III.
A.
THYROID NODULES AND THYROID CANCER RISK FROM FALLOUT
Tabulation of Thyroid Effects and Age at Exposure
Medical records were reviewed by Adams of the Marshall Islands Medical Program for age at exposure and thyroid nodule data. Conard had performed a similar tabulation (Co80). Memoranda between the evacuation teams and their supe-
riors were reviewed for age-at~exposure data (0C68).
Data gathered by the
Marshall Islands Radiological Safety Program related to age at the time of
whole-body counting measurements were reviewed. When discrepancies between the
three sources of age data became apparent (about April 1982) Medical Program re-
cords were assumed correct.
Differences, between age groupings reported in
Table 1 of the Conard report (Co80) and Tables 24 or 25 as reported here, exist
because of the age-at-exposure discrepancies.
Two young Rongelap males with severe growth retardation and gradually
developed atrophy of the thyroid gland were not included in Tables 24 and 25 because thyroid nodules would not develop in these individuals. Surgical examination of nodules detected by palpation in the field revealed four persons without
nodules at the time of surgery (see Table 24).
In addition, two persons too old
to undergo surgery were included among those having nodules.
B.
Tabulation of Risk of Thyroid Cancer
The unexposed population incidence rate of thyroid cancer was higher at
Rongelap and Utirik than for other sea-level populations, 2 cancers per hundredthousand person-years observed at Singapore (Le82) versus 59 cancers per hundredthousand person-years observed in the Marshall Islands comparison group. The
comparison group makeup has been described previously by Conard (Co80). Thyroidcancer in the United States has been diagnosed at the rate of 2.5 per hundred-
thousand person-years observed (De75).
Our estimated value for thyroid cancer risk from fallout for the exposed
population is 150 cancers per million person-gray-years at risk (1.5 cancers per
million person-rad-years). This estimate was based on the Marshall Islands’ comparison group incidence, thyroid doses estimated here, and thyroid observations
in the exposed groups at Rongelap, Utirik, and Sifo Islands (see Table 25). It
could be assumed that the larger studies of cancer gave a more accurate reflection of the spontaneous incidence rate of thyroid cancer at Rongelap and Utirik
than the comparison group statistics. For a population of 251 people examined
over 29 years, about 0.15 thyroid cancers would be expected on the basis of the
rate given at Singapore (Le82). This value when incorporated into the estimate
of cancer risk from radiation exposure leads to a Rongelap-Utirik-Sifo population value of 210 thyroid cancers per million person-gray-years at risk (2.1 thyroid cancers per million person~rad-years at risk), 40% higher than the value we
estimated using Marshallese comparison statistics.
C.
Comparison to Other Estimates of Thyroid Cancer Risk
The value for the radiation-induced. incidence of thyroid cancer was 147
per million person-gray-years (1.47 cancers per million person-rad-years) for
= 62
-