Editor’s note: Dr. Gale is an authority on medical response to nuclear and radiation accidents and participated in rescue efforts at the Chernobyl disaster, as well as at Goiania, Tokaimura, and Fukushima, among other radiation and nuclear accidents.
Anyone reading the popular press or even scientific articles will surely be confused regarding long-term health consequences of the Chernobyl nuclear power facility accident 35 years ago, on April 26, 1986. Some sources report thousands or even hundreds of thousands of cancers and cancer deaths caused by radiation from the Chernobyl accident. Other sources claim little if any harm. But where does the truth lie?
Robert Peter Gale, MD, PhD, DSc (hc), FACP, FRCPI (hon), FRSM
Ionizing Radiation and Cancer Risk
To answer this we need some background about exposure to ionizing radiation and cancer risk. Unfortunately, this relationship is both complex and controversial. Much of what we know is based on data from 94,000 A-bomb survivors and 27,000 matched controls who were not in Hiroshima or Nagasaki the day the bombs exploded. These persons have been carefully followed since 1950, with cancer incidences and deaths recorded and analyzed. With a matched control cohort, we can reasonably accurately estimate the proportion of cancers in the exposed cohort caused by or contributed to by radiation from the A-bomb exposures.
What do these data show? First, the risk of developing a radiation-induced cancer is dose-related—the higher the dose, the greater the probability a cancer in an A-bomb survivor was caused by radiation exposure. Second, the A-bomb data allow us to determine the briefest interval from radiation exposure to cancer diagnosis. For leukemias, this is about 2 years, and for solid cancers, about 10 years. These increased risks, especially those for solid cancers, were most easily detected after 30 years and remain over a person’s lifetime.
Looking at specific cancer types, we know about one-third of the 312 leukemias developing in these 94,000 survivors were caused by or contributed to by their A-bomb radiation exposure. The relative impact of radiation on the development of solid cancers is proportionally less—about 10% of approximately 9,000 cancers were caused by A-bomb radiation exposures. At doses of 5 to 100 milliGray (mGy), the increase in cancer risk to an exposed survivor is about 2%, whereas at doses over 2,000 mGy, the increase in cancer risk is greater than 60%. These conclusions from the A-bomb survivor studies are mostly confirmed by studies of other radiation-exposed populations.
Cautious Application of Data
Can we use these data to estimate how many persons exposed to radiation from Chernobyl have or will develop and/or die from cancer. Yes, but with caveats. For example, the A-bomb survivors were exposed to acute, high-dose whole-body radiation, whereas the roughly 6.4 billion people exposed to radiation from the Chernobyl accident (the population of the northern hemisphere) was (and remains) chronic, low-dose exposure. Additional differences include external vs external plus internal exposure, genetic background, and others. As such, one must be cautious when applying estimates based on the A-bomb survivors to persons exposed from the Chernobyl nuclear power facility accident.
The consensus among most scientists (bearing in mind consensus does not equal truth) is that there is a linear, no-threshold relationship between radiation dose and cancer risk. This is not to say one photon can cause cancer but that one photon increases cancer risk—different concepts. The linear, no-threshold relationship, albeit controversial, is the most conservative interpretation of the data we have and the most protective of public health.
However, there are several problems in accepting this hypothesis. For example, people around the world are exposed to very different background radiation doses. People living in Denver are exposed to background radiation levels much higher than persons living in Los Angeles because of different soil compositions (the Rocky Mountains vs sand) and elevations reflecting distance from the sun, a source of cosmic radiation. Nevertheless, cancer incidences in Denver and Los Angeles are similar except for skin cancers in Denver. Other places in the world have 10- or 50-fold greater background radiation doses than either Denver or Los Angeles with no detectable increased cancer incidence. These data are more consistent with a threshold between radiation dose and cancer risk; namely, up to a certain dose of radiation, there is no increase in cancer risk. Because of these contradictory hypotheses, we need to exercise caution in applying our radiation risk cancer estimators to relatively low-dose chronic radiation exposure typical of persons exposed via the Chernobyl accident.
Back to Chernobyl. From a medical perspective, the two most important radionuclides released by the accident were iodine-131 and caesium-137. Iodine-131 has a half-life of about 8 days, so exposures were over about 2 months after the accident (10 half-lives). However, caesium-137 has a half-life of 30 years, meaning it will be a concern for about 300 years.
How do these data translate to increased cancer risk? This requires some background for context. The average American is exposed to about 6 millisieverts (mSv) of radiation per year. About one-half of this dose comes from naturally occurring background radiation from the earth’s crust, the cosmos, granite countertops, and porcelain teeth, if you have them. The other one-half is man-made, mostly coming from radiologic procedures ordered by physicians such as computed tomography and positron-emission tomography scans.
Now let’s consider the impact of radiation exposure from the Chernobyl accident on millions of people living in Ukraine, Belarus, and Russia including 300,000 evacuees and about 1 million people living in contaminated lands. Their average lifetime excess dose from the Chernobyl accident is about 10 to 30 mSv, a dose that is equivalent to the background dose received from living for 3 to 10 years in Los Angeles. As you can calculate from the data I discuss above, we would expect few if any excess cancers caused by radiation exposure from the Chernobyl accident.
What some scientific bodies such as the United Nations (UN) Scientific Committee on Atomic Radiation (UNSCEAR) have done is to apply the cancer risk estimator from the A-bomb survivors to these persons. Doing this, UNSCEAR predicted about 4,000 excess cancers, which translates to about 2,000 cancer deaths over 50 years. The UN Chernobyl Forum predicted 5,000 excess cancer deaths. Other sources, often without scientific credentials and/or with a political agenda predict thousands, hundreds of thousands, or even millions of cancers and cancer deaths.
What to believe? Let’s look at the data 35 years after the Chernobyl accident, keeping in mind many of these epidemiologic studies are poor-quality, have uncontrolled covariates (eg, increased drinking and smoking in the former Soviet Union), ascertainment biases, and so forth. They are also inappropriately colored by politics: anti- and pro-Russian positions, anti– and pro–nuclear energy positions, and so on.
First, there were about 7,000 excess thyroid cancers in children and adolescents living in Ukraine, Belarus, and Russia proximal to the accident site—an estimated 100-fold increased incidence. Fortunately, most were not fatal. These cancers were caused by exposure to iodine-131, predominately from milk consumption. This increased risk continues today but is decreasing. Admittedly, some but not most of these cancers reflect ascertainment bias.
What of other cancers? Leukemias (except for chronic lymphocytic leukemia [CLL]) were the first cancers detected after the A-bomb explosions. You can think of leukemias after radiation exposure as a canary used to detect carbon monoxide in a coal mine. If the canary keels over, get out! Consequently, we and others looked carefully for an increase in leukemias in the 10 years after the accident but found no convincing evidence of one. However, one group reported a modest increase in CLL among persons assigned to mitigate the accident (“liquidators” in Russian), who received much higher radiation doses than the general population. Finding no increase in leukemias was encouraging in the context of predicting a possible increase in other cancers.
What of solid cancers? Again, there are no convincing data of an increase in solid cancers except one report of a small increase in breast cancer, again in heavily exposed women. Again, good news.
The bottom line is there are few data to suggest that radiation released from Chernobyl increased cancer globally.— Robert Peter Gale, MD, PhD, DSc (hc), FACP, FRCPI (hon), FRSM
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No Easy Answers
The bottom line is there are few data to suggest that radiation released from Chernobyl increased cancer globally. Admittedly, failing to detect an increase does not prove there was no increase. However, any increase must be relatively small. We should recall that about 40% of us will develop cancer in our lifetimes. Even if there were an increase from Chernobyl-related radiation, it would be less than 0.1% using current radiation risk models. I should mention there are also no convincing data that birth defects or genetic abnormalities were increased by radiation released from Chernobyl, despite what you might read elsewhere. This is confirmed by a recent NIH report in Science.
Lastly, we need to put the Chernobyl accident and nuclear energy in the context of other energy alternatives and in the context of global climate change. And we should consider that there are more than 400 nuclear power reactors operating worldwide, providing about 10% of global energy production, with 50 more under construction. France produces more than 70% of its electricity from nuclear sources. The figure in the United States is 20%. Most people do not realize that burning fossil fuels releases more radiation into the environment per kilowatt energy generated than a nuclear power facility. Mining to provide pipes for a solar power station brings more radiation to the earth’s surface than radiation released from a nuclear power facility. And construction of the Aswan High Dam in Egypt to provide electrical power has caused many thousands of bladder cancers from Shistosoma haematobium infection because of slowed flow of the Nile river. There are no easy answers.
In summary, the Chernobyl nuclear power facility accident was a global tragedy. For The ASCO Post, I focused on health consequences, specifically cancer. But we should not forget the important social, psychological, and economic consequences of the Chernobyl accident. The Chernobyl accident was preventable, resulting from human error and faulty reactor design. We now have safer nuclear reactors and have learned important lessons from the Chernobyl and Fukushima accidents. Perhaps it’s time for the United States and the world to reevaluate the potential role of nuclear energy in helping us deal with global climate change.
DISCLOSURE: Dr. Gale has received research funding from the UK National Institute of Health Research funding scheme.
DISCLAIMER: This commentary represents the views of the authors and may not necessarily reflect the views of ASCO or The ASCO Post.
Dr. Gale is Visiting Professor of Haematology at the Imperial College London and a Foreign Member of the Russian Federation and China Academies of Science and Medical Science.The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.