rate effects: what is new and what is needed?

Despite decades of research to understand the biological effects of ionising radiation, there is still much uncertainty over the role of dose rate. Motivated by a virtual workshop on the "Effects of spatial and temporal variation in dose delivery" organised in November 2020 by the Multidisciplinary Low Dose Initiative (MELODI), here, we review studies to date exploring dose rate effects, highlighting significant findings, recent advances and to provide perspective and recommendations for requirements and direction of future work. A comprehensive range of studies is considered, including molecular, cellular, animal, and human studies, with a focus on low linear-energy-transfer radiation exposure. Limits and advantages of each type of study are discussed, and a focus is made on future research needs.

ICRP PUBLICATION 152 Approved by the Commission in November 2021

Radiation detriment is a concept developed by the International Commission on Radiological Protection to quantify the burden of stochastic effects from low-dose and/or low-dose-rate exposures to the human population. It is determined from the lifetime risks of cancer for a set of organs and tissues and the risk of heritable effects, taking into account the severity of the consequences. This publication provides a historical review of detriment calculation methodology since ICRP Publication 26, with details of the procedure developed in ICRP Publication 103, which clarifies data sources, risk models, computational methods, and rationale for the choice of parameter values. A selected sensitivity analysis was conducted to identify the parameters and calculation conditions that can be major sources of variation and uncertainty in the calculation of radiation detriment. It has demonstrated that sex, age at exposure, dose and dose-rate effectiveness factor, dose assumption in the calculation of lifetime risk, and lethality fraction have a substantial impact on radiation detriment values. Although the current scheme of radiation detriment calculation is well established, it needs to evolve to better reflect changes in population health statistics and progress in scientific understanding of radiation health effects. In this regard, some key parameters require updating, such as the reference population data and cancer severity. There is also room for improvement in cancer risk models based on the accumulation of recent epidemiological findings. Finally, the importance of improving the comprehensibility of the detriment concept and the transparency of its calculation process is emphasised.© 2022 ICRP. Published by SAGE.

Methodological improvements to meta-analysis of low dose rate studies and derivation of dose and dose-rate effectiveness factors

Epidemiological studies of cancer rates associated with external and internal exposure to ionizing radiation have been subject to extensive reviews by various scientific bodies. It has long been assumed that radiation-induced cancer risks at low doses or low-dose rates are lower (per unit dose) than those at higher doses and dose rates. Based on a mixture of experimental and epidemiologic evidence the International Commission on Radiological Protection recommended the use of a dose and dose-rate effectiveness factor for purposes of radiological protection to reduce solid cancer risks obtained from moderate-to-high acute dose studies (e.g. those derived from the Japanese atomic bomb survivors) when applied to low dose or low-dose rate exposures. In the last few years there have been a number of attempts at assessing the effect of extrapolation of dose rate via direct comparison of observed risks in low-dose rate occupational studies and appropriately age/sex-adjusted analyses of the Japanese atomic bomb survivors. The usual approach is to consider the ratio of the excess relative risks in the two studies, a measure of the inverse of the dose rate effectiveness factor. This can be estimated using standard meta-analysis with inverse weighting of ratios of relative risks using variances derived via the delta method. In this paper certain potential statistical problems in the ratio of estimated excess relative risks for low-dose rate studies to the excess relative risk in the Japanese atomic bomb survivors are discussed, specifically the absence of a well-defined mean and the theoretically unbounded variance of this ratio. A slightly different method of meta-analysis for estimating uncertainties of these ratios is proposed, motivated by Fieller's theorem, which leads to slightly different central estimates and confidence intervals for the dose rate effectiveness factor. However, given the uncertainties in the data, the differences in mean values and uncertainties from the dose rate effectiveness factor estimated using delta-method-based meta-analysis are not substantial, generally less than 70%.

Risk of cancer associated with low-dose radiation exposure: comparison of results between the INWORKS nuclear workers study and the A-bomb survivors study

The Life Span Study (LSS) of Japanese atomic bomb survivors has served as the primary basis for estimates of radiation-related disease risks that inform radiation protection standards. The long-term follow-up of radiation-monitored nuclear workers provides estimates of radiation-cancer associations that complement findings from the LSS. Here, a comparison of radiation-cancer mortality risk estimates derived from the LSS and INWORKS, a large international nuclear worker study, is presented. Restrictions were made, so that the two study populations were similar with respect to ages and periods of exposure, leading to selection of 45,625 A-bomb survivors and 259,350 nuclear workers. For solid cancer, excess relative rates (ERR) per gray (Gy) were 0.28 (90% CI 0.18; 0.38) in the LSS, and 0.29 (90% CI 0.07; 0.53) in INWORKS. A joint analysis of the data allowed for a formal assessment of heterogeneity of the ERR per Gy across the two studies (P = 0.909), with minimal evidence of curvature or of a modifying effect of attained age, age at exposure, or sex in either study. There was evidence in both cohorts of modification of the excess absolute risk (EAR) of solid cancer by attained age, with a trend of increasing EAR per Gy with attained age. For leukemia, under a simple linear model, the ERR per Gy was 2.75 (90% CI 1.73; 4.21) in the LSS and 3.15 (90% CI 1.12; 5.72) in INWORKS, with evidence of curvature in the association across the range of dose observed in the LSS but not in INWORKS; the EAR per Gy was 3.54 (90% CI 2.30; 5.05) in the LSS and 2.03 (90% CI 0.36; 4.07) in INWORKS. These findings from different study populations may help understanding of radiation risks, with INWORKS contributing information derived from cohorts of workers with protracted low dose-rate exposures.

Dose limits for occupational exposure to ionising radiation and genotoxic carcinogens: a German perspective

This paper summarises the view of the German Commission on Radiological Protection ("Strahlenschutzkommission", SSK) on the rationale behind the currently valid dose limits and dose constraints for workers recommended by the International Commission on Radiological Protection (ICRP). The paper includes a discussion of the reasoning behind current dose limits followed by a discussion of the detriment used by ICRP as a measure for stochastic health effects. Studies on radiation-induced cancer are reviewed because this endpoint represents the most important contribution to detriment. Recent findings on radiation-induced circulatory disease that are currently not included in detriment calculation are also reviewed. It appeared that for detriment calculations the contribution of circulatory diseases plays only a secondary role, although the uncertainties involved in their risk estimates are considerable. These discussions are complemented by a review of the procedures currently in use in Germany, or in discussion elsewhere, to define limits for genotoxic carcinogens. To put these concepts in perspective, actual occupational radiation exposures are exemplified with data from Germany, for the year 2012, and regulations in Germany are compared to the recommendations issued by ICRP. Conclusions include, among others, considerations on radiation protection concepts currently in use and recommendations of the SSK on the limitation of annual effective dose and effective dose cumulated over a whole working life.

Sensitivity analysis of parameters and methodological choices used in calculation of radiation detriment for solid cancer

Purpose: Radiation detriment is a concept used by the International Commission on Radiological Protection (ICRP) to quantify the harmful health effects of radiation exposure in humans. The current approach of radiation detriment calculation has been defined in ICRP Publication 103 in 2007. It is determined from lifetime risk of cancer and heritable effects for a composite reference population, taking into account the severity of the disease in terms of lethality, quality of life and years of life lost. Many parameters are used in the calculations and the variation of these parameters can have effects on the cancer detriment, which needs to be investigated.Materials and methods: In this paper, we conducted a sensitivity analysis for examining the impact of 12 different parameters or methodological choices on the calculation of solid cancer detriment, such as the lifetime risk calculation method, survival curve, dose and dose-rate effectiveness factor (DDREF), age-at-exposure, sex, reference population, risk transfer model, latency, attained age, lethality, minimum quality of life factor and relative cancer-free life lost. Sensitivity calculations have been performed systematically for each of 10 solid cancer sites, by changing each one of the parameters in turn.Results: This sensitivity analysis demonstrated a large impact on estimated detriment from DDREF, age-at-exposure, sex and lethality, a noticeable impact of risk transfer model associated to variation of baseline rates, and a limited impact of risk calculation method, survival curve, latency, attained age, quality of life and relative years of life lost.Conclusion: These results could have implications for radiation protection standards, and they should help define priorities for future research in the field of low radiation dose and dose rate research. The present sensitivity analysis is part of a global effort of ICRP to review the bases of radiation detriment calculation and assess potential evolutions to improve the radiation protection system.

History of radiation detriment and its calculation methodology used in ICRP Publication 103

Over the past decades, the International Commission on Radiological Protection (ICRP) has used radiation detriment, which is a multidimensional concept to quantify the overall harm to health from stochastic effects of low-level radiation exposure of different parts of the body. Each tissue-specific detriment is determined from the nominal tissue-specific risk coefficient, weighted by the severity of the disease in terms of lethality, impact on quality of life and years of life lost. Total detriment is the sum of the detriments for separate tissues and organs. Tissue weighting factors for the calculation of effective dose are based on relative contributions of each tissue to the total detriment. Calculating radiation detriment is a complex process that requires information from various sources and judgements on how to achieve calculations. As such, it is important to document its calculation methodology. To improve the traceability of calculations and form a solid basis for future recommendations, the ICRP Task Group 102 on detriment calculation methodology was established in 2016. As part of its mission, the history of radiation detriment was reviewed, and the process of detriment calculation was detailed. This article summarises that work, aiming to clarify the methodology of detriment calculation currently used by ICRP.