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Low doses of ionizing radiation may not carry as much cancer risk as we’ve thought, according to researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory in Berkeley, California.

Breast-cancer researcher Mina Bissell, PhD, explained:

Our data show that at lower doses of ionizing radiation, DNA repair mechanisms work better than at higher doses. This nonlinear DNA damage response casts doubt on the general assumption that any amount of ionizing radiation is harmful and additive.

Dr. Bissell, who holds the title of distinguished scientist at the Berkeley Lab’s Life Sciences Division, was quoted in a lab news release. The findings have particular significance for those who undergo such low-dose medical imaging procedures as mammograms.

The lab revealed the results of its research in a freely available study published online Monday in Proceedings of the National Academy of Sciences.

The decreased risk at low doses derives not from the amount of damage caused by the radiation but rather from how the body fixes the damage. The researchers found that after low doses of radiation, the body repairs double-strand DNA breaks (meaning the double helix is completely severed) on site. It’s a relatively simple procedure with relatively little chance of error.

But as doses increase, creating more double-strand breaks, the broken strands congregate at “radiation-induced foci” (RIF), which are aggregations of repair proteins. With lots of repairs going on at once, the opportunities for mistakes increase.

“We hypothesize that, contrary to what has long been thought, double-strand breaks are not static entities but will rapidly cluster into preferred regions of the nucleus we call DNA repair centers as radiation exposure increases,” said Sylvain Costes, PhD, a biophysicist who led the study.

“As a result of this clustering,” Dr. Costes said, “a single RIF may reflect a center where multiple double-strand breaks are rejoined. Such multiple repair activity increases the risks of broken DNA strands being incorrectly rejoined, and that can lead to cancer.”

The researchers are looking into whether their results, achieved with a single human breast cell line, will hold up with different lines and different breast cells. Stay tuned.

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