Comment on "dose-responses from multi-model inference for the non-cancer disease mortality of atomic bomb survivors" (Radiat. Environ. Biophys (2012) 51:165-178) by Schöllnberger et al

Evaluation of risks of cardiovascular disease from radiation exposure linked to computed tomography scans in the UK

Epidemiological studies of patient populations have shown that high doses of radiation increase risks of cardiovascular disease (CVD). Results from a recent meta-analysis of 93 epidemiological studies covering a wide range of doses provided evidence of a causal association between radiation exposure and CVD, and indicated excess relative risk per Gy for maximum dose below 500 mGy or delivered at low dose rates. These doses cover the range of organ doses expected from multiple diagnostic computed tomography (CT) scans. Dose-effect factors for the excess absolute risk of mortality from CVD following radiation exposure were derived from the meta-analysis. The present study uses these factors to estimate excess risks of mortality for various types of CVD, including cerebrovascular disease (CeVD), from CT scans of the body and head, assuming that the meta-analytic factors were accurate and represented a causal relationship. Estimates are based on cumulative doses to the heart and brain from CT scans performed on 105 574 patients on 12 CT scanners over a period of 5½ years. The results suggest that the excess number of deaths from CeVD could be 7 or 26 per 100 000 patients depending whether threshold brain doses of 200 mGy or 50 mGy, respectively are assumed. These results could have implications for head CT scans. However, the results rely on the validity of risk factors derived in the meta-analysis informing this assessment and which include significant uncertainties. Further incidence studies should provide better information on risk factors and dose thresholds, particularly for CeVD following head CT scans.

Ionising radiation and cardiovascular disease: systematic review and meta-analysis

OBJECTIVE

To systematically review and perform a meta-analysis of radiation associated risks of cardiovascular disease in all groups exposed to radiation with individual radiation dose estimates.

DESIGN

Systematic review and meta-analysis.

MAIN OUTCOME MEASURES

Excess relative risk per unit dose (Gy), estimated by restricted maximum likelihood methods.

DATA SOURCES

PubMed and Medline, Embase, Scopus, Web of Science Core collection databases.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

Databases were searched on 6 October 2022, with no limits on date of publication or language. Animal studies and studies without an abstract were excluded.

RESULTS

The meta-analysis yielded 93 relevant studies. Relative risk per Gy increased for all cardiovascular disease (excess relative risk per Gy of 0.11 (95% confidence interval 0.08 to 0.14)) and for the four major subtypes of cardiovascular disease (ischaemic heart disease, other heart disease, cerebrovascular disease, all other cardiovascular disease). However, interstudy heterogeneity was noted (P<0.05 for all endpoints except for other heart disease), possibly resulting from interstudy variation in unmeasured confounders or effect modifiers, which is markedly reduced if attention is restricted to higher quality studies or those at moderate doses (<0.5 Gy) or low dose rates (<5 mGy/h). For ischaemic heart disease and all cardiovascular disease, risks were larger per unit dose for lower dose (inverse dose effect) and for fractionated exposures (inverse dose fractionation effect). Population based excess absolute risks are estimated for a number of national populations (Canada, England and Wales, France, Germany, Japan, USA) and range from 2.33% per Gy (95% confidence interval 1.69% to 2.98%) for England and Wales to 3.66% per Gy (2.65% to 4.68%) for Germany, largely reflecting the underlying rates of cardiovascular disease mortality in these populations. Estimated risk of mortality from cardiovascular disease are generally dominated by cerebrovascular disease (around 0.94-1.26% per Gy), with the next largest contribution from ischaemic heart disease (around 0.30-1.20% per Gy).

CONCLUSIONS

Results provide evidence supporting a causal association between radiation exposure and cardiovascular disease at high dose, and to a lesser extent at low dose, with some indications of differences in risk between acute and chronic exposures, which require further investigation. The observed heterogeneity complicates a causal interpretation of these findings, although this heterogeneity is much reduced if only higher quality studies or those at moderate doses or low dose rates are considered. Studies are needed to assess in more detail modifications of radiation effect by lifestyle and medical risk factors.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42020202036.

Dose-responses for mortality from cerebrovascular and heart diseases in atomic bomb survivors: 1950-2003

The scientific community faces important discussions on the validity of the linear no-threshold (LNT) model for radiation-associated cardiovascular diseases at low and moderate doses. In the present study, mortalities from cerebrovascular diseases (CeVD) and heart diseases from the latest data on atomic bomb survivors were analyzed. The analysis was performed with several radio-biologically motivated linear and nonlinear dose-response models. For each detrimental health outcome one set of models was identified that all fitted the data about equally well. This set was used for multi-model inference (MMI), a statistical method of superposing different models to allow risk estimates to be based on several plausible dose-response models rather than just relying on a single model of choice. MMI provides a more accurate determination of the dose response and a more comprehensive characterization of uncertainties. It was found that for CeVD, the dose-response curve from MMI is located below the linear no-threshold model at low and medium doses (0-1.4 Gy). At higher doses MMI predicts a higher risk compared to the LNT model. A sublinear dose-response was also found for heart diseases (0-3 Gy). The analyses provide no conclusive answer to the question whether there is a radiation risk below 0.75 Gy for CeVD and 2.6 Gy for heart diseases. MMI suggests that the dose-response curves for CeVD and heart diseases in the Lifespan Study are sublinear at low and moderate doses. This has relevance for radiotherapy treatment planning and for international radiation protection practices in general.

Cardiovascular diseases related to ionizing radiation: The risk of low-dose exposure (Review)

Traditionally, non-cancer diseases are not considered as health risks following exposure to low doses of ionizing radiation. Indeed, non-cancer diseases are classified as deterministic tissue reactions, which are characterized by a threshold dose. It is judged that below an absorbed dose of 100 mGy, no clinically relevant tissue damage occurs, forming the basis for the current radiation protection system concerning non-cancer effects. Recent epidemiological findings point, however, to an excess risk of non-cancer diseases following exposure to lower doses of ionizing radiation than was previously thought. The evidence is the most sound for cardiovascular disease (CVD) and cataract. Due to limited statistical power, the dose-risk relationship is undetermined below 0.5 Gy; however, if this relationship proves to be without a threshold, it may have considerable impact on current low-dose health risk estimates. In this review, we describe the CVD risk related to low doses of ionizing radiation, the clinical manifestation and the pathology of radiation-induced CVD, as well as the importance of the endothelium models in CVD research as a way forward to complement the epidemiological data with the underlying biological and molecular mechanisms.

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.

Cardiovascular disease mortality of A-bomb survivors and the healthy survivor selection effect

The latest A-bomb survivor data for cardiovascular diseases are analysed to investigate whether in the first years after the bombings the baseline rates of proximal survivors were markedly different compared with those of the distal survivors. This phenomenon relates to a healthy survivor selection effect. This question is important for the decision whether to include or exclude the early years of follow-up when analysing the biological effects from acute low and high dose exposures following the nuclear weapons explosions in Hiroshima and Nagasaki. The present study shows that for cerebrovascular diseases and heart diseases the baseline rates are not significantly different in the first two decades of follow-up. Thus, for these two detrimental health outcomes, there is no need to exclude distal survivors and the first decades of follow-up time when investigating the shapes of the related dose-responses.

Ischemic heart disease in workers at Mayak PA: latency of incidence risk after radiation exposure

We present an updated analysis of incidence and mortality from atherosclerotic induced ischemic heart diseases in the cohort of workers at the Mayak Production Association (PA). This cohort constitutes one of the most important sources for the assessment of radiation risk. It is exceptional because it comprises information on several other risk factors. While most of the workers have been exposed to external gamma radiation, a large proportion has additionally been exposed to internal radiation from inhaled plutonium. Compared to a previous study by Azizova et al. 2012, the updated dosimetry system MWDS-2008 has been applied and methods of analysis have been revised. We extend the analysis of the significant incidence risk and observe that main detrimental effects of external radiation exposure occur after more than about 30 years. For mortality, significant risk was found in males with an excess relative risk per dose of 0.09 (95% CI: 0.02; 0.16) [Formula: see text] while risk was insignificant for females. With respect to internal radiation exposure no association to risk could be established.

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.