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RE: Damage stays from low-dose X-rays
New Scientist reports
Routine X-ray safety called into question
22:00 31 March 03 NewScientist.com news service
http://www.newscientist.com/news/news.jsp?id=ns99993568
The safety of routine X-rays has been called into question following
the unexpected discovery that cells exposed to low doses avoid or
delay repairing damaged DNA.
Puzzlingly, cells given higher doses of X-rays were faster and more
efficient at patching up any damage. But the German researchers who
made the discovery say it is not clear whether the sloppy repairs
that follow low level exposure is a good or bad thing.
Kai Rothkamm and Markus Löbrich, at the University of Saarland in
Homburg, acknowledge that unrepaired breaks in DNA could well lead to
cells becoming cancerous. But it is equally possible, they say, that
the failure to repair low-level DNA damage has evolved as a safety
measure.
"Repairing a DNA break is dangerous, as the repair could itself be
faulty," says Rothkamm. "If, say, only one in ten cells has received
DNA damage, it may be safer just to let it die and be replaced."
It makes sense that cells must repair themselves after exposure to
higher doses, because more cells are damaged. "Tissue can't eliminate
every cell, so repair becomes a necessity," he says.
The discovery was made by exposing human lung cells to a range of X-
ray doses. The lowest dose of 1.2 milligrays was typical of routine
medical and dental X-rays. The highest - 200 milligrays - is 10 times
higher than the doses received by patients undergoing computer
tomography scans.
The variation in DNA repair efficiency was totally unexpected. High
dose cells repaired 95 per cent of their damage within a day. At
doses of 5.0 milligrays, the DNA repaired fell to 80 per cent. But
cells exposed to the lowest doses avoided repairs altogether, or
delayed it for days.
Rothkamm suggests there may be a threshold level below which damage
is ignored. Chemical signals within or between cells could determine
when damage has reached such a level that repairs are essential. But
more research is needed, he says.
Michael Clark, scientific spokesman for the UK's National
Radiological Protection Board, agrees. "This needs to be taken
further, to see if there are implications in vivo," he says. "But we
don't feel it has any immediate effect on our risk estimates for X-
rays. People should not stop having them."
Journal reference: Proceedings of the National Academy of Sciences
(doi/10.1073/pnas.0830918100)
Andy Coghlan
========================
fwp_dawson@hotmail.com
-----Original Message-----
From: owner-radsafe@list.vanderbilt.edu
[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of Jim Hoerner
Sent: 02 April 2003 01:12
To: Know_Nukes@yahoogroups.com
Cc: downwinders@yahoogroups.com; radsafe@list.vanderbilt.edu
Subject: Damage stays from low-dose X-rays
Well, it is April 1st, but I am not making this up...
Study: Damage stays from low-dose X-rays
Tuesday, April 1, 2003 Posted: 10:49 AM EST (1549 GMT)
WASHINGTON (Reuters) -- Low doses of X-rays such as those patients receive
in the dentist's chair may do more long-lasting damage than higher doses,
German scientists reported on Monday in a study that turns common wisdom on
its head.
Their findings, based on experiment with cell cultures, will have to be
duplicated by other labs and then repeated in living animals before doctors
can offer guidance on the effects of low-dose X-rays on humans.
The team, led by Markus Lobrich at the Universitat des Saarlandes, said its
reasearch suggests that doses of X-rays generally considered harmless may in
fact do long-lasting damage.
But they said they had developed a test that would help doctors look for
genetic damage in people exposed to low doses of X-rays, such as cancer
patients undergoing radiotherapy, patients getting X-rays and professionals
working with X-ray equipment.
Writing in the Proceedings of the National Academy of Sciences, Lobrich's
team said they exposed human cell cultures to varying X-ray doses in the
laboratory.
To their surprise, they found that damage from low radiation levels lingered
days to weeks longer than damage caused by more powerful levels.
Ionizing radiation like the kind produced by X-rays and some nuclear
breakdown products can cause leukemia and other cancers. The radiation can
cause breaks in DNA that go across both strands of its double helix
structure.
Scientists had assumed that the body moves to repair these breaks at the
same rate, no matter what the dose of radiation.
But Lobrich's team found this may not be true. It could be, they propose,
that the body simply does not recognize lower levels of damage and does not
move to repair it.
When these damaged cells divide and multiply, the unrepaired damage
multiplies along with them, they suggested.
Copyright 2003 Reuters. All rights reserved. This material may not be
published, broadcast, rewritten, or redistributed.
http://www.cnn.com/2003/HEALTH/04/01/health.xrays.reut/index.html
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