gt.
30)
Estimates of Thyroid Risk.
Several attempts have been made to predict the risk
of thyroid neoplasia from irradiation.
Beach and Dolphin (1962), using data
from several studies, described the thyrofd response to dose as linear, corresponding to a rate of 1.7% thyrofd malignancies per 500 rads to the thyroid, or
34 mal ignanci es/106 persons/rad.
Since the time at risk was taken to be
20 years, their annual risk for thyroid malignancies can be expressed as about
107/106 persons/rad/year.
Hempelmann (1968), with data fran other studies,
predicted a risk for malignancies of 0 to 5.5 cases/106 persons/rad/year, and
for thyroid nodules of 38-53 cases/106 persons/rad/year.
Dolphin (1968), fron
other data, estimated the risk for thyrofd canceto be 100/106 persons/rad,
or, considering the time at risk to be 20 years, 5 cases/]0® persons/rad/year.
The National Academy of Sciences BEIR Report (NAS, 1972) estimated the risk of
thyroid cancer to be of the order of 1.6 to 9.3 cases/106 persons/rad/year.
Later, the Natfonal Academy of Sciences (NAS, 1980) estimated the risk of thyroid cancer to be approximately 4 cases of thyroid carcinana and J2 cases of
thyrofd adenoma/106/rad/year.
In the Reactor Safety Study (US NRC, 1975),
absolute risks for thyroid malignancies and nodules, based upon a large number
of studies, were estimated to be 4.3 and 8.1 cases/}06 persons/rem/year, respectively, or a total of 12.4 cases of thyroid neoplasia/106 persons/ran/year.
For comparison, these numbers can be compared to an average annual incidence of
thyrotd cancer for both sexes, al] ages and races conbinei, of 3.6 per 100,000
for locations represented fn national surveys (NCI, 1975).
-_
Jerr
a
Since these estimates for thyroid neoplasia are primarily derived fron
external frradfation, estimates for !311 risks should be lower to account for
the differences in effectiveness of x-ray and I31I, as discussed above.
The lfket{hood of hypothyroidism 1s low at low doses.
Macon et al. (1977)
reviewed the medical literature and found that hypothyroicism was renurted