Cancer risk from CTs not as high as previously thought, study finds

Patients’ risk of developing cancer from CT scans is not as high as previously thought, but the rate still doubled over the time period studied, according to a study presented Wednesday at the RSNA meeting. 

Many studies have examined cancer risk from radiation, usually drawing on data from people exposed to high amounts of it, such as from the atomic bomb or radiotherapy. The current study, by Dr. Pat Basu, a radiologist at Stanford University and a White House fellow, instead looks at more than 10 million Medicare claims from 1998 through 2005.

Basu and colleagues analyzed the distribution of CT scans, determined the ionizing radiation exposure associated with the exams, and estimated the associated risk in a population of older adults.

Previous estimates of radiation-induced cancer rates were between 1.5% and 2%, but the current study shows they are much lower than that. The researchers found the rate from 1998 to 2001 was 0.02%, while from 2002 to 2005 it was 0.04%.

“When we calculated this, we were surprised,” Basu said. “We expected the number to be higher, especially since in the last five years or so the number floating around was 2%.”

They found 42% of all patients underwent a CT scan between 1998 and 2001. The number jumped to 50% between 2002 and 2005.

The researchers also stratified CT dose by procedure. While CT scans of the head were most common (25% in the 1998 to 2001 bracket and 30% in the 2002 to 2005 bracket), abdominal CTs delivered the most radiation at 8 mSv. Abdominal CTs accounted for approximately 40% of the total radiation exposure.

Some limitations of the study are that it does not address radiation exposure in patients younger than 20, who are most at risk for developing cancer from radiation. There is also no way to assess the frequency of the scans, Basu said. The reason for the scans and the patients’ clinical status were also unavailable.

The risk estimate may have changed since 2005 considering efforts revolving around dose modulation and campaigns to avoid unnecessary radiation exposure have ramped up, Basu said. Therefore, more research is needed to adequately assess the current risk.