"There are no studies at present, that we know of, showing that medical imaging or radiation from those scans cause cancer," Farley adds.

While in the study the doctors acknowledge no evidence of a direct link between cancer and CT scans, they say that radiation's cancer effects are well studied, not just in Japanese bomb survivors, but in nuclear power plant workers and patients who receive multiple medical imaging exams. Because of this, they say there's enough evidence to suggest that 5-10 rads, the radiation dose exposure of a patient who receives several medical imaging scans, is enough to trigger cancer.


What's more, Dr. Berrington and colleagues note that a paper released this year in the Journal of Radiological Protection suggests that because of how they affect biological tissue, at least in the lab, low-dose X-rays might actually cause more DNA damage than gamma rays, the principal radioactive element of a nuclear bomb explosion.

Study design

But that wasn't ACR's only concern with the papers. Another caveat is that Dr. Berrington's study might exaggerate future CT-cancer cases because of a flaw in its design.

Though the researchers excluded from their data patients who died within 5 years of getting a CT scan (as it is believed to take at least 5 years for a cancer to develop from the radiation exposure), ACR says the study still assumes patients getting CT scans have comparable life spans to those who don't get them.

"The estimate of 29,000 is going to be high, because even though they threw out information from patients who had cancer within 5 years of the end of life, which is a good thing, they continue to go on the assumption to that someone getting CT scans has the same life expectancy [as someone who doesn't]," says Farley. "Whereas people had CT scans ... for [an] underlying reason, and were in a higher-risk group."

This is a concern because, presumably, many of these people with a shorter life span would die before ever developing cancer (medical imaging related or not).

In their paper, the researchers do acknowledge that death rates higher than those assumed in the study could cause a 5 to 20 percent drop in cancer-risk projections.

However, they also say that another design feature of the study could actually underestimate cancer risk.

In the study, they excluded people receiving CT scans who had an associated Medicare or insurance code for cancer diagnosis, as a pre-CT cancer would obviously distort any post-CT results. Nonetheless, they say it's possible no cancers turned up in their CT scan, and that the scan itself could have created cancers.

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