CT scans can help doctors find and treat disease, but their relatively large radiation dose has caused some health experts to worry that the growing use of the modality will lead to tens of thousands of new cancers.

But the cancer risk from CT scans might be several orders of magnitude less than previously estimated, according to a study presented Wednesday at the Radiological Society of North America's 2010 annual meeting.

Researchers analyzing more than 10 million Medicare records over a seven-year period estimate the radiation-induced cancer risk from the scans to be less than one in 2,500. Earlier estimates placed the risk range around one out of 50 to one out of 60, the researchers said.
However, the cancer risk, though low, doubled over the period studied as the use of CT scans increased.

"We were surprised," study co-author Dr. Pat A. Basu told reporters during a press conference Wednesday. "We expected the numbers to be much higher."

Basu, the course director of health policy, finance and economics at Stanford University, was recently the first radiologist to be appointed a White House Fellow.

In the study, the researchers examined 5.3 million Medicare records from 1998 to 2001, and 5.6 million from 2002 to 2005 - these records amounted to about one-fifth of all Medicare enrollees.

The researchers found that roughly half of enrollees had CT scans, around 42 percent in the earlier period and 49 percent in the later one. Of the patients getting CT scans, those of the head were the most common - between a quarter and a third of scans were of the head - although abdominal CT accounted for most of the radiation exposure, nearly 40 percent in both groups.

However, only 6 percent or fewer received "significant" radiation doses in the low range, 50 mSv - 100 mSv, or in the high range, 100 mSv or greater. To put it in perspective, Basu said that the cosmic radiation exposure from 10 roundtrip cross-country flights equaled about 1 mSv. (And a cosmonaut should expect to receive 200 mSv from a year in space.)

Using cancer prediction models based on radiation exposure, the researchers estimate the cancer rates from the CT scans were only 0.02 percent for 1998-2001 and 0.04 percent from 2002-2005. That's about 949 cancers in the first group, and 2,079 in the second, Basu said.

In the Archives of Internal Medicine last year researchers predicted that the CT scans performed in 2007 alone could lead to 29,000 new cancers, and ultimately 15,000 deaths.

Basu said his study had several limitations. For one, the researchers weren't able to assess the frequency of the scans. This is important, Basu said, because acute radiation exposure is worse than protracted exposures. That is, 10 CT scans in 10 hours have a greater cancer risk than 10 scans spaced out over 10 years.

Also, the researchers couldn't figure in radiation exposure given to children or adults under 20 years of age, when the cancer risks are greater.

Basu and his colleagues think radiologists should still closely monitor CT utilization and ensure patients understand the risks and benefits.

"Having educated patients is the best thing for everybody," Basu said.

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