The risk of thyroid cancer rises with increasing radiation dose, according to the most thorough risk analysis for thyroid cancer to date among people who grew up in the shadow of the 1986 Chernobyl power-plant disaster.
The incidence of thyroid cancer was 45 times greater among those who received the highest radiation dose as compared to those in the lowest-dose group, according to a team of American and Russian researchers led by Dr. Scott Davis and colleagues in the Public Health Sciences Division. They report their findings in the September issue of Radiation Research.
"This is the first study of its kind to establish a dose-response relationship between radiation dose from Chernobyl and thyroid cancer," said Davis, referring to the observation that as radiation doses increase, so does the risk of thyroid cancer. "We found a significant increased risk of thyroid cancer among people exposed as children to radiation from Chernobyl, and that the risk increased as a function of radiation dose."
Having such information in hand, Davis said, may help officials better predict what long-term health effects to expect in the event of a similar nuclear accident or terrorist attack.
"Another potential benefit of the study is that it allows officials to more accurately understand and document the magnitude of the thyroid-cancer burden that has resulted from Chernobyl. This information will be important in designing and maintaining programs targeted toward the victims of the disaster."
5 million exposed to radiation
While about 30 people were killed immediately from the blast, which remains the worst accident of its kind in history, an estimated 5 million people were exposed to the resulting radiation.
"Prior to Chernobyl, thyroid cancer in children was practically nonexistent. Today we see dozens and dozens of cases a year in the regions contaminated by the disaster, and the incidence continues to rise," Davis said. "This provides some evidence that there's an excess of thyroid cancer in children and in people who were children at the time of the accident. However, until now nobody had taken the next step to find out just how much a risk there is and whether it rises along with radiation dose."
While previous Chernobyl studies have relied on broad-stroke estimates of radiation exposure based on such factors as ground contamination, geographic proximity to the northern Ukraine plant or other surrogate measures of exposure, this study is the first of its kind to factor into the equation individualized estimates of radiation dose based on in-person interviews about diet and other lifestyle factors, Davis said.
"After all these years, many efforts have been made by various research groups around the world to study the health effects of Chernobyl, and hundreds of scientific papers have been published. But ours is the first report that provides quantitative estimates of thyroid-cancer risk in relation to individual estimates of radiation dose," said Davis, also chairman of the Department of Epidemiology at the University of Washington School of Public Health and Community Medicine in Seattle.
Dr. Kenneth Kopecky, a biostatistician in PHS, was the study's co-investigator and directed the data analysis. PHS staff managed and coordinated all aspects of the project. They included Theresa Taggart (project manager), Lynn Onstad (statistician), Teri Kopp (administration) and Laurie Shields (research coordinator).
The Fred Hutchinson team organized a collaborative effort with a dozen scientists at four Russian institutions to conduct this research: the Medical Radiological Research Center in Obninsk, the Bryansk Diagnostic Center, the Bryansk Institute of Pathology and the National Center of Hematology in Moscow. All investigators were members of the International Consortium for Research on the Health Effects of Radiation funded by the U.S. Office of Naval Research.
The researchers focused their efforts on western part of the Bryansk Oblast of Russia. This region, located about 66 miles northeast of Chernobyl, is the most heavily contaminated area in the Russian Federation. This was the first study of this type among residents of the Russian Federation exposed to Chernobyl radiation.
Working through a local cancer registry, the researchers identified 26 people with thyroid cancer who were younger than 20 years old when the Chernobyl accident occurred; the majority were under 16 when their thyroid cancers were diagnosed. They then identified 52 healthy control subjects from the general population for comparison purposes. The controls and cancer cases were matched by age and place of residence at the time of the accident.
The researchers then set about collecting information from these individuals and their mothers or fathers that would allow them to estimate each person's radiation dose using computer models. Interviews took place in the home and were conducted by Russian physicians.
Individual doses depended largely on the ingestion patterns of food contaminated with radioactive iodine-131 (I-131), which concentrates in the thyroid gland. The primary source of food-based I-131 was milk from cows that grazed on contaminated pastures. Radiation doses to the thyroid increased along with the amount of milk and dairy products consumed. External, airborne radiation and contamination of other foods also contributed somewhat to the overall dose, depending on the person's proximity to the plant at the time of the accident. These doses were all received within the first few months after the accident, before the I-131 in the environment decayed into nonradioactive elements. While other radioactive contaminants remain in the area, they do not cause appreciable doses of radiation to the thyroid.
In addition to the study's ability to estimate individual radiation doses based on personal interviews, other strengths of the study included the fact that all cases of thyroid cancer were confirmed independently by a panel of expert pathologists, and the study focused on people exposed as young children and adolescents, a group that is likely to be most susceptible to the effects of radiation exposure to the thyroid gland. Limitations of the study included its small sample size and its reliance on individual recall for reporting factors such as milk-consumption patterns that were used to estimate radiation dose.
Efforts are under way to investigate a larger population in a similar fashion to see if these findings can be replicated, Davis said.
Davis and colleagues also have extended their cancer-risk studies to older Chernobyl survivors and are investigating how the damage caused to DNA by radiation influences the risk of developing thyroid cancer.
For his contributions to the field, earlier this year Davis became the first foreign epidemiologist elected to the Russian Academy of Medical Sciences. The group's status in that country is on par with the U.S. National Academy of Sciences. In May, he received an honorary diploma in Moscow.