NEWS

Final Report of Hanford Thyroid Disease Study Released

Tom Reynolds

Northwest U.S. residents with childhood radiation exposures from Washington state’s Hanford nuclear site had similar risk levels for thyroid cancer and other thyroid disease regardless of their radiation dose, according to the final report of the federal government’s Hanford Thyroid Disease Study.

The limited available evidence also suggests that disease incidence among study participants is comparable to incidence in nonexposed populations. This means that if Hanford radiation releases had any health effects on the thyroid gland, they are probably too small for epidemiology to detect, the investigators concluded.

About 100 people attended an open meeting June 21 in Richland, Wash., where the report was released and investigators fielded questions from the public, said Paul Garbe, D.V.M., scientific adviser for the Hanford study at the Centers for Disease Control and Prevention in Atlanta, the sponsoring agency. The study was conducted under contract by researchers at the Fred Hutchinson Cancer Research Center in Seattle.

Some who heard the negative findings are unlikely to be satisfied. More than 5,000 "downwinders" —residents of Washington, Oregon, and Idaho who believe Hanford radiation caused their thyroid, breast, and other cancers—have two lawsuits pending against five corporations that ran the facility, including General Electric and DuPont. On June 18, a ruling by the 9th U.S. Circuit Court of Appeals gave the litigants new momentum, overturning a lower court ruling that dismissed most of their cases.

History

The controversy dates to 1986, when the U.S. Department of Energy began to release previously classified documents detailing radiation releases from the plant, which manufactured plutonium for early nuclear weapons, including the bomb that leveled Nagasaki, Japan, in World War II. From 1944 through 1957, the plutonium production process at the Hanford facility caused the release of a variety of radioisotopes, which drifted on the wind and the Columbia River, settled on vegetation, and were consumed by grazing animals. The vast majority of radioactive releases came in a single year—1945. A CDC advisory panel recommended a study to estimate radiation doses to area residents, and another study to examine the feasibility of studying potential health effects of iodine-131, the radioisotope that accounted for most of the exposures.

In the first study, the Hanford Environmental Dose Reconstruction Project (HEDR), scientists looked at radiation exposure routes, most importantly consumption of cow and goat milk, but it also including vegetables and fruits (see map, p. 1047). They analyzed distribution patterns from local dairies to determine where residents got their milk and how long it took to get from cow to consumer (since I-131’s 8-day half-life figures in the calculations). Those who drank goat milk had higher doses because goats concentrate iodine in their milk more efficiently than cows. The results of HEDR were published in 1994.



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The Hanford Nuclear Site is in southeastern Washington. A 246 by 306 mile area around the site was studied for the Hanford Environmental Dose Reconstruction Project.

 
In 1988, Congress mandated the thyroid disease study, which would take nearly 13 years to complete and cost $18 million. The study used computer programs from HEDR and interviews with participants to estimate I-131 doses for 3,440 people born between 1940 and 1946 to mothers living in seven Washington counties. This group was chosen because children are believed to be most vulnerable to the effects of radiation on the thyroid. Studies of the Hiroshima and Nagasaki atom bomb blasts, the 1986 Chernobyl reactor explosion in Ukraine, and the Marshall Islands and Nevada nuclear test sites have all found increased rates of thyroid cancer or thyroid nodules, though the routes of exposure differed from Hanford’s.

The subjects were interviewed about where they lived and how much milk and food they consumed as children and were tested and examined for thyroid disease by physicians who were unaware of their exposure history. In an attempt to reconstruct 50-year-old exposures, the subjects’ mothers or other adults who had raised them were also interviewed about diet whenever possible.

In addition to participants who lived in the Hanford area during the releases, the data analysis included a group who had moved out of the area before Hanford’s operations began. Doses could not be calculated for this last group, but their exposures were likely low to nonexistent, and they were included in the outcome analyses for comparison.

In 1999, CDC released a draft report for public comment and peer review. Using a dose-response analysis, the investigators found no evidence for any effects of the participants’ intake of I-131 on the incidence of thyroid cancer, benign thyroid nodules, hypothyroidism, hyperparathyroidism, or ultrasound-detected thyroid abnormalities. (Ultrasound is used to detect abnormalities too small to be palpated, although the clinical significance of these is not well understood.)

Draft Report Criticized

The draft report raised the hackles of critics, mostly downwinders who complained that the study should have included people residing in other areas or born at other times and looked for health problems other than thyroid disease or evaluated exposures to other radionuclides besides I-131. CDC maintains that its strategy—focusing on the most likely health effects from the most likely source of exposure—allows the best chance for detecting an effect, if there is one.

The study did find what one scientific reviewer called "a surprising amount of thyroid disease," including hypothyroidism in 27% of female participants. But because they relied on the dose-response relationship instead of an external control group, the investigators could draw few conclusions regarding these rates. They decided against including a control group from outside the Hanford area because they felt that group differences unrelated to Hanford radiation might confound the findings.

Arthur B. Schneider, M.D., Ph.D., chief of endocrinology at the University of Illinois, Chicago, served on a CDC advisory committee and reviewed the draft report in 1999. He said that screening bias—finding more disease when looking intensively for it—makes the high rates found among study participants difficult to interpret. "It’s a key issue in thyroid cancer or any thyroid disease," Schneider said, "because many thyroid cancers would not become clinically evident except for the intervention—in this case thyroid ultrasound. So it’s not a very useful comparison to make" between rates in the study and general population rates.

Schneider agreed with the critics on one point: the study’s power was relatively limited because it included few people who had high estimated radiation doses. This was despite intentional "oversampling" of areas close to Hanford to get as many highly exposed individuals as possible. Less than 1% had doses over 1000 milliGray (mGy), while the mean dose was 186 mGy. By comparison, I-131 doses averaging 1100 mGy are used in diagnostic imaging, typically in adults, and have not been associated with an increased risk of thyroid cancer.

"They might have used the absolutely perfect design, but maybe there just weren’t enough people with high doses to detect a dose-response," Schneider said.

Despite these caveats, he said the Hanford investigators used the proper design. "There’s no stronger way to do this kind of evaluation than using the dose-response," which has proven fruitful in uncovering radiation effects in many other studies.

"To me the biggest question is how to explain the differences between these and the post-Chernobyl findings," he added. "Why did those kids develop cancer while the Hanford kids didn’t?" (Many in the Chernobyl region had higher doses than anyone got from Hanford, but were exposed over a shorter time period. Scientists estimate that the Chernobyl accident has caused more than 2,000 thyroid cancer cases, and some believe that iodine deficiency in the diet may play a role by allowing more I-131 to get to the thyroid.)

NAS Review

A panel convened by the National Academy of Sciences reviewed the draft report, giving the study high marks for design and execution but identifying problems in data analysis and presentation of the results. The panel said the investigators had failed to take into account several sources of uncertainty surrounding the dose reconstruction and had presented the results in a way that made them appear more conclusive than was warranted. The panel also made recommendations on the best way to communicate the results to the public.

In response, the investigators estimated doses using several different methods, including some that were not based on the HEDR model but simply classified subjects by place of residence or residence plus milk consumption. They also used two different statistical approaches to estimate the impact of dose uncertainty on the risk estimates. The new analyses did not change the overall conclusion of no significant dose-response for any type of thyroid disease.

"We analyzed the data a number of ways, and the results were the same," said Scott Davis, Ph.D., Fred Hutchinson’s principal investigator for the study.

The investigators also searched the literature for population data on thyroid disease that might provide comparisons. A 1997 review of autopsy studies totaling 3,744 cases found that the mean prevalence of thyroid cancer was 3.6%, mostly "occult" microscopic cases. The Hanford study, by contrast, found 12 cases of thyroid cancer among 1,747 women and 7 cases among 1,693 men—less than 1%. (See Stat Bite at right.) But since the researchers were not able to slice up participants’ thyroids to look for occult cancer, the comparison has little meaning.

More apt comparative data are available for noncancerous thyroid disease. For benign nodules, the Framingham (Mass.) Heart Study found a cumulative incidence over 15 years of 8.1% in women and 2.4% in men. The Hanford study found 9.7% and 4.7%, respectively, but many cases were detected through ultrasound, and if these are excluded, the numbers fall to 4.8% and 2.0%. For hypothyroidism, the authors found that prevalence estimates from several major studies are "in reasonable agreement with each other, and define a range that encompasses the estimates derived in the Hanford Thyroid Disease Study," 11.7% in females and 3.7% in males.

Despite the negative findings, many downwinders remain unsatisfied and angry, finding the individual cases of thyroid disease in their families more compelling than the study’s statistics. Their lawsuits, which if successful could cost the Hanford contractors millions of dollars, were given new life when the circuit court reversed a 1998 ruling by the U.S. District Court for Eastern Washington. The district judge had ruled that each plaintiff would have to demonstrate that he or she had received a radiation dose that doubled the risk of thyroid disease. The new ruling dismissed that standard, but also limited damages for emotional stress to plaintiffs diagnosed with disease. The plaintiff’s lawyers are preparing to seek class-action status for the suits, according to the Seattle Times.

To those who remain convinced that Hanford caused their illness, Garbe said, "we try to explain to them that the study is about populations, not individuals, and there is no way we can say for certain whether their disease is or is not related to Hanford."



             
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