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That finding, scheduled to be presented Wednesday at the annual meeting of the Radiological Society of North America in Chicago, is based on an eight-year analysis of Medicare records covering nearly 11 million patients.
"What we found is that overall between two and four out of every 10,000 patients who undergo a CT scan are at risk for developing secondary cancers as a result of that radiation exposure," said Aabed Meer, an M.D. candidate in the department of radiology at Stanford University in Palo Alto, Calif.
"And that risk, I would say, is lower than we expected it to be," said Meer.
As a result, patients who need a CT scan should not be fearful of the consequences, Meer stated.
"If you have a stroke and need a CT scan of the head, the benefits of that scan at that moment outweigh the very minor possibility of developing a cancer as a result of the scan itself," he explained. "CT scans do amazing things in terms of diagnosis. Yes, there is some radiation risk. But that small risk should always be put in context."
The authors set out to quantify that risk by sifting through the medical records of elderly patients covered by Medicare between 1998 and 2005.
The researchers separated the data into two periods: 1998 to 2001 and 2002 to 2005. In the earlier period, 42% of the patients had undergone CT scans. For the period 2002 to 2005, that figure rose to 49%, which was not surprising given the increasing use of scans in U.S. medical care.
Within each group, the research team reviewed the number and type of CT scans administered to see how many patients received low-dose radiation (50 to 100 millisieverts) and how many got high-dose radiation (more than 100 millisieverts). They then estimated how many cancers were induced using standard cancer risk models.
Yet despite the upward trend in the overall use of CT scans, with an apparent doubling of both low- and high-dose radiation exposure within the two time frames, the researchers determined that there was a "significantly lower risk of developing cancer from CT than previous estimates."
Cancers associated with radiation exposure were estimated to be 0.02% of the first group and 0.04% of the second.
Previous estimates ranged from 1.5% to 2%, said the authors.
While the results are good news, the consequences of CT scans should continue to be monitored, the authors concluded.
Dr. Robert Zimmerman, executive vice chair of radiology at Weill Cornell Medical Center in New York City, said that assessing CT scan risk is a tricky endeavor. He believes patient needs should be assessed on a case-by-case basis so as to limit exposure as much as possible.
"It doesn't surprise me that the secondary cancer risk is low," he said. "But it's a very complicated epidemiological notion to deal with. Does every amount of cancer radiation exposure increase your risk, or is there a level of exposure that your body can always tolerate and recover from? It's very, very hard to say," Zimmerman pointed out.
"For better or worse we are now conducting an experiment on the entire population of the U.S. as to whether or not low-dose radiation exposure is going to raise risk of developing cancer," he said.
Reducing radiation doses across the board should be the goal, regardless of the study's finding, he noted. "We always want to make sure that the dose used when scanning is as low as possible, and that scanning only takes place when necessary and beneficial to the patient," he said.
Because this study was presented at a medical meeting, the findings should be viewed as preliminary until they are published in a peer-reviewed journal.
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