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

Responses of the carotid artery to acute, fractionated or chronic ionizing irradiation, and differences from the aorta

The circulatory system receives ionizing radiation at various dose rates. Given mounting epidemiological evidence of elevated radiation risks for diseases of the circulatory system (DCS), the International Commission on Radiological Protection recently recommended the first ever dose threshold for DCS. However, very little knowledge exists about whether radiation effects differ with dose rates and among tissues of the circulatory system. Here, we investigated the impact of dose rates in the carotid artery (CA) and compared it with the aorta. CA was obtained from mice irradiated with the same total dose that was delivered either acutely, 25 fractions, 100 fractions or chronically. CA underwent immunofluorescence and histochemistry staining. Irradiation led to vascular damage, inflammation and fibrosis in CA. The integrative analysis for 14 prelesional endpoints revealed that the magnitude of carotid changes was greater in 25 fractions, smaller in 100 fractions, and much smaller in chronic irradiation, compared with acute irradiation. Radiation responses of the aorta were qualitatively similar to, but quantitatively greater than those of CA. Irradiation causes sparing and enhancing dose protraction effects in a manner that is not a simple function of dose rate, and that radiosensitivity varies within the circulatory system.

Inverse dose protraction effects of high-LET radiation: Evidence and significance

Biological effects of ionizing radiation vary with radiation quality, which is often expressed as the amount of energy deposited per unit length, i.e., linear energy transfer (LET). For acute irradiation, high-LET radiation generally produces greater biological effects than low-LET radiation, but little knowledge exists as to how dose protraction modifies effects. In this regard, inverse dose protraction effects (IDPEs) are phenomena in which dose protraction enhances effects, contrasting with sparing dose protraction effects in which dose protraction reduces effects. Here, we review the current knowledge on IDPEs of high-LET radiation. To the best of our knowledge, since 1967, 80 biology or epidemiology papers have reported IDPEs following external or internal high-LET irradiation with neutrons, deuterons, α-particles, light ions, or heavy ions. IDPEs of high-LET radiation have been described for biochemical changes in cell-free macromolecules, neoplastic transformation, cell death, DNA damage responses and gene expression changes in mammalian cell cultures of human or rodent origin, gene mutations, cytogenetic changes, cancer, non-cancer effects (e.g., testicular effects, cataracts, cardiovascular diseases) and life shortening in non-human mammals (rodents and dogs), and induction of lung cancer and bone tumors in humans. For external irradiation of mammalian cells in vitro and mammals in vivo, IDPEs of low- and high-LET radiation have been reported for radiation doses spanning in excess of three or four orders of magnitude in slightly different ranges, and for radiation dose rates both spanning over six orders of magnitude in different ranges. IDPEs of high-LET radiation in humans have been reported following internal exposure, but not external exposure. Manifestations and mechanisms of IDPEs of high-LET radiation are far less understood than those of low-LET radiation, warranting further studies that will be pivotal to assess the implications for radiation protection.

Inverse dose protraction effects of low-LET radiation: Evidence and significance

Biological effects of ionizing radiation vary not merely with total dose but also with temporal dose distribution. Sparing dose protraction effects, in which dose protraction reduces effects of radiation have widely been accepted and generally assumed in radiation protection, particularly for stochastic effects (e.g., solid cancer). In contrast, inverse dose protraction effects (IDPEs) in which dose protraction enhances radiation effects have not been well recognized, nor comprehensively reviewed. Here, we review the current knowledge on IDPEs of low linear energy transfer (LET) radiation. To the best of our knowledge, since 1952, 157 biology, epidemiology or clinical papers have reported IDPEs following external or internal low-LET irradiation with photons (X-rays, γ-rays), β-rays, electrons, protons or helium ions. IDPEs of low-LET radiation have been described for biochemical changes in cell-free macromolecules (DNA, proteins or lipids), DNA damage responses in bacteria and yeasts, DNA damage, cytogenetic changes, neoplastic transformation and cell death in mammalian cell cultures of human, rodent or bovine origin, mutagenesis in silkworms, cytogenetic changes, induction of cancer (solid tumors and leukemia) and non-cancer effects (male sterility, cataracts and diseases of the circulatory system), tumor inactivation and survival in non-human mammals (rodents, rabbits, dogs and pigs), and induction of cancer and non-cancer effects (skin changes and diseases of the circulatory system) in humans. In contrast to a growing body of phenomenological evidence for manifestations of IDPEs, there is limited knowledge on mechanistic underpinnings, but proposed mechanisms involve cell cycle-dependent resensitization and low dose hyper-radiosensitivity. These necessitate continued studies for further mechanistic developments and assessment of implications of scientific evidence for radiation protection (e.g., in terms of a dose rate effectiveness factor).

Testicular exposure to ionizing radiation and sperm epigenetic alterations as possible mechanisms of hereditary effects: perspectives from the viewpoint of radiation protection

PURPOSE

Since the genotoxicity of ionizing radiation was demonstrated in the 1920s, its hereditary effects have remained a serious concern for human society. The International Commission on Radiological Protection has highlighted the need for appropriate protection against hereditary effects of radiation in humans. In this paper, we review the literature on the possible multigenerational and transgenerational effects following testicular exposure to radiation, focusing on sperm epigenetic alterations as possible mechanisms.

RESULTS

This mini-review highlights that hereditary effects following testicular exposure occur via epigenetic changes of germ cells in animal models, providing implications on human radiation protection.

CONCLUSIONS

A great amount of epigenomic research data has emerged rapidly since the beginning of this century; thus, a revision of the radiological protection protocols against the hereditary effects of radiation would be no longer inevitable. The collection and analysis of evidence on these effects must be enhanced and further accelerated to formulate appropriate protection protocols in the future.

A Million Person Study Innovation: Evaluating Cognitive Impairment and other Morbidity Outcomes from Chronic Radiation Exposure Through Linkages with the Centers for Medicaid and Medicare Services Assessment and Claims Data

The study of One Million U.S. Radiation Workers and Veterans, the Million Person Study (MPS), examines the health consequences, both cancer and non-cancer, of exposure to ionizing radiation received gradually over time. Recently the MPS has focused on mortality patterns from neurological and behavioral conditions, e.g., Parkinson's disease, Alzheimer's disease, dementia, and motor neuron disease such as amyotrophic lateral sclerosis. A fuller picture of radiation-related late effects comes from studying both mortality and the occurrence (incidence) of conditions not leading to death. Accordingly, the MPS is identifying neurocognitive diagnoses from fee-for-service insurance claims from the Centers for Medicare and Medicaid Services (CMS), among Medicare beneficiaries beginning in 1999 (the earliest date claims data are available). Linkages to date have identified ∼540,000 workers with available health information. Such linkages provide individual information on important co-factor and confounding variables such as smoking, alcohol consumption, blood pressure, obesity, diabetes and many other health and demographic characteristics. The total person-level set of time-dependent variables, outcomes, organ-specific dose measures, co-factors, and demographics will be massive and much too large to be evaluated with standard software. Thus, development of specialized open-source software designed for large datasets (Colossus) is nearly complete. The wealth of information available from CMS claims data, coupled with individual dose reconstructions, will thus greatly enhance the quality and precision of health evaluations for this new field of low-dose radiation and neurocognitive effects.

Evaluation of relative biological effectiveness for diseases of the circulatory system based on microdosimetry

In the next decade, the International Commission on Radiological Protection (ICRP) will issue the next set of general recommendations, for which evaluation of relative biological effectiveness (RBE) for various types of tissue reactions would be needed. ICRP has recently classified diseases of the circulatory system (DCS) as a tissue reaction, but has not recommended RBE for DCS. We therefore evaluated the mean and uncertainty of RBE for DCS by applying a microdosimetric kinetic model specialized for RBE estimation of tissue reactions. For this purpose, we analyzed several RBE data for DCS determined by past animal experiments and evaluated the radius of the subnuclear domain best fit to each experiment as a single free parameter included in the model. Our analysis suggested that RBE for DCS tends to be lower than that for skin reactions, and their difference was borderline significant due to large variances of the evaluated parameters. We also found that RBE for DCS following mono-energetic neutron irradiation of the human body is much lower than that for skin reactions, particularly at the thermal energy and around 1 MeV. This tendency is considered attributable not only to the intrinsic difference of neutron RBE between skin reactions and DCS but also to the difference in the contributions of secondary γ-rays to the total absorbed doses between their target organs. These findings will help determine RBE by ICRP for preventing tissue reactions.

Radiation Adverse Outcome pathways (AOPs): examining priority questions from an international horizon-style exercise

PURPOSE

The Organisation for Economic Co-operation and Development (OECD) Adverse Outcome Pathway (AOP) Development Programme is being explored in the radiation field, as an overarching framework to identify and prioritize research needs that best support strengthening of radiation risk assessment and risk management strategies. To advance the use of AOPs, an international horizon-style exercise (HSE) was initiated through the Radiation/Chemical AOP Joint Topical Group (JTG) formed by the OECD Nuclear Energy Agency (NEA) High-Level Group on Low Dose Research (HLG-LDR) under the auspices of the Committee on Radiological Protection and Public Health (CRPPH). The intent of the HSE was to identify key research questions for consideration in AOP development that would help to reduce uncertainties in estimating the health risks following exposures to low dose and low dose-rate ionizing radiation. The HSE was conducted in several phases involving the solicitation of relevant questions, a collaborative review of open-ended candidate questions and an elimination exercise that led to the selection of 25 highest priority questions for the stated purpose. These questions were further ranked by over 100 respondents through an international survey. This final set of questions was judged to provide insights into how the OECD's AOP approach can be put into practice to meet the needs of hazard and risk assessors, regulators, and researchers. This paper examines the 25 priority questions in the context of hazard/risk assessment framework for ionizing radiation.

CONCLUSION

By addressing the 25 priority questions, it is anticipated that constructed AOPs will have a high level of specificity, making them valuable tools for simplifying and prioritizing complex biological processes for use in developing revised radiation hazard and risk assessment strategies.

Radiotherapy for non-cancer diseases: benefits and long-term risks

PURPOSE

The discovery of X-rays was followed by a variety of attempts to treat infectious diseases and various other non-cancer diseases with ionizing radiation, in addition to cancer. There has been a recent resurgence of interest in the use of such radiotherapy for non-cancer diseases. Non-cancer diseases for which use of radiotherapy has currently been proposed include refractory ventricular tachycardia, neurodegenerative diseases (e.g. Alzheimer's disease and dementia), and Coronavirus Disease 2019 (COVID-19) pneumonia, all with ongoing clinical studies that deliver radiation doses of 0.5-25 Gy in a single fraction or in multiple daily fractions. In addition to such non-cancer effects, historical indications predominantly used in some countries (e.g. Germany) include osteoarthritis and degenerative diseases of the bones and joints. This narrative review gives an overview of the biological rationale and ongoing preclinical and clinical studies for radiotherapy proposed for various non-cancer diseases, discusses the plausibility of the proposed biological rationale, and considers the long-term radiation risks of cancer and non-cancer diseases.

CONCLUSIONS

A growing body of evidence has suggested that radiation represents a double-edged sword, not only for cancer, but also for non-cancer diseases. At present, clinical evidence has shown some beneficial effects of radiotherapy for ventricular tachycardia, but there is little or no such evidence of radiotherapy for other newly proposed non-cancer diseases (e.g. Alzheimer's disease, COVID-19 pneumonia). Patients with ventricular tachycardia and COVID-19 pneumonia have thus far been treated with radiotherapy when they are an urgent life threat with no efficient alternative treatment, but some survivors may encounter a paradoxical situation where patients were rescued by radiotherapy but then get harmed by radiotherapy. Further studies are needed to justify the clinical use of radiotherapy for non-cancer diseases, and optimize dose to diseased tissue while minimizing dose to healthy tissue.