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Radiation Risk May Not Decrease With Age

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The older you get, the less you have to worry that any new exposure to radiation might eventually trigger cancer, right?

Not so fast, says a new study published online last week in the Journal of the National Cancer Institute. “Overall,” the researchers write, “the weight of the epidemiological evidence suggests that for adult exposures, radiation risks do not generally decrease with increasing age at exposure.”

David J. Brenner, PhD, director of the Center for Radiological Research at Columbia University in New York, and colleagues took another look at the famous long-term studies of atomic bomb survivors in Japan. They noticed that some of the data were inconsistent with the assumption that the relative risks of radiation exposure decrease the older the patient’s age at exposure.

In trying to figure out why, the researchers assumed two different pathways by which radiation exposure leads to cancer. One is gene mutations that convert normal stem cells into premalignant cells that eventually become cancerous. The other is promotion into cancer of premalignant cells already existing in the body. Radiation exposure in middle age is more likely to trigger cancer via that second pathway, the researchers theorized.

They developed a model based on those effects and applied it to the Japanese atomic-bomb survivor data. Sure enough, the model reproduced the cancer risk patterns among the atomic bomb survivors—and among the U.S. population from about 30 to 60 in age.

The model helps explain why risks of middle-age radiation exposure vary depending on the type of cancer. For example, the relative contribution of the initiation effect versus promotion of preexisting premalignant cells is 10 times greater for breast cancer than for lung cancer. Radiation-induced cancer risks for breast cancer decrease with age, but radiation-induced cancer risks for lung cancer do not.

The researchers note that their findings could have particular implications for certain diagnostic X-ray tests commonly performed on middle-aged adults, and for occupations that involve radiation exposure during middle age.

Related seminar: Radiology Review

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