CT-scan radiation exposure may be higher than earlier reported

The use of CT scans has increased dramatically, and recently published studies indicate that the amount of radiation exposure may be greater than earlier reported, which may result in an increase in the number of radiation-related cancer cases and cancer-related mortality.

Two studies, published in the Archives of Internal Medicine, attempted to quantify the effect of CT on cancer incidence and mortality.

“Every day, more than 19,500 CT scans are performed in the United States, subjecting each patient to the equivalent of 30 to 442 chest radiographs per scan,” Rita Redberg, MD, a professor of medicine and director of the women’s cardiovascular services, at the University of California, San Francisco (UCSF) Medical Center, wrote in an accompanying editorial. “What is becoming clear [is] that the large doses of radiation from such scans will translate, statistically, into additional cancers,” she wrote.

Radiation doses

The first study, conducted by Rebecca Smith-Bindman, MD, of UCSF, and colleagues, was a retrospective cross-sectional investigation examining the radiation doses used with the 11 most common types of diagnostic CT scans. They examined data for 1,119 consecutive adults at four centers in the San Francisco Bay area between January and May 2008.

They found a mean 13-fold variation between the highest radiation dose and the lowest for each type of CT scan. Although the radiation exposure in some procedures was as low as 2mSv, the dose increased to 31mSv for a multiphase abdomen and pelvis CT scan.

“Even the median doses are four times higher than they are supposed to be, according to the currently quoted radiation dose for these tests,” Redberg wrote in her editorial. “Just one CT coronary angiogram, on average, delivers the equivalent of 309 chest radiographs. By their calculations, one in every 270 40-year-old women undergoing CT coronary angiogram will develop cancer from the procedure.” This is compared to one in every 8,100 women of the same age who had a routine head CT scan.

Radiation-related cancers

In the second study, Amy Berrington de González, PhD, an adjunct assistant professor at Johns Hopkins Bloomberg School of Public Health, and colleagues constructed a risk model to estimate age-specific cancer risk for each CT scan type. They derived data from the National Research Council’s report, “Biological Effects of Ionizing Radiation,” national surveys and insurance claims.

They found that approximately 29,000 future cases of cancer could be due to CT scans that took place in 2007. Abdomen, pelvis, chest, head and CT angiography were the largest contributors to this risk.

About 35% of these cancers were estimated to be from CT scans performed on people when they were 35 to 54 years-old. Lung cancer was estimated to be the most common radiation-related cancer, followed by colon cancer and leukemia.

“Presumably, as the number of CT scans increases from the 2007 rate, the number of excess cancers also will increase,” Redberg wrote. “In light of these data, physicians and their patients cannot be complacent about the hazards of radiation or we risk creating a public health time bomb.”

References:
Berrington de González A, Mahesh M, Kim KP, et al. Arch Intern Med. 2009;169 (22): 2071-2077.
Redberg R. Arch Intern Med. 2009;169 (22):2049-2050.
Smith-Bindman R, Lipson J, Marcus R, et al. Arch Intern Med. 2009;169 (22): 2078-2086.