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Chernobyl Study Reveals First Direct Evidence that Risk of Thyroid Cancer
Rises with Increasing Radiation Dose
Press Release by Fred Hutchinson Cancer Research Center
SEATTLE — Sep. 1, 2004 — 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.
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 non-radioactive 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.
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 a par with the esteemed National
Academy of Sciences in the United States. In May he received an honorary
diploma in Moscow.
Davis and colleagues 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.
This work is part of Fred Hutchinson's Global Health Initiative, which
focuses on international collaboration to understand and solve some of the
most widespread health problems in the world, including cancer and
infectious diseases.
Media Note
To
arrange an interview or obtain a copy of the paper, "Risk of Thyroid Cancer
in the Bryansk Oblast of the Russian Federation after the Chernobyl Power
Station Accident," please contact Kristen Woodward in Fred Hutchinson media
relations at (206) 667-5095.
________________________________________
It All Started with a Russian Helicopter Pilot who was
Treated for Leukemia at Fred Hutchinson
Providing some long-awaited answers to Chernobyl survivors has been a
rewarding research endeavor for Scott Davis, Ph.D., and colleagues at Fred
Hutchinson Cancer Research Center, but it hasn't been a straightforward one.
Some of the team's greatest achievements were simply establishing the
working relationships and infrastructure to get the studies off the ground.
"Within the first year of the 1986 accident, we were very interested in
seeing if we could get involved and participate in long-term studies of
health effects," Davis said. "But at the time of the accident, our
government and that of the former Soviet Union were not so friendly, so
establishing connections through that route didn't work."
But in 1990, an opportunity surfaced when a Russian helicopter pilot
involved in the initial efforts to contain the Chernobyl radiation developed
leukemia and came to Fred Hutchinson for a bone-marrow transplant. After his
treatment, an informal exchange program began between Fred Hutchinson and
the National Center for Hematology in Moscow, whose director approached the
center for assistance in developing a research and treatment institute for
victims of the accident. Davis and colleague Kenneth Kopecky, Ph.D., made
their first trip to Moscow that year.
Then, in 1992, the Soviet Union collapsed. "We were back to square one in
terms of negotiations," Davis said.
But, thanks to efforts by Fred Hutchinson's then-president and director,
Robert W. Day, M.D., and by the late Adm. Elmo Zumwalt, a former center
trustee and former chief of naval operations for the U.S. Navy, new
relationships were established. In 1992, a research consortium consisting of
three international teams working in Russia, Belarus and Ukraine was created
to study long-term health effects of the radiation released at Chernobyl.
"Our initial work in Russia was simply to conduct small pilot studies to
establish in concrete terms whether we could carry out all phases of an
epidemiological study," Davis said. "There was no history of doing this kind
of research in Russia or the other two countries. We had to set it all up
from scratch."
Challenges included purchasing Russian vehicles for the field teams using
federal dollars — an unprecedented bureaucratic challenge for the
researchers — importing all laboratory equipment and supplies, and then
figuring out a way to maintain them without the standard resources that one
takes for granted in the United States.
"It's been a long haul and an enormous amount of time and work," Davis said,
whose 30-plus trips to the former Soviet Union include walking the grounds
of the evacuated plant and surveying the desolated 30-kilometer evacuation
zone.
Once the team established the capability to do the research, the group began
its studies of thyroid cancer, a disease linked to radiation exposure. By
the early 1990s, many new cases of the disease, particularly among young
children, were diagnosed in regions near the blast. Since then, reports show
several hundred cases of thyroid cancer in young children in the three
countries contaminated by Chernobyl, a trend that appears to be continuing.
Despite the lack of resources available to initiate these studies, Davis
said that scientists and citizens of the three countries were eager for the
research from the start. "Our collaborators in Russia have been terrific
colleagues," he said. "We now have very close ties with our partner
institutions."
He
also credited the strong encouragement and support from Fred Hutchinson's
senior administration for helping him establish stable working relationships
with their overseas colleagues.
"The incredible support and flexibility of the center, especially in the
early stages, really made this happen. That can't be overstated," Davis
said.
Media Contact Kristen Woodward (206) 667-5095
kwoodwar@fhcrc.org
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Fred Hutchinson Cancer Research Center
The Fred Hutchinson Cancer Research Center, home of two Nobel Prize
laureates, is an independent, nonprofit research institution dedicated to
the development and advancement of biomedical technology to eliminate cancer
and other potentially fatal diseases. Fred Hutchinson receives more funding
from the National Institutes of Health than any other independent U.S.
research center. Recognized internationally for its pioneering work in
bone-marrow transplantation, the center's four scientific divisions
collaborate to form a unique environment for conducting basic and applied
science. Fred Hutchinson, in collaboration with its clinical and research
partners, the University of Washington Academic Medical Center and
Children's Hospital and Regional Medical Center, is the only National Cancer
Institute-designated comprehensive cancer center in the Pacific Northwest
and is one of 38 nationwide.
For more information, visit the center's Web site at
www.fhcrc.org.
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