December 17, 2009 — Computed tomography (CT) scans are widely used and are an invaluable tool for medical imaging. However, the possible overuse of CT scans and the variability in radiation doses might subsequently lead to thousands of cases of cancer, according to findings from 2 new studies published in the December 14/28 issue of the Archives of Internal Medicine.
In the first study, researchers found that radiation doses from common CT procedures are higher and more variable than what is typically cited. For example, the authors note that the median effective dose of an abdomen and pelvis CT scan is often cited as 8 to 10 mSv, but they found that the median dose of this type of scan was actually 66% higher, and the median dose of a multiphase CT scan of the abdomen and pelvis was nearly 4 times higher.
The authors also found a considerable range in doses within and across the institutions included in their study, with a mean 13-fold variation between the highest and lowest dose for each CT type studied.
In the second study, researchers estimated future cancer risks from current CT scan use in the United States, and projected that 29,000 future cancers will be directly attributable to CT scans that were performed in 2007. It is expected that the majority of these projected cancers will be caused by scans of the abdomen and pelvis (n = 14,000), chest (n = 4100), and head (n = 4000), and by CT coronary angiography (n = 2700).
More than 19,500 CT scans are performed every day in the United States; these expose each patient to the equivalent of 30 to 442 chest radiographs per scan, notes Rita F. Redberg, MD, MSc, professor of medicine at the University of California, San Francisco School of Medicine and editor of the Archives of Internal Medicine, in an accompanying editorial.
However, there is a question of benefit — whether these scans will lead to "demonstrable benefits through improvements in longevity or quality of life are hotly debated," she writes. "What is becoming clear, however, is that the large doses of radiation from such scans will translate, statistically, into additional cancers."
"We need to do something now, not wait 10 or 20 years to see the effects. It's not like radiation exposure can be undone after we find out that it does cause cancer," Dr. Redberg told Medscape Oncology.
Estimates Too High?
In response to this news about the cancer risk from CT scans, which has been widely reported in the lay media, the American College of Radiology (ACR) has questioned the methodology used in the 2 studies.
In a statement, the ACR acknowledges that the widespread use of imaging exams has resulted in increased radiation exposure, and advises that no imaging exam be performed unless there is a clear medical benefit that outweighs any associated risk. They support the concept of "as low as reasonably achievable," which urges providers to use the minimum level of radiation needed in imaging exams to achieve the necessary results.
However, in their statement, the ACR notes that "no published studies show that radiation from imaging exams causes cancer." They also question how the risk was measured, pointing out that the "conclusions of the authors of the Archives' studies rely largely on data that equate radiation exposure and effects experienced by atomic-bomb survivors in Japan to present-day patients who receive CT scans."
Most CT scans are conducted in controlled settings, which results in limited radiation exposure to a small portion of the body, whereas atomic-bomb survivors experienced instantaneous exposure to their entire body, they write. CT exams also only expose patients to x-rays, whereas survivors of the atomic bomb were exposed not only to x-rays, but also to particulate radiations, neutrons, and other radioactive materials.
Thus, the known biologic effects are very different for these 2 scenarios, they note, and "cancer assumptions based on this paradigm should be considered, but not accepted as medical fact."
The ACR also noted that, after excluding patients with cancer or within 5 years of the end of life, the studies assume that the patients undergoing CT scanning have the same life expectancy as the general population. "This is not accurate, so the estimates are undoubtedly high," they write.
In direct contrast to some of these comments, Dr. Redberg said, in an interview, that "the evidence is very strong as far as the link between radiation exposure and cancer." She supports the evidence presented in these papers, and believes that it makes it clear that more attention needs to paid to radiation exposure from medical CT scanning. "Both studies underwent rigorous peer review and I have confidence in their findings and scientific methods."
All scans are not life-saving, and it would be hard to argue against there being variation in the radiation doses, or that all scans are necessary, Dr. Redberg said.
"Whenever people first question the safety of a standard practice — whether it's driving without seatbelts or x-raying children's feet to assess their correct shoe size — there will be others who say things are just fine as they are," she said. "We felt these data were accurate and significant enough to raise concern in the medical community."
Doses Higher and More Variability
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 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).
Women Face Greater Risk
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.
Thousands of Future Cancers?
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."
The study by Dr. Smith-Bindman and colleagues was funded by the National Institutes of Health (NIH), a National Institute of Biomedical Imaging and BioEngineering training grant, NIH National Cancer Institute grants, and the University of California, San Francisco School of Medicine Bridge Funding Program. The authors have disclosed no relevant financial relationships.
The study by Dr. Berrington de González and colleagues received no outside funding. Study coauthor Mahadevappa Mahesh, MS, PhD, from Johns Hopkins University School of Medicine, in Baltimore, Maryland, reports receiving funding from Siemens Medical Systems. None of the other authors have disclosed any relevant financial relationships.
Arch Intern Med. 2009;169:2049-2050, 2071-2077, 2078-2086.