HTDS Guide - How the Study Was Conducted
- The HTDS studied a group of people from the Hanford region who were young children at the time of the largest radiation releases from Hanford.
- Each participant attended a medical clinic for a complete diagnostic evaluation for thyroid disease.
- Scientists analyzed whether study participants with higher radiation doses had more thyroid disease than those with lower doses.
- Based on information from Native American Tribes and Nations, a study such as the HTDS in Native American populations alone was not feasible because it would have too little chance of detecting any health effects from Hanford's iodine-131.
The HTDS study population represents a sampling of people born between 1940 and 1946 to mothers who lived in seven counties in Washington State: Benton, Franklin, Adams, Walla Walla, Okanogan, Ferry and Stevens.
All of the participants were young children at the time of the largest radiation releases from Hanford. It is believed that young children receive a higher dose to the thyroid for the same level of exposure than do adolescents and adults, and that the thyroid gland in young children may be more sensitive to the effects of radiation.
The study participants represent a range of possible doses of iodine-131 from Hanford, from the highest doses to very low doses.
Starting from birth certificates of 5,199 people born between 1940 and 1946, investigators were able to locate 94 percent of the group (4,350 people still living and 527 deceased). Of these, 3,440 were willing and able to participate fully.
Of the 3,440 study participants, 249 moved out of the Hanford region before Hanford operations began and did not move back into the region any time before the end of 1957. They are referred to as "out of area" participants in the HTDS. Because their thyroid doses could not be estimated with the computer models used by the HTDS, out of area participants were included in the data analyses as a separate group (see Figures 1-4 in Findings and Interpretations).
In analyzing the data, researchers looked for what is called a "dose-response." A dose-response is when risk of disease increases with increasing dose of radiation. If a study finds a dose-response, it provides very strong evidence linking radiation to the disease.
In the case of the HTDS, researchers studied how rates of thyroid disease in the study group varied in relation to participants' radiation doses from Hanford's iodine-131. The HTDS is an "internally controlled study." This approach enables researchers to compare groups of people who have similar characteristics (such as age, diet, lifestyle or environment) but different levels of exposure.
This approach of using one population composed of individuals with different levels of exposure has been used extensively in assessing the effects of radiation exposure in human populations.
A less effective approach would be to compare the study group to a separate population presumed to be unexposed to radiation ("unexposed control group"). That approach would be less desirable because thyroid disease may be a function of a number of factors other than exposure to radiation, and those factors may differ considerably between different populations.
Had the HTDS used an unexposed control group from another part of the country, scientists could not have known whether any differences in the rates of disease between the groups were due to a difference in exposure levels or some other factor.
Scientists specifically designed the HTDS to assess the health effects of iodine-131 from Hanford. However, to be sure that any apparent health effects of Hanford's iodine-131 were not actually due to other causes, researchers needed to consider other possible radiation exposures.
For the HTDS, the research team considered exposures due to fallout from nuclear testing at the Nevada Test Site in the 1950s. They also considered exposures that individual study participants may have received from diagnostic or therapeutic irradiation, or from jobs that involved working with radioactive materials. By explicitly accounting for such exposures in the analyses of the data, researchers were able to single out the effects of Hanford radiation.
The HTDS research team conducted a feasibility study to determine whether it would be possible to conduct a study like the HTDS of the Native American populations that were exposed to Hanford's iodine-131. Those Tribes and Nations include: Colville, Couer d'Alene, Kalispell, Kootenai, Nez Perce, Spokane, Umatilla, Warm Springs and Yakama.
A separate feasibility study was conducted because it was recognized that aspects of the lifestyles of Native Americans differed from non-Natives, particularly regarding diet, food sources and seasonal residence changes. All of these factors could have affected the dose people received.
Eight of the nine tribes provided tribal-specific information for use in estimating the number of tribal members exposed and the likely magnitude of their radiation dose to the thyroid. Based on these estimates, it was determined that a separate study like the HTDS among Native Americans would not be feasible because it would have too little chance of detecting any health effects from Hanford's iodine-131.
Nevertheless, Native Americans were included in the HTDS if they were identified in the group of 5,199 that made up the study cohort.