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 Scott Davis, Ph.D., and colleagues at Fred Hutchinson Cancer Research Center. 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 findings 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."
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, said Davis, a member of Fred Hutchinson's Public Health Sciences Division.
"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.
Kenneth Kopecky, Ph.D., a biostatistician in Fred Hutchinson's Public Health Sciences Division, was the study's co-investigator and directed the data analysis. Public Health Sciences Division 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 Byransk Diagnostic Center and the Bryansk Institute of Pathology (both in Bryansk), 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 less 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.