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incidence proportional to dose, they derived a cumulative
lifetime incidence of 35 cancers per million infants
each receiving one rad.
However, they were careful
to point out that their analysis did not exclude
the possibility of a curved dose-response relation.
The data in their figure five can also be interpreted
to suggest a "threshold" at perhaps 50-100 rads,
below which cancers would not be induced.
Perhaps
roughly half of the radiation~induced thyroid cancers
may appear within the first fifteen years (compare
values in Ref. 11 with those in Ref. 12), but this
fraction is uncertain.
There is some experimental evidence that iodine
131 is less effective than X-rays in inducing cancer,
Doniach found that the iodine 131 dose had to be about
ten times greater than an abrupt dose of X-rays to
cause equal effects in adult rats.
However, adult
rats are not infant humans and the relative effectiveness
of iodine 131 vs. X-rays in children is yet to be
established.
Perhaps the iodine 131 exposures in
Utah could provide information on this point.
The dose-response relation is very difficult to
establish at very low doses because the incidence of
effects is so very small.
woome radiobiologists feel
that cancer induction is not proportional to dose, but
that a certain "threshold" level must be exceeded before
any cancers can be induced.
If such a threshold exists
and if the doses fall below this level, no induced
cancers will appear.
cancers will
result
It is entirely possible that no
from the Utah exposures.
Theoretical estinates of the number of radiationinduced thyroid cancers will now be calculated for
each of the following three assumptions:
(A) a linear
dose-response with iodine 131 irradiation equally as
effective as X-irradiation,
and causing a lifetine
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