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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 two new studies published in the 14/18 December 2009 issue of the medical journal 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 x rays 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." Dr Redberg went on to call for action now rather than waiting for these predictions to be confirmed in 10 – 20 years time.
Is the risk overestimated?
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.
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 be paid to radiation exposure from medical CT scanning. "Both studies underwent rigorous peer review and I have confidence in their findings and scientific methods."
"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," Dr Redberg said. "We felt these data were accurate and significant enough to raise concern in the medical community."