Radiation dose associated with renal failure mortality: a potential pathway to partially explain increased cardiovascular disease mortality observed after whole-body irradiation

TLR4-A Pertinent Player in Radiation-Induced Heart Disease?

The heart is one of the organs that is sensitive to developing delayed adverse effects of ionizing radiation (IR) exposure. Radiation-induced heart disease (RIHD) occurs in cancer patients and cancer survivors, as a side effect of radiation therapy of the chest, with manifestation several years post-radiotherapy. Moreover, the continued threat of nuclear bombs or terrorist attacks puts deployed military service members at risk of exposure to total or partial body irradiation. Individuals who survive acute injury from IR will experience delayed adverse effects that include fibrosis and chronic dysfunction of organ systems such as the heart within months to years after radiation exposure. Toll-like receptor 4 (TLR4) is an innate immune receptor that is implicated in several cardiovascular diseases. Studies in preclinical models have established the role of TLR4 as a driver of inflammation and associated cardiac fibrosis and dysfunction using transgenic models. This review explores the relevance of the TLR4 signaling pathway in radiation-induced inflammation and oxidative stress in acute as well as late effects on the heart tissue and the potential for the development of TLR4 inhibitors as a therapeutic target to treat or alleviate RIHD.

Considerations of Medical Preparedness to Assess and Treat Various Populations During a Radiation Public Health Emergency

During a radiological or nuclear public health emergency, given the heterogeneity of civilian populations, it is incumbent on medical response planners to understand and prepare for a potentially high degree of interindividual variability in the biological effects of radiation exposure. A part of advanced planning should include a comprehensive approach, in which the range of possible human responses in relation to the type of radiation expected from an incident has been thoughtfully considered. Although there are several reports addressing the radiation response for special populations (as compared to the standard 18-45-year-old male), the current review surveys published literature to assess the level of consideration given to differences in acute radiation responses in certain sub-groups. The authors attempt to bring clarity to the complex nature of human biology in the context of radiation to facilitate a path forward for radiation medical countermeasure (MCM) development that may be appropriate and effective in special populations. Consequently, the focus is on the medical (as opposed to logistical) aspects of preparedness and response. Populations identified for consideration include obstetric, pediatric, geriatric, males, females, individuals of different race/ethnicity, and people with comorbidities. Relevant animal models, biomarkers of radiation injury, and MCMs are highlighted, in addition to underscoring gaps in knowledge and the need for consistent and early inclusion of these populations in research. The inclusion of special populations in preclinical and clinical studies is essential to address shortcomings and is an important consideration for radiation public health emergency response planning. Pursuing this goal will benefit the population at large by considering those at greatest risk of health consequences after a radiological or nuclear mass casualty incident.

Synergistic Effects of Particle Radioactivity (Gross β Activity) and Particulate Matter ≤2.5 μm Aerodynamic Diameter on Cardiovascular Disease Mortality

Background Although the effects of fine particulate matter (particulate matter ≤2.5 μm aerodynamic diameter [PM_2.5]) on cardiovascular disease (CVD) morbidity and mortality are well established, little is known about the CVD health effects of particle radioactivity. In addition, there are still questions about which of the PM_2.5 physical, chemical, or biological properties are mostly responsible for its toxicity. Methods and Results We investigated the association between particle radioactivity, measured as gross β activity from highly resolved spatiotemporal predictions, and mortality for CVD, myocardial infarction, stroke, and all-cause nonaccidental mortality in Massachusetts (2001-2015). Within both difference-in-differences model and generalized linear mixed model frameworks, we fit both single-exposure and 2-exposure models adjusting for PM_2.5 and examined the interaction between PM_2.5 and gross β activity. We found significant associations between gross β activity and PM_2.5 and each mortality cause. Using difference-in-differences and adjusting for PM_2.5, we found the highest associations with myocardial infarction (rate ratio, 1.16 [95% CI, 1.08-1.24]) and stroke (rate ratio, 1.11 [95% CI, 1.04-1.18]) for an interquartile range increase (0.055 millibecquerels per cubic meter) in gross β activity. We found a significant positive interaction between PM_2.5 and gross β activity, with higher associations between PM_2.5 and mortality at a higher level of gross β activity. We also observed that the associations varied across age groups. The results were comparable between the 2 statistical methods also with and without adjusting for PM_2.5. Conclusions This is the first study that, using highly spatiotemporal predictions of gross β-activity, provides evidence that particle radioactivity increases CVD mortality and enhances PM_2.5 CVD mortality. Therefore, particle radioactivity can be an important property of PM_2.5 that must be further investigated. Addressing this important question can lead to cost-effective air-quality regulations.

Epidemiology of stroke and transient ischemic attacks in the population of the territories adjacent to the former Semipalatinsk Nuclear Test Site, Kazakhstan

The issue of radiation exposure as a potential cause of cerebrovascular disease raises many concerns. The aim of the present study was to investigate the epidemiology of stroke and transient ischemic attacks (TIA) along with the associated risk factors among the population of East Kazakhstan exposed to ionising radiation from the former Semipalatinsk Nuclear Test Site (SNTS) in comparison with the unexposed population of the same region. This 5-year retrospective cross-sectional study included the data on 10,970 patients, of whom the majority (62.3%) suffered from ischemic stroke, 11.7% had hemorrhagic stroke and the remaining 26.0% had TIA. At the moment when stroke/TIA happened, exposed patients were younger than the unexposed (mean age 63 years versus 64 years, p < 0.001) and showed higher rates of nearly all associated comorbidities, which commonly were more severe. Besides, exposed patients showed a higher risk of stroke lethality in contrast with the unexposed. The observed features might indicate that people residing in the vicinity of the SNTS are vulnerable to cerebrovascular disease and thus, this study contributes to timely recognition of this public health problem. In addition, a longitudinal study has to be envisaged to clarify whether there is any cause-effect relationship between exposure to radiation from the SNTS and the development of stroke or transient ischemic attacks.

Kidney problems in disaster situations

Mass disasters, particularly earthquakes, cause many medical problems, including kidney problems, but an organized approach to cope with them was initiated only at the end of previous century, subsequent to the Armenian Spitak earthquake in 1988. Originally, interventions were focused on acute kidney injury (AKI) following crush injury and rhabdomyolysis in victims who had been trapped under the debris of collapsed buildings. However, similar problems were also registered in the context of other catastrophic events, especially man-made disasters like wars and torture. Other kidney-related problems, such as the preservation of treatment continuity in chronic kidney disease (CKD), especially in maintenance dialysis patients, deserved attention as well. Specific therapeutic principles apply to disaster-related kidney problems and these may differ from usual day-to-day clinical practice. Those approaches have been formulated in global and specific country-related guidelines and recommendations. It is clear that a well-conceived and organized management of kidney diseases in disasters benefits outcomes. Furthermore, it may be useful if the model and philosophy that were applied over the last three decades could be adapted by broadening the scope of disasters leading to intervention. Actions should be guided and coordinated by a panel of experts steering ad hoc interventions, rather than applying the "old" static model where a single coordinating center instructs and uses volunteers listed long before a potential event occurs.

Low- and moderate-dose non-cancer effects of ionizing radiation in directly exposed individuals, especially circulatory and ocular diseases: a review of the epidemiology

PURPOSE

There are well-known correlations between high and moderate doses (>0.5 Gy) of ionizing radiation exposure and circulatory system damage, also between radiation and posterior subcapsular cataract. At lower dose correlations with circulatory disease are emerging in the Japanese atomic bomb survivors and in some occupationally exposed groups, and are still to some extent controversial. Heterogeneity in excess relative risks per unit dose in epidemiological studies at low (<0.1 Gy) and at low-moderate (>0.1 Gy, <0.5 Gy) doses may result from confounding and other types of bias, and effect modification by established risk factors. There is also accumulating evidence of excess cataract risks at lower dose and low dose rate in various cohorts. Other ocular endpoints, specifically glaucoma and macular degeneration have been little studied. In this paper, we review recent epidemiological findings, and also discuss some of the underlying radiobiology of these conditions. We briefly review some other types of mainly neurological nonmalignant disease in relation to radiation exposure.

CONCLUSIONS

We document statistically significant excess risk of the major types of circulatory disease, specifically ischemic heart disease and stroke, in moderate- or low-dose exposed groups, with some not altogether consistent evidence suggesting dose-response non-linearity, particularly for stroke. However, the patterns of risk reported are not straightforward. We also document evidence of excess risks at lower doses/dose-rates of posterior subcapsular and cortical cataract in the Chernobyl liquidators, US Radiologic Technologists and Russian Mayak nuclear workers, with fundamentally linear dose-response. Nuclear cataracts are less radiogenic. For other ocular endpoints, specifically glaucoma and macular degeneration there is very little evidence of effects at low doses; radiation-associated glaucoma has been documented only for doses >5 Gy, and so has the characteristics of a tissue reaction. There is some evidence of neurological detriment following low-moderate dose (∼0.1-0.2 Gy) radiation exposure in utero or in early childhood.

Irradiation of the kidneys causes pathologic remodeling in the nontargeted heart: A role for the immune system

Cardiac disease is a frequent and significant adverse event associated with radiotherapy for cancer. Identifying the underlying mechanism responsible for radiation injury to the heart will allow interventions to be developed. In the present study, we tested if local kidney irradiation results in remodeling of the shielded, nontargeted heart. One kidney, two kidneys, or the total body of male WAG and Dahl SS rats were irradiated with 10 Gy of X-rays. Local kidney irradiation resulted in systemic hypertension, increased BUN, infiltration of T lymphocytes, natural killer cells, and macrophages into the renal cortex and medulla, and renal fibrosis. Local irradiation of kidneys in WAG rats resulted in remodeling in the nontargeted heart after 120 days, manifested by perivascular fibrosis and increased interventricular septal thickness, but was not seen in Dahl SS rats due to a high baseline level of fibrosis in the sham-irradiated animals. Genetic depletion of T cells mitigated the nephropathy after local kidney irradiation, indicating a role for the immune system in mediating this outcome. Local kidney irradiation resulted in a cascade of pro-inflammatory cytokines and low-molecular weight metabolites into the circulation associated with transmission of signals resulting in pathologic remodeling in the nontargeted heart. A new model is proposed whereby radiation-induced cardiac remodeling in susceptible animals is indirect, with lower hemi body organs such as the kidney exporting factors into the circulation that cause remodeling outside of the irradiated field in the shielded, nontargeted heart. This nontargeted effect appears to be mediated, in part, by the immune system.

Radiation induced cardiovascular disease: An odyssey of bedside-bench-bedside approach

The journey to Mars will be an ambitious, yet arduous task as it will entail culmination of all the information we have gathered over many decades. While the mission is of utmost importance, preservation of astronaut's well-being is paramount also. To that end, mitigation of radiation risk especially afflicting cardiovascular disease (CVD) is of great interest and challenge. Current data from astronauts on low earth orbit and Apollo missions provides insight on the risk of CVD from radiation exposure. However, data is limited given the small cohort size of astronauts who embarked on just nine prolonged missions. Therefore, a cerebral approach to understanding and mitigating risks are essential. This paper discusses the need for a predictive preclinical model to help understand and mitigate the effects of radiation on astronauts. We will discuss strengths and limitations of preclinical models and the methods of validating and constructing a model to predict human clinical outcomes. Our bedside-bench-bedside approach focuses on adapting the preclinical model through common investigative tools used between humans and animals. The result will be an optimization of preclinical model to a point of being a surrogate clinical model capable of predicting CVD outcomes in astronauts exposed to radiation.

Effects of single-dose protons or oxygen ions on function and structure of the cardiovascular system in male Long Evans rats

PURPOSE

Studies are required to determine whether exposures to radiation encountered during manned missions in deep space may have adverse effects on the cardiovascular system. Most of the prior studies on effects of simulated space radiation on the heart and vasculature have been performed in mouse models. To provide data from a second animal species, two studies were performed to assess effects of high-energy charged particle radiation on the heart and abdominal aorta in a rat model.

MATERIALS AND METHODS

In study A, male Long Evans rats were exposed to whole-body protons (250 MeV, 0.5 Gy) or oxygen ions (¹⁶O, 600 MeV/n, 0.5 Gy), and ultrasonography was used to measure in vivo cardiac function and blood flow parameters at 3, 5, 9 and 12 months after radiation, followed by tissue collection at 12 months. In study B, male Long Evans rats were exposed to ¹⁶O (1 GeV/n, 0.01-0.25 Gy), and hearts collected at 6 to 7 and 12 months for histology and western-blots.

RESULTS

Both protons (250 MeV) and ¹⁶O (600 MeV/n) caused a decrease in left ventricular posterior wall thickness at 3-5 months, but did not change echocardiographic measures of cardiac function. In Pulsed-wave Doppler assessment of the abdominal aorta, an increase was seen in mean velocity, peak velocity, and velocity time integral at 12 months after ¹⁶O (600 MeV/n), suggesting a change in vascular function. There were no significant changes in histopathology or histological quantification of total collagens in heart or aorta. On the other hand, an increase was seen in a 75 kDa peptide of collagen type III in the left ventricle of rats exposed to protons (250 MeV) and ¹⁶O (600 MeV/n and 1 GeV/n), suggesting that radiation caused remodeling of existing collagens in the heart. ¹⁶O (600 MeV/n and 1 GeV/n) caused increases in left ventricular protein levels of immune cell markers CD2, CD4, CD8, and CD68.

CONCLUSION

A single low dose of whole body protons or ¹⁶O in male Long Evans rats did not change cardiac function or induce gross pathological changes in the heart or aorta, but induced mild changes in vascular function and remodeling of existing collagens in the heart. Altogether, studies in prior mouse models and the current work in rats indicate minor changes in cardiac function and structure after a low dose of single-ion radiation.

Short-term ambient particle radioactivity level and renal function in older men: Insight from the Normative Aging Study

BACKGROUND

Whole-body and thoracic ionizing radiation exposure are both associated with the development of renal dysfunction. However, whether low-level environmental radiation from air pollution affects renal function remains unknown.

OBJECTIVES

We investigated the association of particle radioactivity (PR) with renal function defined by the estimated glomerular filtration rate (eGFR) and chronic kidney disease (CKD) in the Normative Aging Study.

METHODS

This longitudinal analysis included 2491 medical visits from 809 white males enrolled between 1999 and 2013. The eGFR was calculated using the CKD-EPI and MDRD equations, and CKD cases were identified as those with an eGFR <60 mL/min/1.73 m². Gross β activity measured by five monitors of the U.S. Environmental Protection Agency's RadNet monitoring network was utilized to represent PR.

RESULTS

Ambient PR levels from 1 to 28 days prior to clinical visit demonstrated robust negative associations with both forms of eGFR, but not with the increased odds of CKD. An interquartile range higher 28-day average ambient PR level was significantly associated with 0.83-mL/min/1.73 m² lower eGFR estimated by the CKD-EPI equation (95% confidence interval: -1.46, -0.20, p-value = 0.01). Controlling for PM_2.5 or black carbon in the model slightly attenuated the PR effects on eGFR. However, in individuals with the highest levels (3rd tertile) of C-reactive protein (CRP) or fibrinogen, we observed robust associations of PR with eGFR and CKD, suggesting that systemic inflammation may modify the PR-eGFR and PR-CKD relationships.

CONCLUSIONS

Our study reveals adverse health effects of short-term low-level ambient PR on the renal function, providing evidence to guide further study of the interplay between PR, inflammation, and renal health.

Age at Exposure to Radiation Determines Severity of Renal and Cardiac Disease in Rats

Radiotherapy with sparsely ionizing photons is a cornerstone of successful cancer treatment. Age at time of exposure to radiation is known to influence biological outcomes for many end points. The effect of dose and age at exposure upon the occurrence of radiogenic cardiovascular disease is poorly understood. The goal of this work was to determine the response of maleWAG/RijCmcr rats at 6 months of age to gamma rays, and at 6 months or 6 weeks of age to X rays, using clinically relevant biomarkers of cardiovascular disease and kidney injury. Overall, there were significant radiation-induced effects on the levels of bicarbonate (P=0.0016), creatinine (P=0.0002), calcium (P = 0.0009), triglycerides (P = 0.0269) and blood urea nitrogen, albumin, protein, AST, alkaline phosphatase, total cholesterol and HDL (all P < 0.0001). Of those variables with a significant radiation-dose effect, there were significant modifications by age at time of exposure for bicarbonate (P = 0.0033), creatinine (P = 0.0015), AST (P = 0.0040), total cholesterol (P = 0.0006) and blood urea nitrogen, calcium, albumin, protein, alkaline phosphatase and HDL (all P < 0.0001). Cardiac perivascular collagen content was significantly increased in rats that were 8.0 Gy X-ray irradiated at 6 weeks of age (P < 0.047) but not at 6 months of age. While systemic blood pressure was elevated in both cohorts after 8.0 Gy X-ray irradiation (compared to agematched sham-irradiated controls), the magnitude of the increase above baseline was greater in the younger rats (P < 0.05). These findings indicate that dose and age at time of irradiation determine the timeline and severity of cardiac and renal injury.

Cardiac and Renal Delayed Effects of Acute Radiation Exposure: Organ Differences in Vasculopathy, Inflammation, Senescence and Oxidative Balance

We have previously shown significant pathology in the heart and kidney of murine hematopoietic-acute radiation syndrome (H-ARS) survivors of 8.7-9.0 Gy total-body irradiation (TBI). The goal of this study was to determine temporal relationships in the development of vasculopathy and the progression of renal and cardiovascular delayed effects of acute radiation exposure (DEARE) at TBI doses less than 9 Gy and to elucidate the potential roles of senescence, inflammation and oxidative stress. Our results show significant loss of endothelial cells in coronary arteries by 4 months post-TBI (8.53 or 8.72 Gy of gamma radiation). This loss precedes renal dysfunction and interstitial fibrosis and progresses to abnormalities in the arterial media and adventitia and loss of coronary arterioles. Major differences in radiation-induced pathobiology exist between the heart and kidney in terms of vasculopathy progression and also in indices of inflammation, senescence and oxidative imbalance. The results of this work suggest a need for different medical countermeasures for multiple targets in different organs and at various times after acute radiation injury to prevent the progression of DEARE.

EPIDEMIOLOGICAL STUDY OF HEALTH EFFECTS IN FUKUSHIMA NUCLEAR EMERGENCY WORKERS-STUDY DESIGN AND PROGRESS REPORT

Between 14 March and 16 December 2011, the radiation dose limit for emergency work was tentatively raised from an effective dose of 100-250 mSv by the Japanese Government after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The objective of this study is to clarify the long-term health effects of radiation on the emergency workers involved during that period, based on a detailed evaluation of the radiation exposures and long-term monitoring. The potential subjects of the study are the approximately 20 000 workers who were engaged in emergency operations at FDNPP during the period described above. During the first phase of this project, the first 5 years from 2014, we plan to set up a research scheme and establish a cohort. To date, the establishment of the scheme for general health examinations is nearly complete. As of 31 January 2017, 5419 emergency workers (27.4% of the subjects) have agreed to participate in the study. We will continue our efforts to recruit additional potential subjects during the first phase to maximize the size of the cohort.

Increased prevalence of essential hypertension in areas previously exposed to fallout due to nuclear weapons testing at the Semipalatinsk Test Site, Kazakhstan

This study examines the association between environmental radiation exposure and essential hypertension in a series of investigated geographical districts adjacent to the Semipalatinsk nuclear test site in Kazakhstan. The sample consists of 2000 volunteers participants in screening examinations in three administrative districts close to the nuclear test site, which was carried out as part of the Government Programs on Environmental Health Hazard. The cross-sectional study compares prevalence ratios in a population sample with long-term exposure in the low and intermediate dose range. Age-adjusted odds ratios for hypertension were found significantly increased with higher exposure groups. After accounting for main cardiovascular risk factors into the model and stratifying by gender, the prevalence odds ratios for radiation remained significantly increased, with a significant dose-response effect observed for some but not all subgroups. The results support existing evidence of cardiovascular health effects of radiation exposure and of persisting environmental health issues that require attention in both epidemiological surveys and healthcare provision.

Associations Between Ambient Particle Radioactivity and Blood Pressure: The NAS (Normative Aging Study)

BACKGROUND

The cardiovascular effects of low-level environmental radiation exposures are poorly understood. Although particulate matter (PM) has been linked to cardiovascular morbidity and mortality, and elevated blood pressure (BP), the properties promoting its toxicity remain uncertain. Addressing a knowledge gap, we evaluated whether BP increased with higher exposures to radioactive components of ambient PM, herein referred to as particle radioactivity (PR).

METHODS AND RESULTS

We performed a repeated-measures analysis of 852 men to examine associations between PR exposure and BP using mixed-effects regression models. As a surrogate for PR, we used gross β activity, measured by the US Environmental Protection Agency's radiation monitoring network. Higher PR exposure was associated with increases in both diastolic BP and systolic BP, for exposures from 1 to 28 days. An interquartile range increase in 28-day PR exposure was associated with a 2.95-mm Hg increase in diastolic BP (95% confidence interval, 2.25-3.66; P<0.001) and a 3.94-mm Hg increase in systolic BP (95% confidence interval, 2.62-5.27; P<0.001). For models including both PR and PM ≤2.5 µm, the PR-BP associations remained stable and significant. For models including PR and black carbon or PR and particle number, the PR-BP associations were attenuated; however, they remained significant for many exposure durations.

CONCLUSIONS

This is the first study to demonstrate the potential adverse effects of PR on both systolic and diastolic BPs. These were independent and similar in magnitude to those of PM ≤2.5 µm, black carbon, and particle number. Understanding the effects of particle-bound radionuclide exposures on BP may have important implications for environmental and public health policy.

Circulatory disease in French nuclear fuel cycle workers chronically exposed to uranium: a nested case–control study

Objectives

There is growing evidence of an association between low-dose external γ-radiation and circulatory system diseases (CSDs), yet sparse data exist about an association with chronic internal uranium exposure and the role of non-radiation risk factors. We conducted a nested case–control study of French AREVA NC Pierrelatte nuclear workers employed between 1960 and 2005 to estimate CSD risks adjusting for major CSD risk factors (smoking, blood pressure, body mass index, total cholesterol and glycaemia) and external γ-radiation dose.

Methods

The study included 102 cases of death from CSD and 416 controls individually matched on age, gender, birth cohort and socio-professional status. Information on CSD risk factors was collected from occupational medical records. Organ-specific absorbed doses were estimated using biomonitoring data, taking into account exposure regime and uranium physicochemical properties. External γ-radiation was measured by individual dosimeter badges. Analysis was conducted with conditional logistic regression.

Results

Workers were exposed to very low radiation doses (mean γ-radiation dose 2 and lung uranium dose 1 mGy). A positive but imprecise association was observed (excess OR per mGy 0.2, 95% CI 0.004 to 0.5). Results obtained after adjustment suggest that uranium exposure might be an independent CSD risk factor.

Conclusions

Our results suggest that a positive association might exist between internal uranium exposure and CSD mortality, not confounded by CSD risk factors. Future work should focus on numerous uncertainties associated with internal uranium dose estimation and on understanding biological pathway of CSD after protracted low-dose internal radiation exposure.

The dose-response relationship for cardiovascular disease is not necessarily linear

The probability for a complication after radiotherapy is usually a function of dose and volume in the organ or tissue of interest. In most epidemiological studies the risk for a complication is stratified in terms of dose, but not irradiated volume. We show that the obtained risk cannot generally be applied to radiotherapy patients.The epidemiological data of Darby et al. (N Engl J Med 368:2527, 2013) who found a linear relationship between the excess relative risk of major coronary events as function of mean heart dose in patients treated with tangential breast irradiation are analyzed. We have used the relative seriality model for a partly irradiated heart (“a lot to a little”) which models radiation therapy using two tangential fields. The relative seriality model was then used to predict NTCP of cardiovascular disease for a homogenously irradiated heart (“a little to a lot”). The relative seriality model was fitted to the data of Darby et al. (N Engl J Med 368:2527, 2013) for tangential breast irradiation. For the situation “a little to a lot” it was found that the dose-response relationship is sigmoidal and contradicts the findings of Darby et al. (N Engl J Med 368:2527, 2013). It was shown in this work that epidemiological studies which predict a linear dose-response relationship for cardiovascular disease can be reproduced by bio-physical models for normal tissue complication. For irradiation situations which were not included in the epidemiological studies, e.g. a homogenous irradiation of the heart (“a little to a lot”) the dose-response curve can be different. This could have consequences whether or not IMRT should be used for treating breast cancer. We believe that the results of epidemiological studies should not be generally used to predict normal tissue complications. It is better to use such data to optimize bio-physical models which can then be applied (with caution) to general treatment situations.

Heart Disease Mortality in the Life Span Study, 1950-2008

Based on the findings from the Radiation Effects Research Foundation's studies of the cohort of Japanese atomic bomb survivors, it has been reported that total-body irradiation at 0.5-1.0 Gy could be responsible for increased rates of mortality from broad-based categories of cardiovascular disease (CVD), i.e., stroke and heart disease. However, CVD consists of various subtypes that have potentially different radiation dose responses, as well as subtype-specific risks that have not been fully evaluated. Potential problems with changes in the coding rules for the International Classification of Diseases (ICD) and the underlying causes and trends in CVD mortality in Japan also need to be considered. The goal of this study was to clarify the radiation risk of subtype-specific heart disease over different time periods. Radiation dose response was examined for mortality from several heart disease subtypes in 86,600 members of the Life Span Study (LSS) cohort during 1950-2008. These subtypes included ischemic heart disease (IHD), valvular heart disease (VHD), hypertensive organ damage (HOD) and heart failure (HF). Individual radiation doses ranged between 0 and 4 Gy. In addition to analyses for the total period, we examined specific periods, 1950-1968, 1969-1980, 1981-1994 and 1995-2008, corresponding to major developments in medical technologies and ICD code revisions. We observed significant positive associations between radiation dose and mortality from heart disease overall in 1950-2008 [excess relative risk or ERR/Gy (95% CI) = 0.14 (0.06, 0.22)]. Subtype-specific ERRs also positively increased with dose: 0.45 (0.13, 0.85) for VHD, 0.36 (0.10, 0.68) for HOD and 0.21 (0.07, 0.37) for HF, respectively. No significant departure from linearity was shown for the dose-response model. Although there was no evidence for a threshold in a model function, the lowest dose ranges with a statistically significant dose response were 0-0.7 Gy for heart disease overall and VHD, 0-1.5 Gy for HOD and 0-0.4 Gy for HF. No significant association between radiation exposure and IHD was observed in any model, although a quadratic model fit the best. The risk of HOD and rheumatic VHD increased significantly in the earliest periods [ERR/Gy = 0.59 (0.07, 1.32) and 1.34 (0.24, 3.16), respectively]. The risk of nonrheumatic VHD increased with calendar time and was significant in the latest period [ERR/Gy = 0.75 (0.02, 1.92)]. The risk of IHD, especially for myocardial infarction, tended to be elevated in the most recent period after 2001, where cautious interpretation is needed due to the uncertain validity of death diagnosis. Radiation risks of heart disease mortality in the LSS appeared to vary substantially among subtypes, indicating possible differences in radiation-induced pathogenesis. Trends in CVD rates in Japan during the long observation period may also impact risk analyses.

Radiation and circulatory disease

Exposure to therapeutic doses of ionizing radiation is associated with damage to the heart and coronary arteries. However, only recently have studies with high-quality individual dosimetry data allowed this risk to be quantified while also adjusting for concomitant chemotherapy, and medical and lifestyle risk factors. At lower levels of exposure the evidence is less clear. In this article I review radiation-associated risks of circulatory disease in groups treated with radiotherapy for malignant and non-malignant disease, and in occupationally- or environmentally-exposed groups receiving rather lower levels of radiation dose, also for medical diagnostic purposes. Results of a meta-analysis suggest that excess relative risks per unit dose for various types of heart disease do not exhibit statistically significant (p>0.2) heterogeneity between studies. Although there are no marked discrepancies between risks derived from the high-dose therapeutic and medical diagnostic studies and from the moderate/low dose occupational and environmental studies, at least for ischemic heart disease and stroke there are indications of larger risks per unit dose for lower dose rate and fractionated exposures. Risks for stroke and other types of circulatory disease are significantly more variable (p<0.0001), possibly resulting from confounding and effect-modification by well known (but unobserved) risk factors. Adjustment for any of mean dose, dose fractionation or age at exposure results in the residual heterogeneity for cerebrovascular disease becoming non-significant. The review provides strong evidence in support of a causal association between both low and high dose radiation exposure and most types of circulatory disease.

Individual response to ionizing radiation

The human response to ionizing radiation (IR) varies among individuals. The first evidence of the individual response to IR was reported in the beginning of the 20th century. Considering nearly one century of observations, we here propose three aspects of individual IR response: radiosensitivity for early or late adverse tissue events after radiotherapy on normal tissues (non-cancer effects attributable to cell death); radiosusceptibility for IR-induced cancers; and radiodegeneration for non-cancer effects that are often attributable to mechanisms other than cell death (e.g., cataracts and circulatory disease). All the molecular and cellular mechanisms behind IR-induced individual effects are not fully elucidated. However, some specific assays may help their quantification according to the dose and to the genetic status. Accumulated data on individual factors have suggested that the individual IR response cannot be ignored and raises some clinical and societal issues. The individual IR response therefore needs to be taken into account to better evaluate the risks related to IR exposure.

Effects of ionizing radiation on the heart

This article provides an overview of studies addressing effects of ionizing radiation on the heart. Clinical studies have identified early and late manifestations of radiation-induced heart disease, a side effect of radiation therapy to tumors in the chest when all or part of the heart is situated in the radiation field. Studies in preclinical animal models have contributed to our understanding of the mechanisms by which radiation may injure the heart. More recent observations in human subjects suggest that ionizing radiation may have cardiovascular effects at lower doses than was previously thought. This has led to examinations of low-dose photons and low-dose charged particle irradiation in animal models. Lastly, studies have started to identify non-invasive methods for detection of cardiac radiation injury and interventions that may prevent or mitigate these adverse effects. Altogether, this ongoing research should increase our knowledge of biological mechanisms of cardiovascular radiation injury, identify non-invasive biomarkers for early detection, and potential interventions that may prevent or mitigate these adverse effects.

Hemodynamic Flow-Induced Mechanotransduction Signaling Influences the Radiation Response of the Vascular Endothelium

Hemodynamic shear stress is defined as the physical force exerted by the continuous flow of blood in the vascular system. Endothelial cells, which line the inner layer of blood vessels, sense this physiological force through mechanotransduction signaling and adapt to maintain structural and functional homeostasis. Hemodynamic flow, shear stress and mechanotransduction signaling are, therefore, an integral part of endothelial pathophysiology. Although this is a well-established concept in the cardiovascular field, it is largely dismissed in studies aimed at understanding radiation injury to the endothelium and subsequent cardiovascular complications. We and others have reported on the differential response of the endothelium when the cells are under hemodynamic flow shear compared with static culture. Further, we have demonstrated significant differences in the gene expression of static versus shear-stressed irradiated cells in four key pathways, reinforcing the importance of shear stress in understanding radiation injury of the endothelium. This article further emphasizes the influence of hemodynamic shear stress and the associated mechanotransduction signaling on physiological functioning of the vascular endothelium and underscores its significance in understanding radiation injury to the vasculature and associated cardiac complications. Studies of radiation effect on endothelial biology and its implication on cardiotoxicity and vascular complications thus far have failed to highlight the significance of these factors. Factoring in these integral parts of the endothelium will enhance our understanding of the contribution of the endothelium to radiation biology. Without such information, the current approaches to studying radiation-induced injury to the endothelium and its consequences in health and disease are limited.

Radiation-related risks of non-cancer outcomes in the atomic bomb survivors

Risks of non-cancer outcomes after exposure to atomic bomb (A-bomb) radiation have been evaluated among the Life Span Study (LSS) cohort and its subcohort, the Adult Health Study (AHS). Information regarding non-cancer outcomes in the LSS is obtained from death certificates. In the AHS, members undergo clinical examinations biennially to determine their health status. Many AHS studies have been limited to participants attending the clinic over a limited period, and therefore have varying degrees of inferential utility; as such, care is required for comparison with the LSS results. Disease structure of non-cancer diseases in Japan has changed over the long follow-up period since the end of World War II. The health status of the A-bomb survivors may be associated with the hardships of living in a devastated city and impoverished country following the prolonged war effort, in addition to the direct effects of radiation exposure. Radiation-related risk of cardiovascular disease may have increased due to radiation-related increased risk of hypertension and other secondary associations, and the risk of atherosclerotic disorders has also been reported recently. These results should be interpreted with caution because of changes in disease definitions over the follow-up period. The radiation-related risk of non-cancer respiratory diseases also appears to have increased over the follow-up period, but the shapes of the dose-response curves have shown little consistency.

A priming dose of protons alters the early cardiac cellular and molecular response to (56)Fe irradiation

PURPOSE

Recent evidence suggests that the heart may be injured by ionizing radiation at lower doses than was previously thought. This raises concerns about the cardiovascular risks from exposure to radiation during space travel. Since space travel is associated with exposure to both protons from solar particle events and heavy ions from galactic cosmic rays, we here examined the effects of a "priming" dose of protons on the cardiac cellular and molecular response to a "challenge" dose of (56)Fe in a mouse model.

METHODS

Male C57BL/6 mice at 10 weeks of age were exposed to sham-irradiation, 0.1 Gy of protons (150 MeV), 0.5 Gy of (56)Fe (600 MeV/n), or 0.1 Gy of protons 24 hours prior to 0.5 Gy of (56)Fe. Hearts were obtained at 7 days post-irradiation and western-blots were used to determine protein markers of cardiac remodeling, inflammatory infiltration, and cell death.

RESULTS

Exposure to (56)Fe caused an increase in expression of α-smooth muscle cell actin, collagen type III, the inflammatory cell markers mast cell tryptase, CD2 and CD68, the endothelial glycoprotein thrombomodulin, and cleaved caspase 3. Of all proteins investigated, protons at a dose of 0.1 Gy induced a small increase only in cleaved caspase 3 levels. On the other hand, exposure to protons 24 hours before (56)Fe prevented all of the responses to (56)Fe.

CONCLUSIONS

This study shows that a low dose of protons may prime the heart to respond differently to a subsequent challenge dose of heavy ions. Further investigation is required to identify responses at additional time points, consequences for cardiac function, threshold dose levels, and mechanisms by which a proton priming dose may alter the response to heavy ions.

A recommended epidemiological study design for examining the adverse health effects among emergency workers who experienced the TEPCO fukushima daiichi NPP accident in 2011

Results from medical examinations conducted in 2012 of workers who were engaged in radiation work in 2012 as a result of the 2011 Fukushima Daiichi Nuclear Power Plant (NPP) accident showed that the prevalence of abnormal findings was 4.21%, 3.23 points higher than the 0.98% that was found prior to the accident in the jurisdiction area of the labor inspection office which holds jurisdiction over the NPP. The Ministry of Health, Labour and Welfare (MHLW) concluded that the 2010 and 2012 data cannot be easily compared because 70% of the enterprises within the jurisdiction of the office that reported the 2012 results were different from those that did so in 2010. In addition, although the radiation workers' estimated average dose weighted by number of workers was 3.66 times higher than decontamination workers' dose, the prevalence among radiation workers was only 1.14 times higher than that among decontamination workers. Based on the results of the medical examinations, however, the MHLW decided to implement an epidemiological study on the health effects of radiation exposure on all emergency workers. This article explains key issues of the basic design of the study recommended by the expert meeting established in the MHLW and also identifies challenges that could not be resolved and thus required further consideration by the study researchers. The major issues included: (a) study methods and target group; (b) evaluation of cumulative doses; (c) health effects (end points); (d) control of confounding factors; and (e) study implementation framework. Identified key challenges that required further deliberation were: (a) preventing arbitrary partisan analysis; (b) ensuring a high participation rate; (c) inquiry about the medical radiation doses; and (d) the preparedness of new analytical technology. The study team formulated and implemented the pilot study in 2014 and started the full-scale study in April 2015 with funding from a research grant from the MHLW.

Space radiation and cardiovascular disease risk

Future long-distance space missions will be associated with significant exposures to ionizing radiation, and the health risks of these radiation exposures during manned missions need to be assessed. Recent Earth-based epidemiological studies in survivors of atomic bombs and after occupational and medical low dose radiation exposures have indicated that the cardiovascular system may be more sensitive to ionizing radiation than was previously thought. This has raised the concern of a cardiovascular disease risk from exposure to space radiation during long-distance space travel. Ground-based studies with animal and cell culture models play an important role in estimating health risks from space radiation exposure. Charged particle space radiation has dense ionization characteristics and may induce unique biological responses, appropriate simulation of the space radiation environment and careful consideration of the choice of the experimental model are critical. Recent studies have addressed cardiovascular effects of space radiation using such models and provided first results that aid in estimating cardiovascular disease risk, and several other studies are ongoing. Moreover, astronauts could potentially be administered pharmacological countermeasures against adverse effects of space radiation, and research is focused on the development of such compounds. Because the cardiovascular response to space radiation has not yet been clearly defined, the identification of potential pharmacological countermeasures against cardiovascular effects is still in its infancy.

Low dose radiation and circulatory diseases: a brief narrative review

Exposure to high doses of ionizing radiation is associated with damage to the heart and coronary arteries. However, only recently have studies with high-quality individual dosimetry data allowed this risk to be estimated while adjusting for concomitant chemotherapy. An association between lower dose exposures and late-occurring circulatory disease has only recently been suspected in the Japanese atomic bomb survivors and in various occupationally exposed cohorts and is still controversial. Excess relative risks per unit dose in moderate- and low-dose epidemiological studies are variable, possibly resulting from confounding and effect-modification by well known (but unobserved) risk factors. Here, we summarize the evidence for a causal association between moderate- and low-level radiation exposure (whether at high or low dose rates) and circulatory disease.

Delayed Effects of Acute Radiation Exposure in a Murine Model of the H-ARS: Multiple-Organ Injury Consequent to <10 Gy Total Body Irradiation

The threat of radiation exposure from warfare or radiation accidents raises the need for appropriate animal models to study the acute and chronic effects of high dose rate radiation exposure. The goal of this study was to assess the late development of fibrosis in multiple organs (kidney, heart, and lung) in survivors of the C57BL/6 mouse model of the hematopoietic-acute radiation syndrome (H-ARS). Separate groups of mice for histological and functional studies were exposed to a single uniform total body dose between 8.53 and 8.72 Gy of gamma radiation from a Cs radiation source and studied 1-21 mo later. Blood urea nitrogen levels were elevated significantly in the irradiated mice at 9 and 21 mo (from ∼22 to 34 ± 3.8 and 69 ± 6.0 mg dL, p < 0.01 vs. non-irradiated controls) and correlated with glomerosclerosis (29 ± 1.8% vs. 64 ± 9.7% of total glomeruli, p < 0.01 vs. non-irradiated controls). Glomerular tubularization and hypertrophy and tubular atrophy were also observed at 21 mo post-total body irradiation (TBI). An increase in interstitial, perivascular, pericardial and peribronchial fibrosis/collagen deposition was observed from ∼9-21 mo post-TBI in kidney, heart, and lung of irradiated mice relative to age-matched controls. Echocardiography suggested decreased ventricular volumes with a compensatory increase in the left ventricular ejection fraction. The results indicate that significant delayed effects of acute radiation exposure occur in kidney, heart, and lung in survivors of the murine H-ARS TBI model, which mirrors pathology detected in larger species and humans at higher radiation doses focused on specific organs.

Long-term effects of radiation exposure on health

Late-onset effects of exposure to ionising radiation on the human body have been identified by long-term, large-scale epidemiological studies. The cohort study of Japanese survivors of the atomic bombings of Hiroshima and Nagasaki (the Life Span Study) is thought to be the most reliable source of information about these health effects because of the size of the cohort, the exposure of a general population of both sexes and all ages, and the wide range of individually assessed doses. For this reason, the Life Span Study has become fundamental to risk assessment in the radiation protection system of the International Commission on Radiological Protection and other authorities. Radiation exposure increases the risk of cancer throughout life, so continued follow-up of survivors is essential. Overall, survivors have a clear radiation-related excess risk of cancer, and people exposed as children have a higher risk of radiation-induced cancer than those exposed at older ages. At high doses, and possibly at low doses, radiation might increase the risk of cardiovascular disease and some other non-cancer diseases. Hereditary effects in the children of atomic bomb survivors have not been detected. The dose-response relation for cancer at low doses is assumed, for purposes of radiological protection, to be linear without a threshold, but has not been shown definitively. This outstanding issue is not only a problem when dealing appropriately with potential health effects of nuclear accidents, such as at Fukushima and Chernobyl, but is of growing concern in occupational and medical exposure. Therefore, the appropriate dose-response relation for effects of low doses of radiation needs to be established.

Mortality from Circulatory System Diseases among French Uranium Miners: A Nested Case-Control Study

A significant association has been observed between radon exposure and cerebrovascular disease (CeVD) mortality among French uranium miners, but risk factors for circulatory system diseases (CSD) have not been previously considered. We conducted new analyses in the recently updated (through 2007) French cohort of uranium miners (n = 5,086), which included 442 deaths from CSD, 167 of them from ischemic heart disease (IHD) and 105 from CeVD. A nested case-control study was then set up to collect and investigate the influence of these risk factors on the relationships between mortality from CSD and occupational external gamma ray and internal ionizing radiation exposure (radon and long-lived radionuclides) in this updated cohort. The nested case-control study included miners first employed after 1955, still employed in 1976 and followed up through 2007. Individual information about CSD risk factors was collected from medical files for the 76 deaths from CSD (including 26 from IHD and 16 from CeVD) and 237 miners who had not died of CSD by the end of follow-up. The exposure-risk relationships were assessed with a Cox proportional hazard model weighted by the inverse sampling probability. A significant increase in all CSD and CeVD mortality risks associated with radon exposure was observed in the total cohort [hazard ratios: HRCSD/100 working level months (WLM) = 1.11, 95% confidence interval (1.01; 1.22) and HRCeVD/100 WLM = 1.25 (1.09; 1.43), respectively]. A nonsignificant exposure-risk relationship was observed for every type of cumulative ionizing radiation exposure and every end point [e.g., HRCSD/100WLM = 1.43 (0.71; 2.87)]. The adjustment for each CSD risk factor did not substantially change the exposure-risk relationships. When the model was adjusted for overweight, hypertension, diabetes, hypercholesterolemia and smoking status, the HR/100WLM for CSD, for example, was equal to 1.21 (0.54; 2.75); and when it was adjusted for risk factors selected with the Akaike information criterion, it was equal to 1.44 (0.66; 3.14). To our knowledge, this is the first study to use a uranium miner cohort to consider the major standard CSD risk factors in assessing the relationships between ionizing radiation exposure and the risk of death from these diseases. These results suggest that the significant relationship between CeVD risk and radon exposure observed in the total French cohort is probably not affected by the CSD risk factors. Extending the collection of information about CSD risk factors to a larger subsample would be useful to confirm this result.

Emerging issues in radiogenic cataracts and cardiovascular disease

In 2011, the International Commission on Radiological Protection issued a statement on tissue reactions (formerly termed non-stochastic or deterministic effects) to recommend lowering the threshold for cataracts and the occupational equivalent dose limit for the crystalline lens of the eye. Furthermore, this statement was the first to list circulatory disease (cardiovascular and cerebrovascular disease) as a health hazard of radiation exposure and to assign its threshold for the heart and brain. These changes have stimulated various discussions and may have impacts on some radiation workers, such as those in the medical sector. This paper considers emerging issues associated with cataracts and cardiovascular disease. For cataracts, topics dealt with herein include (i) the progressive nature, stochastic nature, target cells and trigger events of lens opacification, (ii) roles of lens protein denaturation, oxidative stress, calcium ions, tumor suppressors and DNA repair factors in cataractogenesis, (iii) dose rate effect, radiation weighting factor, and classification systems for cataracts, and (iv) estimation of the lens dose in clinical settings. Topics for cardiovascular disease include experimental animal models, relevant surrogate markers, latency period, target tissues, and roles of inflammation and cellular senescence. Future research needs are also discussed.

Classification of radiation effects for dose limitation purposes: history, current situation and future prospects

Radiation exposure causes cancer and non-cancer health effects, each of which differs greatly in the shape of the dose-response curve, latency, persistency, recurrence, curability, fatality and impact on quality of life. In recent decades, for dose limitation purposes, the International Commission on Radiological Protection has divided such diverse effects into tissue reactions (formerly termed non-stochastic and deterministic effects) and stochastic effects. On the one hand, effective dose limits aim to reduce the risks of stochastic effects (cancer/heritable effects) and are based on the detriment-adjusted nominal risk coefficients, assuming a linear-non-threshold dose response and a dose and dose rate effectiveness factor of 2. On the other hand, equivalent dose limits aim to avoid tissue reactions (vision-impairing cataracts and cosmetically unacceptable non-cancer skin changes) and are based on a threshold dose. However, the boundary between these two categories is becoming vague. Thus, we review the changes in radiation effect classification, dose limitation concepts, and the definition of detriment and threshold. Then, the current situation is overviewed focusing on (i) stochastic effects with a threshold, (ii) tissue reactions without a threshold, (iii) target organs/tissues for circulatory disease, (iv) dose levels for limitation of cancer risks vs prevention of non-life-threatening tissue reactions vs prevention of life-threatening tissue reactions, (v) mortality or incidence of thyroid cancer, and (vi) the detriment for tissue reactions. For future discussion, one approach is suggested that classifies radiation effects according to whether effects are life threatening, and radiobiological research needs are also briefly discussed.

A review of non-cancer effects, especially circulatory and ocular diseases

There is a well-established association between high doses (>5 Gy) of ionizing radiation exposure and damage to the heart and coronary arteries, although only recently have studies with high-quality individual dosimetry been conducted that would enable quantification of this risk adjusting for concomitant chemotherapy. The association between lower dose exposures and late occurring circulatory disease has only recently begun to emerge in the Japanese atomic bomb survivors and in various occupationally exposed cohorts and is still controversial. Excess relative risks per unit dose in moderate- and low-dose epidemiological studies are somewhat variable, possibly a result of confounding and effect modification by well-known (but unobserved) risk factors. Radiation doses of 1 Gy or more are associated with increased risk of posterior subcapsular cataract. Accumulating evidence from the Japanese atomic bomb survivors, Chernobyl liquidators, US astronauts, and various other exposed groups suggests that cortical cataracts may also be associated with ionizing radiation, although there is little evidence that nuclear cataracts are radiogenic. The dose-response appears to be linear, although modest thresholds (of no more than about 0.6 Gy) cannot be ruled out. A variety of other non-malignant effects have been observed after moderate/low-dose exposure in various groups, in particular respiratory and digestive disease and central nervous system (and in particular neuro-cognitive) damage. However, because these are generally only observed in isolated groups, or because the evidence is excessively heterogeneous, these associations must be treated with caution.

Cardiovascular effects after low-dose exposure and radiotherapy: what research is needed?

The authors of this report met at the Head Quarter of the International Atomic Energy Agency (IAEA) in Vienna, Austria, on 2-4 July 2012, for intensive discussions of an abundance of original publications on new epidemiological studies on cardiovascular effects after low-dose exposure and radiotherapy and radiobiological experiments as well as several comprehensive reviews that were published since the previous meeting by experts sponsored by the IAEA in June 2006. The data necessitated a re-evaluation of the situation with special emphasis on the consequences current experimental and clinical data may have for clinical oncology/radiotherapy and radiobiological research. The authors jointly arrived at the conclusions and recommendations presented here.

Cardiac injury after 10 gy total body irradiation: indirect role of effects on abdominal organs

The objective of this study was to determine whether radiation-induced injury to the heart after 10 Gy total body irradiation (TBI) is direct or indirect. Young male WAG/RijCmcr rats received a 10 Gy single dose using TBI, upper hemi-body (UHB) irradiation, lower hemi-body (LHB) irradiation, TBI with the kidneys shielded or LHB irradiation with the intestines shielded. Age-matched, sham-irradiated rats served as controls. The lipid profile, kidney injury, heart and liver morphology and cardiac function were determined up to 120 days after irradiation. LHB, but not UHB irradiation, increased the risk factors for cardiac disease as well as the occurrence of cardiac and kidney injury in a way that was quantitatively and qualitatively similar to that observed after TBI. Shielding of the kidneys prevented the increases in risk factors for cardiac disease. Shielding of the intestines did not prevent the increases in risk factors for cardiac disease. There was no histological evidence of liver injury 120 days after irradiation. Injury to the heart from irradiation appears to be indirect, supporting the notion that injury to abdominal organs, principally the kidneys, is responsible for the increased risk factors for and the occurrence of cardiac disease after TBI and LHB irradiation.

Understanding radiation-induced cardiovascular damage and strategies for intervention

There is a clear association between therapeutic doses of thoracic irradiation and an increased risk of cardiovascular disease (CVD) in cancer survivors, although these effects may take decades to become symptomatic. Long-term survivors of Hodgkin's lymphoma and childhood cancers have two-fold to more than seven-fold increased risks for late cardiac deaths after total tumour doses of 30-40 Gy, given in 2 Gy fractions, where large volumes of heart were included in the field. Increased cardiac mortality is also seen in women irradiated for breast cancer. Breast doses are generally 40-50 Gy in 2 Gy fractions, but only a small part of the heart is included in the treatment fields and mean heart doses rarely exceeded 10-15 Gy, even with older techniques. The relative risks of cardiac mortality (1.1-1.4) are consequently lower than for Hodgkin's lymphoma survivors. Some epidemiological studies show increased risks of cardiac death after accidental or environmental total body exposures to much lower radiation doses. The mechanisms whereby these cardiac effects occur are not fully understood and different mechanisms are probably involved after high therapeutic doses to the heart, or part of the heart, than after low total body exposures. These various mechanisms probably result in different cardiac pathologies, e.g. coronary artery atherosclerosis leading to myocardial infarct, versus microvascular damage and fibrosis leading to congestive heart failure. Experimental studies can help to unravel some of these mechanisms and may identify suitable strategies for managing or inhibiting CVD. In this overview, the main epidemiological and clinical evidence for radiation-induced CVD is summarised. Experimental data shedding light on some of the underlying pathologies and possible targets for intervention are also discussed.

Systematic review and meta-analysis of circulatory disease from exposure to low-level ionizing radiation and estimates of potential population mortality risks

BACKGROUND

Although high doses of ionizing radiation have long been linked to circulatory disease, evidence for an association at lower exposures remains controversial. However, recent analyses suggest excess relative risks at occupational exposure levels.

OBJECTIVES

We performed a systematic review and meta-analysis to summarize information on circulatory disease risks associated with moderate- and low-level whole-body ionizing radiation exposures.

METHODS

We conducted PubMed/ISI Thomson searches of peer-reviewed papers published since 1990 using the terms "radiation" AND "heart" AND "disease," OR "radiation" AND "stroke," OR "radiation" AND "circulatory" AND "disease." Radiation exposures had to be whole-body, with a cumulative mean dose of < 0.5 Sv, or at a low dose rate (< 10 mSv/day). We estimated population risks of circulatory disease from low-level radiation exposure using excess relative risk estimates from this meta-analysis and current mortality rates for nine major developed countries.

RESULTS

Estimated excess population risks for all circulatory diseases combined ranged from 2.5%/Sv [95% confidence interval (CI): 0.8, 4.2] for France to 8.5%/Sv (95% CI: 4.0, 13.0) for Russia.

CONCLUSIONS

Our review supports an association between circulatory disease mortality and low and moderate doses of ionizing radiation. Our analysis was limited by heterogeneity among studies (particularly for noncardiac end points), the possibility of uncontrolled confounding in some occupational groups by lifestyle factors, and higher dose groups (> 0.5 Sv) generally driving the observed trends. If confirmed, our findings suggest that overall radiation-related mortality is about twice that currently estimated based on estimates for cancer end points alone (which range from 4.2% to 5.6%/Sv for these populations).

Experimental radiation-induced heart disease: past, present, and future

Radiation-induced heart disease (RIHD) is a serious side effect of radiotherapy for intrathoracic and chest wall tumors. The threshold dose for development of clinically significant RIHD is believed to be lower than previously assumed. Therefore, research into mechanisms of RIHD has gained substantial momentum. RIHD becomes clinically apparent ten to fifteen years after radiation exposure. Chronic manifestations of RIHD include accelerated atherosclerosis, cardiomyopathy, and valve abnormalities. Reducing exposure of the heart during radiotherapy is the only known method of preventing RIHD, and there are no approaches to reverse RIHD once it occurs. We use a combination of pharmacological and genetic animal models to determine biological mechanisms of RIHD. Major technological advances in small animal research have made this type of study more valuable. The long-term goal of this work is to identify targets for intervention in RIHD, thereby enhancing the efficacy and safety of thoracic radiotherapy.