[ RadSafe ] Risk of cancer due to radiation exposure in middle age may be higher than previously estimated
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Mon Oct 25 23:10:40 CDT 2010
Risk of cancer due to radiation exposure in middle age may be higher than
October 25th, 2010 in Medicine & Health / Cancer
Contrary to common assumptions, the risk of cancer associated with radiation
exposure in middle age may not be lower than the risk associated with exposure
at younger ages, according to a study published online October 25 in the Journal
of the National Cancer Institute.
It is well known that children are more sensitive than adults to the effects of
radiation and that they have a greater risk of developing radiation-induced
cancer than adults. Some data also suggest that, in general, the older a person
is when exposed to radiation, the lower their risk of developing a
radiation-induced cancer. On the other hand, statistical evidence from long-term
studies of atomic bomb survivors in Japan indicates that for radiation exposure
after about age 30, the risk of developing radiation-induced cancer does not
continue to decline.
To explore this issue, David J. Brenner, Ph.D., D.Sc., at Columbia University in
New York, and colleagues reanalyzed the Japanese atomic bomb survivor data
assuming two different pathways through which radiation exposure can ultimately
lead to cancer. The first is initiation of gene mutations that convert normal
stem cells to premalignant cells that could eventually lead to cancer. The
second is radiation induced promotion, or expansion, of the number of existing
premalignant cells in the body. The initiation effect is more likely to play a
role in children than in adults, they reason, because cells initiated at an
early age have a longer time available to expand in number and progress on the
pathway to cancer. The promotion effect, on the other hand, is more likely to be
important for radiation exposures in middle age, because the adult body already
contains larger numbers of premalignant cells.
The researchers developed a model based on these biological effects and applied
the model to the Japanese atomic bomb survivor data. They found that the model
was able to reproduce the cancer risk patterns associated with age at radiation
exposure observed in these survivors. They then applied the same model to
predict cancer risks as a function of age in the U.S. population and found that
the cancer risks predicted by the model were consistent with the data in the age
range from about 30 to 60.
The authors conclude that cancer risk after exposure in middle age may increase
for some tumor types contrary to conventional wisdom. They add that these
findings could have practical implications regarding x-ray diagnostic tests,
which are predominantly performed on middle aged adults, as well as for
occupations that involve radiation exposures, again where most exposures are in
"Overall, the weight of the epidemiological evidence suggests that for adult
exposures, radiation risks do not generally decrease with increasing age at
exposure," they write, "and the mechanistic underpinning described here provides
this conclusion with some biological plausibility."
In an accompanying editorial, John D. Boice, Sc.D., of the International
Epidemiology Institute, Rockville, Md., and Vanderbilt University, Nashville,
notes that there are uncertainties in generalizing the Japanese data to a U.S.
population. He also notes that other data and other models contradict the
results of this study. However, he concludes that this biology-based model
"raises provocative hypotheses and conclusions that, although preliminary, draw
attention to the continued importance of low-dose radiation exposures in our
Provided by Journal of the National Cancer Institute
"Risk of cancer due to radiation exposure in middle age may be higher than
previously estimated." October 25th, 2010.
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