Multiple CT scans can increase risk of cancer

From the doctor's office to the airport, Americans have more opportunity for radiation exposure than ever before.

While low levels of radiation are used in medical imaging and airport body scanners, there is much debate about exposure levels and potential cancer risks. New research from Wake Forest Baptist Medical Center has found that there could be an increased risk of cancer for certain individuals who are exposed to multiple CT scans.

The research, published in the December issue of the journal Radiation Research, was the first to look at how CT scans affected mice specially bred with a lung cancer-susceptibility gene known as Ki-ras. Mutations in this gene have been implicated in about 30 percent of a common form of human lung cancer. In this study, mice that expressed the mutant Ki-ras gene were exposed to low-dose helical computed tomography (CT) radiation. The mice exposed to CT radiation had 43 percent more tumors than unirradiated mice, and females were more susceptible than males.

"We believe we are the first to use an animal model in an actual clinical CT scanner to directly look at the risk of producing tumors after a diagnostic procedure," said lead researchers Michael T. Munley, Ph.D., associate professor of radiation oncology, and Mark S. Miller, Ph.D., professor of cancer biology, at Wake Forest Baptist. "What we found is that there may be an increased risk of tumor formation from CT scans in certain individuals with cancer susceptibility."

Co-author Kenneth T. Wheeler, Ph.D, professor emeritus of radiology, pointed out that the irradiated mice that did not express the mutant Ki-ras gene had no increase in tumors. This suggests that there may be little to no risk associated with low-dose CT radiation if a person does not have a cancer susceptibility gene. But for those patients who have a suspicious lesion on a CT scan or have a known cancer susceptibility gene, "we suggest it may be more appropriate for them to be imaged using a magnetic resonance scanner (MRI) instead of a CT scanner to avoid any possible additional cancer risk," Wheeler said. "This would be the prudent thing to do."

Munley and Miller said they designed their study to mimic the CT parameters of the National Lung Screening Trial (NLST) that compared two ways of detecting early stage lung cancers: low-dose helical CT or a standard chest X-ray. NLST enrolled 53,454 current or former heavy smokers from 33 U.S. sites - including Wake Forest Baptist. Published earlier this year, the NLST study found a 20 percent reduction in deaths from lung cancer among current or former heavy smokers who were screened with CT versus those screened by chest X-ray.

Munley added that other previously published papers have estimated that up to 29,000 cancers per year in the United States may be attributable to CT scans, but said there is much debate about these estimates.

Munley, Miller and Wheeler agree that more research is needed. They are currently working to establish if there is a low threshold dose that is both "safe" and produces a usable diagnostic image. "Our data doesn't show yet that there is a low dose that's okay," Wheeler said.