CT scans implicated in 2% of cancers
■ Some experts question that estimate. Meanwhile, explaining the radiation risks of necessary imaging without unduly frightening patients remains daunting.
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A recent study estimates that between 1.5% and 2% of all cancers can be attributed to radiation from the 62 million computed tomography scans Americans get each year. The finding comes on the heels of earlier, similar risk estimates, and it has some experts saying physicians should think twice about ordering the test.
The review article in the Nov. 29, 2007, New England Journal of Medicine arrives at its estimate by examining the cancer effects on the 25,000 Japanese who survived the 1945 atomic bombs and received radiation doses equivalent to the x-rays emitted by several CT scans.
The authors, David J. Brenner, PhD, and Eric J. Hall, PhD, are professors at the Columbia University Center for Radiological Research and have studied the cancer-causing effects of imaging for years. They write that the evidence of cancer risk from CTs is "reasonably convincing" for adults and "very convincing for children."
CTs deliver radiation doses many times that of other forms of radiography. An abdominal CT, for example, delivers at least 50 times more radiation to the stomach than an abdominal x-ray. While the authors estimate that the lifetime attributable cancer risk of a single head CT scan ranges from 0.08% for a neonate to practically zero for a 70-year-old, they write that "the concern about the risks from CT is related to the rapid increase in its use." The number of CTs that Americans get yearly has increased by 95% since 1980.
Arl Van Moore Jr., MD, chair of the American College of Radiology's Board of Chancellors, said that relying on Japanese atomic bomb survivors to gauge CT cancer risk is "like comparing apples and oranges" because they received a uniform, whole body dose all at once, while patients receive fractionated doses that only affect a single organ and are delivered over time.
The cancer-risk estimate "is not well supported by the data in the article," said G. Donald Frey, PhD, a medical physicist and radiology professor at the Medical University of South Carolina in Charleston. Indeed, no large-scale epidemiological studies of cancer risks associated specifically with CTs have been published.
"If [the study] were to cause people to not get a needed CT scan, it would really be a tragic situation," Dr. Frey said. "What we're really interested in, always, is the individual patient situation, and if a CT scan is ordered appropriately, then the benefits will almost surely exceed the risks."
Drs. Brenner and Hall write in the NEJM that they, too, agree that "most diagnostic CT scans are associated with very favorable ratios of benefit to risk." But they are concerned that the growth of CT use -- especially in children, in the management of blunt trauma, seizures and chronic headaches or in the practice of defensive medicine -- may pose a public health problem in the long term.
The new study highlights physicians' obligation to discuss the benefits and risks of CTs with their patients, said Howard P. Forman, MD, professor of diagnostic radiology and epidemiology and public health at Yale University School of Medicine.A 2006 American Journal of Roentgenology study Dr. Forman co-authored found that ordering physicians at 90 academic medical centers were most likely to discuss only the purpose of the CT, while talk of risks was left to radiology technologists.
"Ordering physicians have been dramatically underestimating the risks of a CT scan when they present it to patients, so much so [that] when you survey patients, many of them are not aware of the fact that radiation is even used," Dr. Forman said. "There is a huge disconnect between those that are being exposed and those in the know."
Howard Brody, MD, MPH, noted that obtaining informed consent for CTs is not easy, because the risk can be hard to explain to patients without unduly frightening them.
"Many people just have no clue what different exposures to radiation means," said Dr. Brody, who is director of the University of Texas Medical Branch Institute for the Medical Humanities and a retired family physician. "If one CAT scan equals this many chest x-rays, doctors know what that means. But if we tell the average person on the street, would they have any clue?"
Dr. Brody added that physicians should consider whether ordering a CT test is necessary or whether another test could be substituted for it.
"We shouldn't be leaving it up to the patient to refuse those [unnecessary] CAT scans if we should not be ordering them," he said. "That's our responsibility as physicians."
The American College of Radiology is developing a radiation dose registry in an effort to track patient exposure and study the long-term effects of radiography. In 2006, the American Medical Association House of Delegates adopted policy supporting such a registry.