Year: 2020
Type: editorial
Abstract: In their study in the IJE, Rivkind and colleagues investigate the risk of breast cancer among women residing in Bryansk, Russia, who were exposed to prolonged internal and external radiation from the 1986 Chernobyl accident.1 Although more than 30 years have passed since the accident, this is the first study to evaluate breast cancer risk in relation to individual radiation dose estimates among residents of areas contaminated by Chernobyl fallout. The primary findings were an increased breast cancer risk in the highest categories of exposure and very high effect estimates compared with previous studies, although admittedly confidence intervals were wide. Whether the differences with previous studies are due to chance, different exposure scenarios, underestimation of doses, population differences or some source of bias relating to the study design is an important area for future investigation. The health effects of radiation may depend on the type of radiation, total dose, dose rate and susceptibility of the individual at the time of exposure. Most of our understanding of the health effects of low to moderate doses comes from the Life Span Study of Japanese atomic bomb survivors who were acutely exposed to primarily external gamma radiation, a type of radiation that can cause damage to multiple tissues and organ systems. The radioactive isotopes released into large areas of Ukraine, Belarus and Russia from the Chernobyl accident included iodine 131 (I-131, half-life of 8 days), caesium 137 (Cs-137, half-life of 30 years), and others. In contrast to the atomic bomb survivors, studies of populations exposed to Chernobyl offer the opportunity to examine prolonged exposure to both internal and external radiation for a number of radionuclides that vary in their uptake and concentration throughout various tissues in the body. Studies examining the radiation-related risk of cancer after the Chernobyl accident, including cohorts established by the National Cancer Institute in collaboration with institutions in Ukraine and Belarus, have largely focused on thyroid cancer in exposed children.2 As the thyroid gland uses iodine to make hormones, iodine, and thus radioidines (e.g. I-131), concentrate in thyroid tissue, making the thyroid gland especially vulnerable to this type of exposure. It is believed that radiosensitivity increases with the rate of cell proliferation; as a result, many studies were designed to focus on children and adolescents.3 In addition to the thyroid, iodine also concentrates to lesser degrees in the salivary glands, stomach, intestine, kidney, ovaries and the mammary glands of nursing women.4 Epidemiological studies for the corresponding cancers have not been conducted in the context of Chernobyl, possibly because the doses may be substantially more modest and these cancers have relatively low incidence or they occur in special populations. Over time, residents of contaminated areas are also exposed to long-lived radionuclides, such as Cs-137, which, once inhaled or ingested, are distributed fairly uniformly throughout the body's soft tissues. In contrast to thyroid cancer, which is both considered rare and associated with low mortality, breast cancer has the highest rates of cancer incidence and mortality in women worldwide and represents an important ongoing focus of radiation risk assessment. Epidemiological data on breast cancer risk after prolonged exposure to low doses of radiation are limited, though of important public health concern because they reflect exposures in occupational settings as well as a likely repeat scenario in the case of another nuclear accident. Rivkind et al.’s case–control study is the first to evaluate the risk of female breast cancer in relation to long-term, low dose rate, low-to-moderate dose exposure from the Chernobyl accident.1 Although 95% confidence intervals (CIs) were wide, reported dose-response estimates [excess odds ratio per centigray (EOR/cGy), 0.57; 95% CI, <0 to 15.5] were substantially larger than those observed most recently among Japanese atomic bomb survivors (ERR/cGy, 0.011; 95% CI, 0.0073 to 0.016 for attained age of 70 after exposure at 30 years),5 chronic low dose-rate exposure from the Mayak nuclear weapons facility from waste discharged into the Techa River (ERR/cGy, 0.02; 95% CI, –0.01 to 0.06)6 or a pooled analysis that included medically exposed women.7 The reason(s) for these differences are not clear. In general, high risk estimates could result from an underestimation of doses. Rivkind and colleagues note the increased uptake of radioiodine during pregnancy and lactation.1 Exposure during these important reproductive periods was accounted for based on the 2004 International Commission on Radiological Protection Publication 95. Customization of breast doses could be considered in light of new research on iodine metabolism, breastfeeding patterns, improved anatomical models (i.e. customized phantoms) and breast cancer aetiology. For example, the majority of breast cancers are believed to originate in the terminal ductal lobular units, which could be specifically considered in dose estimation. High risk estimates can also result from the influence of a highly radiosensitive subgroup. Whereas childhood has been shown to be a period of heightened radiosensitivity to thyroid cancer in both atomic bomb survivors and residents exposed to Chernobyl fallout,8,9 comparable data for breast cancer are limited. Recent data from the Life Span Study reveal a significant radiation dose-response that varies based on age at exposure, and with risk estimates several times higher among women with later age at menarche and those exposed around menarche.5 Rivkind et al. report much higher radiation-related risk for women aged <13 years at the time of the accident compared with those aged ≥13 years, although confidence intervals were very wide and overlapping for these two groups.1 These findings are consistent with the belief that radiosensitivity is increased during times of rapid cell proliferation, which occurs during breast development at the time of puberty. Another important period of susceptibly may come during pregnancy, when the breast undergoes rapid cell proliferation in preparation for lactation.10 Involution (i.e. programmed cell death), which occurs during lactation and weaning, has also been identified as an important factor in breast cancer risk. Unfortunately, there were not enough women in Rivkind et al.’s study to evaluate risk after exposure during specific reproductive periods. Rivkind et al.’s study collected extensive information on breast cancer risk factors and other potential confounders and selected controls from the rolls of community health clinics from which cases arose. In general, case-control studies may be more susceptible to both confounding and selection bias, the sources of which may be difficult to identify or account for in practice. The role of chance, including uncertainty in dose estimation, also cannot be ruled out. In light of the high estimates reported in this study, and limited data on prolonged low dose-rate exposure, the radiation-related risk of breast cancer after the Chernobyl accident remains an important area for future research. The Intramural Research Program, Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, U.S. Department of Health and Human Services. Conflict of Interest: The author has no conflict of interest to disclose.
DOI: 10.1093/ije/dyaa038
Author: Elizabeth K Cahoon
Language: English
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,514
Award: G9000323499
OpenAlex ID: W3013309361