Thousands of new cancer diagnoses predicted, due to soaring use of CT
(18 December 2009: VIDYYA MEDICAL NEWS SERVICE) -- Possible overuse of CT scans and variability in radiation doses might subsequently lead to thousands of cases of cancer, according to findings from two new studies in the December 14-28, 2009 issue of the Archives of Internal Medicine [1,2].
In the first study, researchers found that radiation doses from common CT procedures are higher and more variable than what is typically quoted and that there is a considerable range in doses within and across institutions, with a mean 13-fold variation between the highest and lowest dose for each CT study type included.
In the second study, researchers projected that 29 000 future cancers will be directly attributable to CT scans that had been performed in the US in 2007, including 4100 caused by scans of the chest and 2700 specifically caused by CT coronary angiography.
According to cardiologist Dr Rita Redberg (University of California, San Francisco), who wrote an editorial accompanying the studies , more than 19 500 CT scans are performed every day in the US, which in turn expose each patient to the equivalent of 30 to 442 chest radiographs per scan.
This is "far more radiation from medical CT scans than has been recognized previously, in amounts projected to cause tens of thousands of excess cancers annually," Redberg writes. "Although a guiding principle in medicine is to ensure that the benefit of a procedure or therapy outweighs the risk, the explosion of CT scans in the past decade has outpaced evidence of their benefit."
Moreover, she notes, highly specific tests tend to lead to further testing, which in turn increases radiation exposure.
"We need to do something now, and not wait 10 or 20 years to see the effects. It's not like the radiation exposure can be undone, after we find out that it does cause cancer," Redberg said during an interview.
Doses higher and more variability
In the first paper, Dr Rebecca Smith-Bindman (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 four facilities in California between January 1 and May 30, 2008. These data were then used estimate the lifetime attributable risk of cancer associated with these imaging scans.
The 11 types of CT scans evaluated in the study constituted 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, ranging from a median effective dose of 2 mSv for a routine head CT scan to 31 mSv for a multiphase abdomen and pelvis CT scan. Effective doses also varied significantly within and across institutions and within each type of CT study and tended to be higher and more variable in CT scans of the abdomen and pelvis.
The authors estimated lifetime attributable risks of cancer by study type from these measured doses, and as they expected, the number of CT scans that would result in a cancer varied considerably by gender, age, and type. For example, they estimated that one in 270 women who underwent a CT coronary angiogram at age 40 years will eventually develop cancer, as compared with one in 600 men. Risks were approximately doubled in patients aged 20, but were approximately 50% lower for 60-year-olds.
"The risks declined substantially with age and were lower for men, so radiation-associated cancer risks are of particular concern for younger, female patients," they write.
Thousands of future cancers?
In the second study, Dr Amy Berrington de González (National Cancer Institute, Bethesda, MD) and colleagues conducted a study to determine the estimated risk of future cancer risks from current CT scan use in the US according to age, gender, and scan type.
After excluding scans performed late in life or in patients with a cancer diagnosis, researchers estimated that 57 million CT scans were performed in the US in 2007. By their calculation, the projected number of incident cancers per 10 000 scans generally decreased with increasing age at exposure, and while 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, they estimated that approximately 29 000 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.
"Changes made to practice now could help to avoid the possibility of reaching the level of attributable risk suggested," 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."
Estimates too high?
In response to the studies, which received wide media coverage, the American College of Radiology (ACR) released a statement asserting: "No published studies show that radiation from imaging exams causes cancer."
The ACR also questioned how risk was measured, pointing out that the conclusions of both studies "rely largely on data that equates 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 entail limited radiation exposure to a small portion of the body, whereas atomic-bomb survivors experienced instantaneous exposure to their entire body, the statement reads. Moreover, survivors of the atomic bomb were exposed not only to X-rays, but also to particulate radiation, neutrons, and other radioactive materials. The known biological effects, says the ACR, are very different for these two scenarios, and "cancer assumptions based on this paradigm should be considered, but not accepted as medical fact."
Redberg disagrees: "The evidence is very strong as far as the link between radiation exposure and cancer. . . . Both studies underwent rigorous peer review, and I have confidence in their findings and scientific methods," she said in an interview.
"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," Redberg said. "We felt these data were accurate and significant enough to raise concern in the medical community."
1.Smith-Bindman R, Lipson J, Marcus R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med 2009; 169:2078-2086.
2.Berrington de González A, Mahesh M, Kim KP, et al. Projected cancer risks from computed tomographic scans performed in the United States in 2007. Arch Intern Med 2009; 169:2071-2077.
3.Redberg RF. Cancer risks and radiation exposure from computed tomographic scans. How can we be sure that the benefits outweigh the risks? Arch Intern Med 2009; 169:2049-2050.
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