A Conversation With
A Conversation with Drs. Maureen Hatch and Kiyohiko Mabuchi on the 25th Anniversary of the Chernobyl Nuclear Disaster
April 26 marked the 25th anniversary of the accident at the Chernobyl nuclear power plant in northern Ukraine. In addition to causing 28 near-term deaths due to acute radiation poisoning, the accident exposed 5 million people in Belarus, Russia, and Ukraine to radioactive fallout, mainly iodine-131 and cesium-137. This exposure has led to extensive epidemiology research over the past 25 years by the governments of Ukraine, Belarus, and the United States and their research partners. At NCI, this epidemiology research is led by scientists in the Radiation Epidemiology Branch (REB) of the Division of Cancer Epidemiology and Genetics (DCEG). Dr. Maureen Hatch, former head and current member of the Chernobyl Research Unit (CRU) at DCEG, and Dr. Kiyohiko Mabuchi, deputy branch chief of REB, head of the CRU, and senior scientist, discuss the lessons learned from the Chernobyl accident and summarize some of the ongoing research on the health consequences of the accident.
What lessons have been learned from the Chernobyl accident, in terms of the health consequences?
Dr. Hatch: Prior to the accident, iodine-131 (I-131), the major component of fallout from Chernobyl and a radioisotope that concentrates in the thyroid gland, was believed to have low or no malignant potential. The NCI-supported cohort studies of exposed young people in Ukraine and Belarus [who underwent screening examinations for thyroid cancer every 2 years] have provided solid evidence that I-131 exposure in childhood and adolescence increases the risk of thyroid cancer, confirming the observations based on post-accident case-control and ecologic studies by others.
The magnitude of the increased risks observed in both countries is consistent with that following external radiation in childhood, as is the fact that thyroid cancer risk in Ukraine remains significantly elevated decades after exposure. In addition, a separate study of nearly 2,600 people who were exposed in utero to I-131 fallout from the Chernobyl accident found a high radiation-related risk of thyroid cancer.
Dr. Mabuchi: One of the important questions in radiation research is whether cancer risk from chronic exposure is lower than the risk from acute exposures. In a study of more than 110,000 male cleanup workers in Ukraine who sustained repeated exposures to external radiation, NCI investigators observed a radiation-related risk of leukemia comparable to risks experienced by Japanese atomic bomb survivors with acute exposure.
In addition to the risk for leukemia overall, the NCI study results, in agreement with those from studies of Chernobyl cleanup workers in Belarus, Russia, and the Baltic countries, found a radiation effect on chronic lymphocytic leukemia, a type of leukemia that had not been linked to radiation exposure before the Chernobyl accident.
What lessons have been learned about larger public health considerations, such as possible preventive measures and how best to communicate health concerns to the public?
Dr. Hatch: It’s clearly a challenge to balance the need to communicate protective measures effectively while avoiding widespread anxiety. Residents from the most contaminated area surrounding the Chernobyl plant were evacuated fairly expeditiously. However, the distribution of potassium iodide (KI) to decrease thyroid uptake of I-131 was neither entirely timely nor systematic, in part out of concern that doing so would alarm the population.
In the area surrounding Chernobyl, most of the I-131 exposure to the thyroid among the public resulted from consumption of contaminated cow’s milk. The NCI studies and other epidemiologic studies have clearly indicated the importance of considering the pasture-cow-milk pathway in countermeasures following nuclear accidents. In theory, some thyroid cancer cases might have been averted if the government had issued a broad public health warning and/or banned certain contaminated foods. It is clearly important that governments experiencing disasters of any kind find a way to convey reliable, realistic, and understandable information about evolving events and known health risks without being either alarmist or obfuscating.
The findings of adverse effects on those exposed to Chernobyl fallout prenatally and at young ages also have clinical implications—for example, not using I-131 to treat thyroid disease in pregnant women or young children. The medical community and the public need to be educated accordingly.
What research is the staff in REB doing with regard to the consequences of the Chernobyl accident?
Dr. Mabuchi: At NCI, we are conducting two genetic studies using specimens stored at the Chernobyl Tissue Bank from Ukrainian and Russian subjects who were exposed to fallout from Chernobyl. (Read the story on the Chernobyl Tissue Bank in this issue.) We are also continuing to monitor cancer incidence in members of the Ukrainian and Belarusian thyroid cohorts and among cleanup workers via linkage with the National Cancer Registries in both countries. In addition, we are conducting an additional follow-up examination of Ukrainian subjects who had thyroid nodules identified during one of the earlier biennial thyroid screening cycles.
What questions still remain regarding the health effects of the Chernobyl accident?
Dr. Mabuchi: Questions that remain include the duration and magnitude of post-Chernobyl thyroid cancer risk among exposed young people. We will need continued follow-up in one form or another to learn how long the excess risk will continue. In addition, the thyroid cancer risks to those exposed as fetuses or young adults remain uncertain.
Studies by other investigators of cleanup workers in Ukraine and Russia have reported increases in risk for solid tumors. However, the reported increases have not been convincingly linked to radiation from the accident. There have also been reports of increases in noncancer outcomes, such as cataracts, cardiovascular disease, and mental health problems.
Future epidemiologic studies of the risk of these diseases among Chernobyl cleanup workers will be challenging because of the need to take into account the effects of behavioral and other factors. The indications are that any excess cases attributable to Chernobyl exposure are likely to be small relative to cases arising from non-radiation-related causes.
What have researchers and public health officials learned from the Chernobyl experience that is or will be applicable to the evolving situation at the Fukushima Daiichi nuclear reactor in Japan?
Dr. Hatch: One critical lesson learned is that major accidents like Chernobyl and Fukushima, affecting large segments of the population, need to be followed up by carefully designed epidemiologic studies assessing the nature and magnitude of adverse health effects. To conduct such studies, exposed people must be clearly defined at the earliest possible time, and efforts must be made to estimate accurately the extent of exposure and disease using all available sources of data and fieldwork approaches.
Dose measurement and reconstruction, as well as case ascertainment, should be rigorous and standardized. Such studies require timely planning and multidisciplinary, often multinational, collaboration.
The need to take prompt countermeasures to minimize the absorption of radioiodines by the thyroid to prevent thyroid cancer is another lesson learned, along with the importance of emergency preparedness and clear, complete, and trustworthy communication concerning the extent of exposure and potential health risks.