Nov. 20, 1975
In the population of Utirik (158 persons, all ages) followed for 21
years, the accumulated person-years of observation are estimated at 3140.
In such & group, expection of thyroid cancer would be as follows:
(1)
Based on an average for all registries listed in "Cancer Incidence
on Five Continents" (2.5/100,000 per year):
-08 cases, upper 95% confidence limit .66
(2)
Based on incidence rates in the Marshall Islands, (1952-1962),
excluding cases in exposed individuals (2.0/100,000 per year):
-06 cases, upper 95% confidence limit
(3)
.54
Based on estimates of the risk per rad thyroid dose per year
observed in Rongelap and Ailingnae combined (3.9/1,000,000/
rad/year, thyroid dose 50 rads, 157,000 person-rad-years):
-61 cases, upper 95% confidence limit 2.17
Conclusions:
(1)
The occurrence of even 1 case of thyroid cancer in Utirik is
unlikely to be due to chance at conventional levels of statis-~
tical significance - if the rates do not exceed normal population rates. The occurrence of 2 cases by chance is extremely
unlikely.
(2)
The observed number of thyroid cancers is higher than would be
expected on the basis of experience in the exposed populations
of Rongelap and Ailingnae, but does not exceed the upper 95%
confidence limit of the expected value.
Caveats:
(1)
(2)
In computing expected values, no account has been taken of age
and sex differences between populations. The incorporation of
adjustments for these factors is unlikely to change the above
conclusions.
The above calculations have ignored the possibility that some
unknown proportion of person-years of observation are irrelevant
becuase of the latent period between exposure and detection of
cancer.
Subtraction of these person years would increase the
difference between observed and expected values derived from
non~exposed populations.
(3)
It should be borne in mind that the Marshallese have been more
carefully observed for thyroid disease than the general populations from which expected values have been derived.
(4)
It is curious that the Utirik population evidences no increase
in benign thyroid lesions relative to the unexposed population
of Rongelap.
-
Dr. George B. Hutchison
-
Dr. Brian MacMahon
DOE ARCHIVES