Radiation damages all cells, which then must repair themselves or die. To date, scientists had thought that cell repair takes the same amount of time no matter how much radiation the cell is exposed to, but a new study discovered that cells exposed to lower doses of radiation -- the amount similar to dental X-rays -- actually take from days to weeks longer to repair than cell damage caused by higher levels of radiation, which usually takes only a few hours to one day.
"The difference in repair between low and high doses of X-rays did indeed surprise us," says study author Markus Lobrich, a professor of radiation biology at the University of Saarland in Homburg, Germany. The results appear in the March 31-April 4 Proceedings of the National Academy of Sciences.
The doses given in most diagnostic tests, like dental X-rays, mammograms, and CT scans, are considered low-dose, while the radiation given to cancer patients is high-dose, Lobrich says.
For example, a high dose of radiation, aimed at killing cancer cells, is between 2 and 5 Gy. (Each Gy is one joule of energy absorbed in a mass of 1 kilogram). In cancer therapy, patients are treated with daily doses of about 2 Gy. In contrast, a CT scan is only about 0.02 Gy, a mammogram about 0.01 Gy and a dental exam usually less than 0.01 Gy.
Lobrich says the discrepancy between the rate of cell repair between low- and high-dose radiation could mean an increase in cancer risk to those who have low-dose radiation, or, conversely, it could be a protective mechanism that actually decreases cancer risk.
When cells repair themselves from radiation, they aren't always returned to their exact, original form, he says. In pasting back the broken parts of a cell, called double-strand breaks (DSB), the cells are sometimes changed into a slightly different cells, which are called mutations. While uncommon, these mutations are associated with an increased risk of cancer.
When a large number of cells in one organ are damaged, as in lung cancer radiation, for instance, the body has no choice but to repair the DSBs, or the organ could fail, Lobrich says.
But with low-dose radiation, he says, only a few cells are damaged in a given organ due to the small amount of radiation. The body could basically decide it's better to let those few cells die and be replaced by the division of existing healthy cells in the body than to repair the DSB's and increase risk for a mutation.
"I find this latter a more intriguing idea," he says. "If you believe in the logic of nature, you would expect the body to behave exactly this way, to refrain from repair to avoid the risk of mutations."
"This study is very significant," says Timothy Jorgensen, an associate professor of radiation medicine at Georgetown School of Medicine in Washington, D.C.
Jorgensen says that risk assessment of the effects of radiation has been made from extrapolating how much damage and repair occurs during high-dose radiation and assuming that cell damage and repair follows the same pattern when radiation doses are lower.
"But this study suggests that the mechanism is different, and changes the way we think about how radiation works," he adds.
The question, Lobrich says, will be the subject of further research.
In the meantime, Lobrich and other doctors say, don't stop having the diagnostic X-rays you need.
"I don't want people to get the idea that X-rays are dangerous, because we are all exposed to low doses of X-rays, and they are therapeutic, helping to prevent illness," says Richard B. Setlow, a senior biophysicist at the Brookhaven National Laboratory. Setlow brought the study to the attention of the National Academy of Sciences, which publishes Proceedings.
Lobrich recommends having medically necessary X-rays, but exercising some caution.
"If there is a hereditary indication for an elevated breast cancer risk, like having a mother and/or grandmother who had breast cancer, have the mammography exams done and don't worry too much about the radiation risk," he says. "But if the radiologist is about to take another, or a third image of your chest because he didn't like the quality of the first picture, stop and ask for a medical indication."
Lobrich adds that dental X-rays carry little risk because radiation is not very likely to cause cancer in that part of the body.
For the study, scientists exposed human cell cultures in the laboratory to X-ray levels similar to those in a standard dental exam. It was the first time a study has been done of cell damage following low doses of radiation, Setlow says. Previously, assessments of cell damage had been estimated based on higher doses of radiation.
Using a fluorescent marker, the scientists noted the DSBs in the DNA of the cells following the radiation and monitored them to see how long it took for the cells to repair themselves after radiation.
"Usually DSBs are repaired within a few hours after radiation insult," Lobrich says, "but we observed that DSBs induced by very low doses remained unrepaired for many days. And even two weeks after the radiation, most of the induced DSBs were still present."