Cells exposed to low doses of X-rays may delay or completely avoid repairing damaged DNA, raising questions about the safety of routine X-rays.

Surprisingly, cells exposed to higher doses of X-rays repaired damage at a faster rate. The study’s results were unexpected, but, according to researchers, it is not known whether the reduced repair capacity after low-dose X-rays is a good thing or a bad thing.

While unrepaired breaks in DNA can result in cells becoming cancerous, faulty repairs are also problematic. It may be that allowing a cell to die and be replaced is safer than repairing the cell, researchers said.

This is in-line with the finding that cells exposed to higher doses repair themselves faster; since more cells are damaged by the higher doses, repair becomes a necessity.

In the study, researchers exposed human lung cells to varying X-ray doses. Doses ranged from levels typically received from routine medical and dental X-rays (1.2 milligrays) to levels 10 times higher than are received by patients undergoing CT scans (200 milligrays).

Cells that received high doses repaired 95 percent of their damage within a day. When cells were exposed to doses of 5.0 milligrays, the DNA repaired fell to 80 percent. However, cells exposed to the lowest doses either delayed repair for days or avoided repairs entirely.

Researchers suggest that chemical signals within or between cells could determine whether repairs are essential or can be ignored. They note that while further research is needed to explain these findings, there doesn’t appear to be an immediate effect on X-ray risk estimates and people should not stop having them.

Journal Reference: Proceedings of the National Academy of Sciences April 4, 2003 (doi/10.1073/pnas.0830918100)