te Gr oT

at Hiroshima and Nagasaki.” Studies
of exposures to iodine 181 in humans
have been limited largely to “I therapy
for patients with Graves’ hyperthyroidism. It is unclear from these
studies whether “I alone results in an
excess of thyroid nodules.” Muchless
is known about the health risks of exposure to short-lived radioiodines other
than “I, This information may be important in assessing the impact of ra-

dioiodine exposure from nuclear reactor

While the people from Rongelap and
Utrik have been exhaustively studied
during the last 33 years, these previous
studies of thyroid neoplasia did not include the total geographical extent of
the Republic of the Marshall Islands. To
define more carefully the risk of thyroid
neoplasia from nuclear fallout containing radioactive iodines, we conducted a
retrospective cohort study of thyroid
nodules in 7266 Marshallese people
from 14 atolls, including several southern atolls, which served as the source of
the best available unexposed comparison group.
Study Hypothesis
The objectives of this study were as
follows: (1) to determine the prevalence
of thyroid nodules in people who were
living on 14 northern and southern atolls
at the time of the 1954 BRAVO detonation; (2) to test the null hypothesis that
no difference existed in the prevalence
of thyroid nodules amongthe12 atolls of
this study previously thought unexposed to radioactive fallout; and (3) if
the null hypothesis is rejected, to determine which factors might explain the
variation in rates of thyroid nodules.
Study Location
The Marshall Islands are located 2400
miles southwest of Hawaii; approximately 35000 people (1985) live on 24
atolls spread among 375000 square
miles in the central Pacific Ocean. This
population is distributed roughly in
thirds on the following atolls: Majuro
Atoll, the administrative district of the
government of the Marshall Islands;
Kwajalein Atoll; and the remaining 22
atolls, known collectively as the “outer
islands.” This study took place between
June 1983 and March 1985 on 14 of the 24
inhabited atolls in the Marshall Islands
(Fig 1.

For this study, northern atolls were
defined as those north of Majuro
(Rongelap, Utrik, Mejit Island, Ailuk,
Likiep, Wotje, Maloelap,


Lae, Ujae, and Wotho), and southern
atolls were defined as those south of
Majuro (Jaluit, Ebon, and Mili). These

JAMA, Aug 7, 1987—Vol 258, No. 5

14 study atolls were selected to include

all northern atolls that could have possibly been in the path of fallout and as
many southern atolls as logistically feasible. Atolls that were not studied included five central atolls, two currently
uninhabited northern atolls (Rongerik
and Ailingnae), one southern atoll, and
two atolls west of Bikini.

Study Design and Sample
A population-based retrospective cohort design was employed. Among the
7266 Marshallese people screened in
this study, 2273 persons were alive in
March 1954 and were residing on one of
the 14 study atolls; they were, therefore, potentially exposed to the shortlived radioiodines. Since only these people wereat risk for radioiodine-induced
thyroid neoplasia,it is this group of 2273
persons that makes up the sample in
this study.

During the course of this study, all
residents (age 5 years and older) of each
island selected for screening were invited and encouragedto receive thyroid
examinations. Extensive discussions
with traditional leaders of each atoll
were conducted prior to each trip to
ensure maximal communication to residents of each island. One to two weeks
were spent on each atoll performing the
screening examinations. The population
of each atoll at the timeof screening was
estimated from the 1980 Marshall Islands census data.®
To offset the effect of self-selection by
islanders of each atoll population, we
attempted to screen the entire population of 13 primary atolls. Since migration out of the country is rare, the
primary problem was capturing those
members of the population, especially
the population of 1954, who had moved
to either of the two population centers,
Majuro or Kwajalein. Screening programs were therefore conducted on Majuro and Kwajalein for thoseindividuals

and their families who had lived on any

of the 18 northern or southern atolls
in 1954. However, since we screened

nearly a third of the population of Kwajalein Atoll for thyroid nodules, we also
included Kwajalein as a primary atoll,
making a total of 14 study atolls.
Exposure Criteria
Since the short-lived radioiodines
(#1, I, ™I, ™I) all have half-lives of
less than eight days, the bulk of the
radioiodine exposure from the BRAVO
event occurred during the month of
March 1954. Therefore, the most important historical information concerning
the radioiodine dose was the location of
residence in March 1954. Because most
individuals can provide vivid descrip-

Table 1.—Distance and Direction of Study Atolls
From Bikini Atoll


6, *t

Mejit tslandt
*Distance from atoll ta BRAVO test site on Bikini Atoll
in Statute miles.

tAngle of atoll from 0° latitude line drawn through

Bikini Atoll.

tMejit Island is classified as an atoll for the purposes
of this study.

tions of what they were doing during the
dramatic BRAVOtest, the question was
posed in the following manner: “Where
were you living when the ‘bomb’ caused
the Rongelap and Utrik people to be
moved from their homeland?” Individuals born after March 1, 1954, but before

_ Dec 31, 1954, were classified as in utero
at the timeof the blast, and their atoll of
residence in 1954 wasclassified according to their atoll of birth. Because of the
relative proximity of all the islands
within each atoll and the long distance
between any atoll and the blast site,
all individuals from different islands
within an atoll were classified by the
atoll name for the purposes of 1954
residence status.
Since the people live on small land
masses, the atolls represent discrete

points in the vast ocean area of the
Marshall Islands. The distance from
each atoll to the site of the 1954 BRAVO
test (Bikini Atoll) was therefore selected as a proxy for the radioiodine
dose received in 1954.
A second variable was developed to
better characterize the exposure status
of the Marshallese people. A directional

variable, 8, was selected as a proxy for

meteorologic conditions, such as wind
and precipitation, that may have influenced the distribution of the fallout
cloud. We defined @ as the angle in

degrees, measured clockwise, of each of

the 14 atolls from a 0°latitude line drawn
through Bikini Atoll, using Bikini as the
vertex. Table 1 shows the distance of
each atoll from the BRAVOtestsite as
well as the angle 6 of each atoll from an
east-west line drawn through Bikini.
Diagnostic Criteria
We defined a thyroid nodule as one
that was palpable, discrete, and estimated to be 1.0 cm or greater. Findings
of indiscrete or uncertain lesions and
Thyroid Neoplasia—Hamilton etal

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