Chernobyl legacy: Individual radiation exposure linked to thyroid cancer risk.
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Brenner, AV, MD Tronko, M Hatch, TI Bogdanova, VA Oliynik, JH Lubin, LB Zablotska, VP Tereschenko, RJ McConnell, GA Zamotaeva, P O’Kane, AC Bouville, LV Chaykovskaya, E Greenebaum, IP Paster, VM Shpak and E Ron. 2011. I-131 dose-response for incident thyroid cancers in Ukraine related to the Chornobyl accident. Environmental Health Perspectives http://dx.doi.org/10.1289/ehp.1002674. |
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Scientists working with people living in areas of Ukraine affected by the Chernobyl accident in April 1986 have linked individual radiation exposure to increased thyroid cancer risk. The detailed information collected over the past two decades is important for health researchers to identify high-risk individuals and to understand the health impacts of other nuclear disasters, such as the ongoing crisis in Fukuskima, Japan.
Context
People affected by natural and environmental disasters worry about immediate health effects and the risk of future chronic disease. Answers about possible ensuing health-related problems are hard to speculate at the time of the event, since there is often not enough information about individual exposures to contaminants associated with the disaster. Also, the health effects of different contaminants may not be well-known.
Nuclear accidents are among the most feared disasters. The radioactive materials released can contaminate land and water making them unlivable for generations. They can accumulate in the food chain leading to large-scale, cross-species exposures. Radiation can cause cancers, birth/developmental effects and genetic changes.
One of the most dangerous radioactive materials is radioactive iodine – referred to as I-131. I-131 is produced when atoms are split apart – the fission process – in nuclear fuel rods. Usually, the radiation is contained and stored until it is no longer a threat.
In nuclear accidents, I-131 is released as a gas that can settle on soil and plants and easily dissolve in water. People are primarily exposed by drinking contaminated water or eating contaminated foods.
The thyroid gland needs iodine to function properly. Iodine is taken up quickly by the gland, especially in young children. The thyroid will use both radioactive and non-radioactive iodine.
I-131 has a half-life of eight days – about half of the I-131 decays within that time. As I-131 decays, it releases beta particles that bombard DNA in nearby cells. This can cause mutations and increase the risk that the cells will become cancerous.
The thyroid keeps iodine for about 100 days. The chance that absorbed I-131 will decay while in the thyroid is quite high.
Young children are particularly vulnerable to the damage from beta particles, because they are still developing. The chance that cells will further mutate to become cancerous increases with time.
Many studies show associations between the risk of thyroid cancer and radiation exposure. Many of these studies assume the radiation dose based on where the people lived at the time of the accident.
One of the most dangerous nuclear accidents occurred on April 26, 1986. A series of explosions led to a fire in a nuclear reactor at the Chernobyl nuclear power plant in what is now Ukraine. The accident led to the released of a plume of radioactive material that settled over the former Soviet Union and Europe.
The Chernobyl Forum – a panel of experts from United Nations agencies, including the World Health Organization (WHO) and the International Atomic Energy Agency (IAEA) – expect that about 4,000 radiation-related deaths will result from the Chernobyl disaster. Indeed, more than 4,000 cases of thyroid cancer have been diagnosed in adults who were children living in affected areas at the time of the accident. The survival rate among those diagnosed is about 99 percent.
What did they do?
Researchers at the National Institutes of Health followed 12,500 individuals who live in areas of Ukraine affected by the Chernobyl accident and who were children (younger than 18 years old) at the time of the accident. In order to measure the absorbed dose of I-131, the researchers measured thyroid radioactivity in each participant within two months of the accident and screened them for thyroid cancer up to four times between 1998 and 2007.
Suspected thyroid cancers were checked through a tissue biopsy, and, if affirmed, were surgically removed. The researchers counted only those thyroid cancer cases confirmed by surgery.
The researchers used the dose and cancer diagnosis information to construct mathematical models that predict the risk of thyroid cancer connected with radiation exposure. The mathematical models take into account the rate of thyroid cancer expected in the population and predict the "added risk" of radiation exposure.
What did they find?
Of the 12,500 individuals in the study, 65 had confirmed thyroid cancer.
Similar to previous studies, higher radiation doses were related to an increased risk of thyroid cancer.
For every unit of absorbed radiation – a measure referred to as "one gray" – the added risk of thyroid cancer about doubled. The average exposure was a little less than one gray (0.77 gray).
What does it mean?
This study shows a conclusive risk of thyroid cancer after exposure to radioactive iodine, especially I-131.
The findings are consistent with previous human studies of those affected by the Chernobyl and other nuclear incidents. Many of those studies were smaller or assumed a radiation dose based on where people lived at the time of exposure.
Even after 20 years, the risk for cancer did not decline. This suggests the risk has not disappeared over time and even more cancer cases attributable to the Chernobyl accident are likely.
The recent work published on the lingering health risks as a result of the Chernobyl disaster will provide public health workers and scientists invaluable information on the sort of health impacts to be expected from the recent Fukushima disaster in Japan.
Scientists detected I-131 in the nuclear fallout from the recent accident at the Fukushima nuclear power plant in Japan. The ongoing crisis is a result of the March 11, 2011, earthquake and tsunami. The Japanese nuclear safety agency rated the situation a Level 7: Major Accident on the IAEA's International Nuclear and Radiological Events Scale (INES). This is the second disaster in history to be labeled as such – the first was the Chernobyl accident.
Officials detected I-131 at levels above recommended safety limits in water samples taken in Tokyo, as well as in milk and vegetable samples. The US Food and Drug Administration limited food imports from Japan for this reason.
Soon after the accident the Japanese government began monitoring radioactivity in the thyroid of the children in areas affected by the Fukushima crisis. Very preliminary reports of those measurements suggest that the dose is thousands of times less than children living near the Chernobyl power plant received.
Part of this could be explained by the very iodine-rich diet in Japan. Since the thyroid takes up radioactive iodine in proportion to what is found in the environment, children with iodine-rich diets will absorb less I-131. This is also why potassium iodide tablets are distributed to children affected by nuclear disasters – another measure that public health officials in Japan were quick to take.
The Chernobyl study provides valuable insight into long-term effects of radiation exposure. It also shows that the health effects of the Fukushima disaster may well persist over the next two decades.
ResourcesCardis E and M Hatch. 2011. The Chernobyl accident - an epidemiological perspective. Clinical Oncology 23(4):251-60. The Chernobyl Forum. 2006. Chernobyl’s Legacy: Health, Environmental, and Socio-economic impacts. Saenko V, V Ivanov, A Tsyb, T Bogdanova, M Tronko, Y Demidchik, and S Yamashita. 2011. The Chernobyl accident and its consequences. Clinical Oncology 23(4):234-43. Tronko, M, T Bogdanova, L Voskoboynyk, L Zurnadzhy, V Shpak, and L Gulak. 2010. Radiation induced thyroid cancer: fundamental and applied aspects. Experimental Oncology 32(3):200-4. |
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Based on a work at www.environmentalhealthnews.org.
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