[ RadSafe ] Risk of cancer due to radiation exposure in middle age may be higher than previously estimated

[ROY HERREN]

Risk of cancer due to radiation exposure in middle age may be higher than 
previously estimated
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 
middle age.
"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 
society."
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. 
http://www.physorg.com/news/2010-10-cancer-due-exposure-middle-age.html