year of observation for the purpose of this
calculation because thyroid surgery for the
Comparison group was not offered after 1985.
*In this calculation, to obtain the number of
person-years of observation the individual agesat
the time of the most recent examination were
summed, with the exclusion of all years
subsequent to thyroid nodule surgery in those
cases where it was performed. To use an extreme
example, if a 60 year-old person wasfirst enrolled
in the Comparison group and examined in 1975
and subsequently never appeared for
reexamination, 60 person-years of observation was
calculated. The justification for this approach is
that it is considered unlikely that any clinically
apparent thyroid nodule will spontaneously
disappear. In a recent follow-up study of children
among whom some nodules had been detected
approximately fifteen years previously, only 10
percent of the nodules were no longer palpable
(Rallison et al., 1991).
Factors influencing data interpretation:
It is possible to draw tentative conclusions
relevantto issues of radiation injury to the thyroid
from data available on the exposed populations
alone, thereby avoiding assumptions about the
adequacy of a control group. The data underlying
the following analyses are shown in Appendix D
and grouped and tabulated in Table 3. However,
interpretation of the Marshallese thyroid nodule
data must be done cautiously because of the small
number of observations that were possible. This
is particularly true when the nodules are
subgrouped and analyzed by histologic type. In
addition, thyroid disease is greatly influenced by
gender, thereby further decreasing samplesize for
some analyses, particularly in males. Another
confounding factor, thyroxine suppression, was
initiated in 1965 in an attempt to inhibit or
prevent the growth of benign and malignant
thyroid nodules. This was prescribed only for the
exposed Rongelap population, for the risk of
nodule development resulting from the much
lower Utirik exposure was felt to be small at that
time. It therefore becomes difficult in some
instances to interpret results in which Rongelap
and Utirik data are grouped together. Finally, just
why there were no nodules detected during the
first nine years of medical team visits (1955-1963)
is not clear. Based on the estimgte of nodule
incidence of Maxonet al. (1977) two hiodules would
have been expected to develop in
group by 1963. It is possible, there
absence of nodulesin the early years was merely a
consequence of random distribution pf a relatively
uncommonabnormality, particularly ance the mean
age of the Rongelap people at the tinge of exposure
was rather low, 27.6 years. Once thd first nodules
were detected in 1963 it became thej procedure of
the Marshall Islands Medical Prograrg to include in
its medical team a person highly skilled in thyroid
examination, usually an endocrinologit with special
expertise in thyroid disease or a thyroid surgeon.
This change in procedure introduces q possible bias
that is impossible to quantitate.
It is possible that some nodules ig the exposed
population were naturally occurring rather than
radiation-induced. However, as ther@ is no way to
identify which nodules these were, n@ attempt has
been madeto correct for their presegce.
Issues that can be addressed without Invoking data
from the Comparison group:
1) Is the “epidemic” of thyroid nodulds over?
The number of patients undergoing thyroid
surgery by year over the duration offthe Marshall
Islands Medical Program is shown infFig. 3a. The
same data are shown in Fig, 3b exc@pt that cases
are expressed as percent of the pqpulation that
ntible to new nodule férmation;1.e.,
excluding persgns with prior noduleg and persons
who had died prior to the year for which a percent
was calculated. Clearly the incidenge of nodules
which began in the mid-1960’s inthe exposed
Rongelap group has greatly, if nof completely,
subsided.
Their detection spannad 22 years,
beginning 9 years after exposure. Tle nodules in
the Utirik group, on the other hand had later
onset and a later apparent decline. [Detection of
Utirik nodules has spanned 19 years,{beginning 15
years after exposure.
In the following discussion the [total thyroid
absorbed dose in Figures 4-7 represents the acute
radiation dose to the thyroid occuring during
exposure to fallout-prior to evacuatiog and time to
developmentof nodules (years post exgosure) refers
to the interval in years from exposurejto the initial
clinical detection of the nodule.
Possibly more appropriate questiogs to ask are,
(1) is the epidemic of adenomatous (npnneoplastic)
nodules over, and (2) is the epidemic pf neoplastic
17