Even low doses of radiation may cause damage to genes, German researchers have discovered, questioning the safety of routine X-rays and current models of risk assessment.

Dr Kai Rothkamm and Dr Markus Lobrich of Saarland University in Germany, exposed cell cultures to the kind of radiation levels experienced during routine diagnostic X-rays, and analysed their impact on DNA, the nucleic acid at the heart of cells that carries all our genetic information.

They were surprised to find the X-rays produced damage to cellular DNA which took considerably longer to repair than damage from higher radiation doses. Their results appear in today's issue of the journal, Proceedings of the National Academy of Sciences.

"It's a good paper," said Dr Pam Sykes, a respected medical radiation specialist at Flinders Medical Centre in Adelaide. "The group have been able to measure at a 100-fold lower level than previously achieved."

In the past, research has measured the effects of high X-ray doses, and extrapolated these results to the low-dose rates that are more relevant to public health concerns. But in this latest study, the scientists looked at the lower radiation levels directly.

"The unit the paper is referring to is the equivalent of about 20 chest X-rays, or one spinal X-ray," Sykes told ABC Science Online. "A dental X-ray would be less again."

The researchers used a novel fluorescent marker to count the total number of double-strand breaks in DNA per cell at different times after exposure; this type of damage is usually linked to cancer and hereditary disease.

The researchers were working with cell cultures, not people, but their fluorescent marker has the potential to become a diagnostic tool, she said. "In the future, this new research may lead to the ability for the doctor to take blood and see what damage is being done," predicted Sykes. "It is providing a system where we can start monitoring people."

The test could be used for patients having to undergo a series of X-rays, or for people who have been exposed to high levels of environmental radiation, such as in the case of the 1986 nuclear reactor accident at Chernobyl in the Ukraine, she suggested.

The paper does not indicate that X-rays should not be used, she said. "If you are having a spinal X-ray it is for an important reason. You must weigh the risk of the damage against the outcome of the X-ray."

Sykes noted that the researchers administered the radiation at a much higher rate than would be given in a standard X-ray. As a comparison, the background radiation an average person would receive over a year would be about 2 milligrays (mGy) - a unit of measurement for absorbed radiation. "In the experiment, the [cell] culture was being exposed to 1 mGy over a few minutes," she said.

Repair slower

However, the researchers noted that the damage from low radiation levels lingered days to weeks longer than damage induced by more powerful levels. They observed that when the cell cultures multiplied, the numbers of double-strand breaks decreased.

This might be because instead of repairing the damage, the body simply gets rid of these damaged cells with induced cell death, the duo suggested. In high doses of radiation, more cells b damaged - too many to kill off - so the body repairs the damage instead.

The new finding challenges current models of risk assessment, which assume that DNA repair is equally efficient regardless of X-ray dose. The authors propose that there is a threshold level of damage, above which repair mechanisms operate efficiently, but below which repair of double-strand breaks is impaired.

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