and 80°, which bounds the areaof this study. As shown in Fig 5, these probability contours illustrate that the chance of developing thyroid nodules is influenced by both distance and 6 in a variable manner. For example, for a fixed distance of 300 miles from Bikini, the probability decreases as @ increases. However, for most fixed distances greater than 400 miles from the test site, the probability increases with 6. These results are consistent with previously published fallout patterns showing an initial eastern pathway of the BRAVOfailout cloud.“ They are also consistent with a computer simulation pattern that suggested that after a predominantly eastern direction, toward Utrik, the fallout cloud moved south and west from Utrik.“ Absolute Risk Assessment The absolute risk coefficient has been used to compare the risk for thyroid nodules among exposed populations* and can be expressed as follows: absolute risk coefficient = number of excess cases/Gy/years at risk/1x10* persons (number of excess cases/rad/years at risk/1 million persons), where numberof excess cases is the numberof observed nodules minus the number expected. Using a prevalence of nodules of 2.45% determinedin this study for unexposed Marshallese, we determined a new absolute risk coefficient for the Rongelap and Utrik people exposed to radioactive iodines. Since estimates of the thyroid dose and years at risk for these populations were known from previous studies (see “Methods” section), we calculated a new risk coefficient of 1100 excess cases/Gy/y/1 x 10° persons (11.0 excess cases/rad/y/1 million persons). COMMENT This study demonstrates a stronginverse linear relationship between the probability of thyroid nodules developing in Marshall Islanders and the distance of their 1954 homeatoll from the Bikini test site. The direction of each atoll relative to Bikini was also an important risk factor. Our results indicate that excess thyroid nodules in Marshall Islanders were not limited to the two northern atolls of Rongelap and Utrik but occurred throughout many of the Marshall Islands. These findings suggest that the geographic extent of radioiodine exposure from the 1954 BRAVO test was much broader than previously assumed. Without thyroid dose estimates for people living on 12 of the 14 atolls in this study, radiation exposure cannot be proved as the cause of these neoplasms. > 634 JAMA, Aug 7, 1987—Vol 258, No. 5 Otherrisk factors for thyroid neoplasia, nodules was highestin this region on the atolls of Lae, Ujae, Wotho, and Likiep however, do not appear to be present. There is no evidence for iodine defi(Fig 2). The thyroid doses of people living on ciency in this population: the diet of the atolls previously assumed to be free Marshallese population is well known to from exposure to radioactive fallout have ample iodine content, especially on may have been affected by long expothe outer islands, where thedietis high sure times. While people on Rongelap in fresh fish.** In contrast to the United and Utrik were evacuated 48 to 72 hours States, no head and neckirradiation of after detonation, no such evacuation Marshallese children was employed as took place on otheratolls.“” Thus, peotherapy for benign diseases of childhood ple on those atolls may have had lower such as acne, presumedtonsilar or thymic enlargement, cervical adenitis, or . peak exposures than on Utrik, but befungal infections of the scalp. There are cause of continued exposure for the entire decay processof the radioiodines, no known dietary or environmental their cumulative thyroid doses may goitrogens that are used in the Marshall have been as high as or higher than Islands. If other unknown risk factors those on Utrik. for thyroid disease are present in this population, it must be postulated that One methodologic advantage afthey exert their effects in a pattern such forded by the Marshall Islands is that that the risk from exposure decreases the geography of these islands has prowith distance from Bikini Atoll. Thus, vided considerable variation on our the absence of other known risk factors proxy for exposure. While the thyroid for thyroid nodularity and the presence doses for persons on these atolls is of a strong inverse linear relationship not known, the position of small land between thyroid nodularity and the dismasses across thousands of square tance of each atoll from the BRAVOtest miles of ocean permits us to know the site suggest radioactive fallout as the distance from the blast site exactly. mostlikely cause of these neoplasms. Second, residents of these atolls could Although authors of previousclinical not easily move from atoll to atoll in studies of Marshall! Islanders assumed short periodsof time, especially in 1954, that 12 of the 14 atolls in this study makingit possible to ascertain on which were unexposed, other environmental atolls persons were living during the assessment studies reported evidence exposure period of the BRAVOtest. that suggests that fallout contamination These factors may in part explain why was not limited to Rongelap and Utrik. the variables distance and 6 appear to Robison and colleagues“ reported that be such strong proxies for radiation several inhabited atolls other than dose or conditions that affected the Rongelap and Utrik contained low levdose. els of long-lived radionuclides that were This study has several limitations likely residual from intermediate-range that deserve mention. The ascertainfallout in the Marshall Islands. Alment of exposure, which involves rethough the dose extrapolations from ports from participants, is subject to 1978 to 1954 were not done for these recall bias, especially in cultures such as atolls, the low doses received from the the Marshall Islands that are not time longer-lived isotopes, such as cesium oriented. Asking the question in terms 137 and strontium 90, would not have of where onelived in March 1954 might contributed significantly to the thyroid yield answers of questionable accuracy. dose during these years. However, persons in this study were An additional report documented asked where they lived when the a gamma dose at Ailuk Atoll to be “bomb” exploded, causing the Rongelap 0.01 Gy/h (1.0 rad/h) one hour after the and Utrik people to be evacuated from BRAVOdetonation;* such data suggest their homeland. The detonation of the that this atoll, previously thought to be BRAVO hydrogen test was a dramatic unexposed, received fallout. A comevent: people on many atolls in the puter simulation of the fallout cloud northern Marshalls could see thelight, utilizing all available meteorologic data feel the blast, and see the fallout on predicted that after an initial eastern vegetation hours after the blast. In direction, the maximal point of radiamuch the same way as people recall tion 16 hours after the detonation would clearly what they were doingat the time have been midway between Rongelap of Pearl Harbor or the assassination of and Kwajalein.“ This suggests that the John F. Kennedy, this dramatic hydrofallout cloud may have shifted from an gen bombaffected the Marshallese peoinitial eastern path to a south or southple in a way such that they could provide westdirection. This simulation modelis vivid descriptions of what they were consistent with the results of our doing and where they were living in 1954 study, which show that, except for at the time of the test. For individuals Rongelap, the prevalence of thyroid living on southern atolls who could not Thyroid Neoplasia—Hamilton et al