CT scans: Balancing expected benefit against potential for harm

In the first of the 2 studies, Rebecca Smith-Bindman, MD, a professor in residence of radiology, University of California, San Francisco, and colleagues conducted a retrospective cross-sectional study with the goal of estimating future cancer risks from current CT scan use. They assessed the radiation dose associated with the 11 most common types of diagnostic CT studies that were conducted on 1119 consecutive adult patients at 4 facilities in California between January 1 and May 30, 2008. These data were then used estimate the lifetime attributable risk for cancer associated with these imaging scans.

"It is important to understand how much radiation medical imaging delivers, so this potential for harm can be balanced against the potential for benefit," the authors write. "This is particularly important because the threshold for using CT has declined, and CT is increasingly being used among healthy individuals, in whom the risk of potential carcinogenesis (development of cancer) from CT could outweigh its diagnostic value."

The 11 types of CT scans evaluated in the study comprised approximately 80% of all CT scans performed. The mean patient age was 59 years, and nearly half (48%) were female.

They found that the doses of radiation varied significantly among the different types of CT scans, with the overall median effective doses ranging from 2 mSv for a routine head CT scan to 31 mSv for a multiphase abdomen and pelvis CT scan. The comparison of organ-specific doses showed that CT coronary angiogram delivers a dose to the breast that is equivalent to approximately 15 mammography screenings. It also delivers a radiation dose to the lung that is equivalent to 711 chest x-rays, the authors note.

Effective doses also varied significantly within and across institutions for each type of CT scan. Effective doses tended to be higher and more variable in CT scans of the abdomen and pelvis, and the largest dose range was seen in multiphase abdomen and pelvis CT scanning (range, 6 to 90 mSv).

Greater risk faced by women

The authors estimated lifetime attributable risks for cancer by scan type from these measured doses and, as they expected, the number of CT scans that would result in a cancer varied considerably by sex, age, and type. It would take far fewer CT scans to result in a cancer in women than in men, for example, reflecting a higher cancer risk from radiation.

They estimated that 1 in 270 women who underwent a CT coronary angiogram at the age of 40 years will eventually develop cancer, compared with 1 in 600 men. For a routine head CT, the estimated risk was 1 in 8100 for a 40-year-old woman and 1 in 11,080 for a man of the same age. For 20-year-old patients, these risks were approximately double; for 60-year-old patients, they were approximately 50% lower.

Based on the highest effective dose that was observed, a 20-year-old women who received a CT scan for suspected pulmonary embolism, a CT coronary angiography, or a multiphase abdomen and pelvis CT scan could have an associated increased risk of developing cancer of as high as 1 in 80, note the authors. "The risks declined substantially with age and were lower for men, so radiation-associated cancer risks are of particular concern for the younger, female patient," they write.

Estimated levels of extra cancers attributable to CT scans

In the second study, Amy Berrington de González, DPhil, from the National Cancer Institute, in Bethesda, Maryland, and colleagues conducted a study to determine the estimated risk for future cancer from current CT scan use in the United States according to age, sex, and scan type.

They used risk models based on the National Research Council's "Biological Effects of Ionizing Radiation" report, and organ-specific radiation doses derived from a national survey.

An estimated 72 million CT scans were performed in the United States in 2007. For their calculations, the researchers excluded scans obtained in the last 5 years of life and those with a diagnostic code related to cancer, lowering the number to 57 million.

The number of CT scans performed increased with age at exposure until the age of 45 years, the authors note; it is estimated that 30% of scans are performed in adults 35 to 54 years. In addition, it is estimated that about 60% of the scans were performed in women.

The projected number of incident cancers per 10,000 scans generally decreased with increasing age at exposure, and although the risk varied according to the type of scan, there were consistently high risks for chest or abdomen CT angiography and whole-body CT, they noted.

When age- and sex-specific annual frequencies were combined with the estimated risk per 10,000 scans, the authors estimated that approximately 29,000 (95% uncertainty limits [UL], 15,000 - 45,000) future cancers could be related to the number of CT scans performed in 2007.

When broken down by cancer site, lung cancer was estimated to be the most common projected radiation-related cancer (n = 6200; 95% UL, 2300 -13,000). This was followed by colon cancer (n = 3500; 95% UL, 1000 - 6800) and leukemia (n = 2800; 95% UL, 800 - 4800).

"Changes made to practice now could help to avoid the possibility of reaching the level of attributable risk suggested above," the authors write. "Our detailed estimates highlight several areas of use in which the public health impact may be largest, specifically abdomen and pelvis and chest CT scans in adults aged 35 to 54 years."