A Historical Survey of Key Epidemiological Studies of Ionizing Radiation Exposure

In this article we review the history of key epidemiological studies of populations exposed to ionizing radiation. We highlight historical and recent findings regarding radiation-associated risks for incidence and mortality of cancer and non-cancer outcomes with emphasis on study design and methods of exposure assessment and dose estimation along with brief consideration of sources of bias for a few of the more important studies. We examine the findings from the epidemiological studies of the Japanese atomic bomb survivors, persons exposed to radiation for diagnostic or therapeutic purposes, those exposed to environmental sources including Chornobyl and other reactor accidents, and occupationally exposed cohorts. We also summarize results of pooled studies. These summaries are necessarily brief, but we provide references to more detailed information. We discuss possible future directions of study, to include assessment of susceptible populations, and possible new populations, data sources, study designs and methods of analysis.

Comprehensive exposure assessments from the viewpoint of health in a unique high natural background radiation area, Mamuju, Indonesia

Mamuju is one of the regions in Indonesia which retains natural conditions but has relatively high exposure to natural radiation. The goals of the present study were to characterize exposure of the entire Mamuju region as a high natural background radiation area (HNBRA) and to assess the existing exposure as a means for radiation protection of the public and the environment. A cross-sectional study method was used with cluster sampling areas by measuring all parameters that contribute to external and internal radiation exposures. It was determined that Mamuju was a unique HNBRA with the annual effective dose between 17 and 115 mSv, with an average of 32 mSv. The lifetime cumulative dose calculation suggested that Mamuju residents could receive as much as 2.2 Sv on average which is much higher than the average dose of atomic bomb survivors for which risks of cancer and non-cancer diseases are demonstrated. The study results are new scientific data allowing better understanding of health effects related to chronic low-dose-rate radiation exposure and they can be used as the main input in a future epidemiology study.

Radiation Biology and Its Role in the Canadian Radiation Protection Framework

The linear no-threshold (linear-non-threshold) model is a dose-response model that has long served as the foundation of the international radiation protection framework, which includes the Canadian regulatory framework. Its purpose is to inform the choice of appropriate dose limits and subsequent as low as reasonably achievable requirements, social and economic factors taken into account. The linear no-threshold model assumes that the risk of developing cancer increases proportionately with increasing radiation dose. The linear no-threshold model has historically been applied by extrapolating the risk of cancer at high doses (>1,000 mSv) down to low doses in a linear manner. As the health effects of radiation exposure at low doses remain ambiguous, reducing uncertainties found in cancer risk dose-response models can be achieved through in vitro and animal-based studies. The purpose of this critical review is to analyze whether the linear no-threshold model is still applicable for use by modern nuclear regulators for radiation protection purposes, or if there is sufficient scientific evidence supporting an alternate model from which to derive regulatory dose limits.

Lack of increased DNA double-strand breaks in peripheral blood mononuclear cells of individuals from high level natural radiation areas of Kerala coast in India

The high level natural radiation area (HLNRA) of Kerala is a 55km long and 0.5km wide strip in south west coast of India. The level of background radiation in this area varies from <1.0mGy/year to 45.0mGy/year. It offers unique opportunity to study the effect of chronic low dose/low dose-rate radiation directly on human population. Spontaneous level of DNA double strand breaks (DSBs) was quantified in peripheral blood mononuclear cells of 91 random individuals from HLNRA (N=61, mean age: 36.1±7.43years) and normal level natural radiation area (NLNRA) (N=30, mean age: 35.5±6.35years) using gamma-H2AX as a marker. The mean annual dose received by NLNRA and HLNRA individuals was 1.28±0.086mGy/year and 8.28±4.96mGy/year, respectively. The spontaneous frequency of DSBs in terms of gamma-H2AX foci among NLNRA and HLNRA individuals were 0.095±0.009 and 0.084±0.004 per cell (P=0.22). The individuals from HLNRA were further classified as low dose group (LDG, 1.51-5.0mGy/year, mean dose: 2.63±0.76mGy/year) and high dose group (HDG, >5.0mGy/year, mean dose: 11.04±3.57mGy/year). The spontaneous frequency of gamma-H2AX foci per cell in NLNRA, LDG and HDG was observed to be 0.095±0.009, 0.096±0.008 and 0.078±0.004 respectively. Individuals belonging to HDG of HLNRA showed marginally lower frequency of DSBs as compared to NLNRA and LDG of HLNRA. This could be suggestive of either lower induction or better repair of DSBs in individuals from HDG of HLNRA. The present study indicated that 5.0mGy/year could be a possible threshold dose for DSB induction at chronic low-dose radiation exposure in vivo. However, further studies on DNA damage induction and repair kinetics are required to draw firm conclusions.

Answers to Common Questions About the Use and Safety of CT Scans

Articles in the scientific literature and lay press over the past several years have implied that computed tomography (CT) may cause cancer and that physicians and patients must exercise caution in its use. Although there is broad agreement on the latter point--unnecessary medical tests of any type should always be avoided--there is considerable controversy surrounding the question of whether, or to what extent, CT scans can lead to future cancers. Although the doses used in CT are higher than those used in conventional radiographic examinations, they are still 10 to 100 times lower than the dose levels that have been reported to increase the risk of cancer. Despite the fact that at the low doses associated with a CT scan the risk either is too low to be convincingly demonstrated or does not exist, the magnitude of the concern among patients and some medical professionals that CT scans increase cancer risk remains unreasonably high. In this article, common questions about CT scanning and radiation are answered to provide physicians with accurate information on which to base their medical decisions and respond to patient questions.

Uncertainties in estimating health risks associated with exposure to ionising radiation

The information for the present discussion on the uncertainties associated with estimation of radiation risks and probability of disease causation was assembled for the recently published NCRP Report No. 171 on this topic. This memorandum provides a timely overview of the topic, given that quantitative uncertainty analysis is the state of the art in health risk assessment and given its potential importance to developments in radiation protection. Over the past decade the increasing volume of epidemiology data and the supporting radiobiology findings have aided in the reduction of uncertainty in the risk estimates derived. However, it is equally apparent that there remain significant uncertainties related to dose assessment, low dose and low dose-rate extrapolation approaches (e.g. the selection of an appropriate dose and dose-rate effectiveness factor), the biological effectiveness where considerations of the health effects of high-LET and lower-energy low-LET radiations are required and the transfer of risks from a population for which health effects data are available to one for which such data are not available. The impact of radiation on human health has focused in recent years on cancer, although there has been a decided increase in the data for noncancer effects together with more reliable estimates of the risk following radiation exposure, even at relatively low doses (notably for cataracts and cardiovascular disease). New approaches for the estimation of hereditary risk have been developed with the use of human data whenever feasible, although the current estimates of heritable radiation effects still are based on mouse data because of an absence of effects in human studies. Uncertainties associated with estimation of these different types of health effects are discussed in a qualitative and semi-quantitative manner as appropriate. The way forward would seem to require additional epidemiological studies, especially studies of low dose and low dose-rate occupational and perhaps environmental exposures and for exposures to x rays and high-LET radiations used in medicine. The development of models for more reliably combining the epidemiology data with experimental laboratory animal and cellular data can enhance the overall risk assessment approach by providing biologically refined data to strengthen the estimation of effects at low doses as opposed to the sole use of mathematical models of epidemiological data that are primarily driven by medium/high doses. NASA's approach to radiation protection for astronauts, although a unique occupational group, indicates the possible applicability of estimates of risk and their uncertainty in a broader context for developing recommendations on: (1) dose limits for occupational exposure and exposure of members of the public; (2) criteria to limit exposures of workers and members of the public to radon and its short-lived decay products; and (3) the dosimetric quantity (effective dose) used in radiation protection.

Radiation epidemiology: a perspective on Fukushima

For nearly 100 years, epidemiologic studies of human populations exposed to ionising radiation have provided quantitative information on health risks. High dose deterministic (tissue reaction) effects result when sufficient numbers of functioning cells are killed, such as in bone marrow depression that can lead to death. Lower dose stochastic effects are probabilistic in nature and include an increased risk of cancer later in life and heritable genetic defects, although genetic conditions in the children of irradiated parents have yet to be convincingly demonstrated. Radiation studies are of diverse populations and include not only the Japanese atomic bomb survivors, but also patients treated with radiation for malignant and non-malignant disease; patients exposed for diagnostic purposes; persons with intakes of radionuclides; workers occupationally exposed; and communities exposed to environmental and accidentally released sources of radiation. Much is known about radiation and its risks. The major unanswered question in radiation epidemiology, however, is not whether radiation causes cancer, but what the level of risk is following low dose (<100 mSv) or low dose rate exposures. Paracelsus is credited with first articulating that the 'poison is in the dose', which for radiation epidemiology translates as 'the lower the dose, the lower the risk' and, an important corollary, the lower the dose, the greater the difficulty in detecting any increase in the number of cancers possibly attributable to radiation. In contrast to the Chernobyl reactor accident, the Fukushima reactor accident has to date resulted in no deterministic effects and no worker deaths. Estimates to date of population doses suggest very low uptakes of radioactive iodine which was a major determinant of the epidemic of thyroid cancer following childhood exposures around Chernobyl. The estimates to date of population doses are also much lower (and the distribution much narrower) than the doses for which cancer excesses have been detected among atomic bomb survivors after 60 years of follow-up. Studies of populations exposed to low doses are also limited in their ability to account for important lifestyle factors, such as cigarette smoking and medical x-ray exposures, which could distort findings. Studies of the Fukushima population should be and are being considered for reassurance and health care reasons. Apart from as regards the extreme psychological stress caused by the horrific loss of life following the tsunami and the large-scale evacuation from homes and villages, such studies have limited to no chance of providing information on possible health risks following low dose exposures received gradually over time--the estimated doses (to date) are just too small.

Lauriston S. Taylor lecture: radiation epidemiology--the golden age and future challenges

Epidemiology is the study of the distribution and causes of disease in humans. Studies of human populations exposed to ionizing radiation have been conducted for nearly 100 y during the "Golden Age of Radiation Epidemiology." Radiation epidemiology is now so sophisticated that human studies are the basis for radiation protection standards and for compensation schemes in response to claims of ill health from prior exposures. The studies of exposed human populations are very broad and include not only the Japanese atomic bomb survivors, but also patients given radiotherapy for cancer, patients treated with radiation for nonmalignant disease, patients given diagnostic radiation, persons with intakes of radionuclides, workers exposed to occupational radiation, and communities exposed to environmental sources of radiation. But there is more to be learned, and future knowledge may be advanced from new and continued occupational studies of the early radiation workers, atomic veterans, medically exposed patients, and populations living in areas of high natural background radiation. The interaction between radiation and underlying genetic susceptibilities is an important emerging area of research. It is indeed an honor to be included among the Lauriston S. Taylor Lecturers.