How Dangerous Are CT Scans?

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Jack Hollingsworth / Corbis

A patient undergoes a CT scan.

Computed tomography (CT or CAT) scans help doctors detect everything from cancer to kidney stones. But some physicians are raising concerns about the safety of such procedures — most notably, an increase in cancer risk. A CT scan packs a mega-dose of radiation — as much as 500 times that of a conventional X-ray. If your doctor orders a CT scan for you or your child, should you think twice?

Absolutely, say researchers behind two recent studies that sound the alarm about the increased cancer risk associated with multiple CT scans. In the first study of its kind, physicians at hospitals in Florida and Washington, D.C., evaluated the medical-imaging records of 1,243 randomly selected patients to calculate just how much radiation each patient had sustained in the past five years. Although CT scans were the biggest source of radiation, other offenders included X-rays and mammograms. The results of the study, presented in May at the annual conference of the Society for Academic Emergency Medicine, were disturbing: the average patient had received 45 millisieverts (mSv) of radiation. (The typical chest X-ray dispatches 0.02 mSv of radiation.) And 12% of patients had gotten more than twice that amount — 100 mSv or more. "Our focus is to bring awareness to the fact that people are getting large doses of radiation and it's not innocuous," says Timothy Bullard, the study's lead author and chief medical officer at Orlando Regional Medical Center. "We want people to use the technology appropriately."

"Appropriate" is the key word — especially since a review study published last November in the New England Journal of Medicine determined that as many as one-third of all CT scans performed in the United States are unnecessary. The authors take issue with the "perhaps 20 million adults and, crucially, more than 1 million children per year in the United States [who] are being irradiated unnecessarily." Part of the problem, the authors say, is that patients are being prescribed multiple, unneeded CT scans, a predicament that could be avoided with better communication between physicians. "Having the same CT scan twice is ridiculous," says David Brenner, the review's lead author and director of the Center for Radiological Research at Columbia University Medical Center. "There is no excuse." In one of the review's highlighted studies, among patients undergoing CT scans, 30% were on their third scan, 7% had five or more, and 4% had more than nine. Also to blame: doctors increasingly practicing defensive medicine. "There is an underlying philosophy that you're at fault if you miss anything," says Bullard. "The goal is to be perfect every time." Plus, he notes, CT scans have no immediate negative side effects. "They are quick, painless, and send patients away with the sense of satisfaction that everything's been done."

Exactly how much radiation is too much? Because CT scans came into vogue in the 1980s and radiation-induced cancer takes roughly 20 years to develop, long-term studies of CT scans and cancer are still under way. But scientists are already anticipating future health implications. Indeed, researchers found a population of 25,000 Japanese post-atomic-bomb survivors who were exposed to roughly the same amount of radiation as two CT scans. Based in part on those studies, the Food and Drug Administration estimates that an adult's lifetime risk of developing radiation-induced cancer from a CT scan is roughly 1 in 2,000. Worse, the risk for children is even higher.

Compared with adults, children are more sensitive to radiation because they have longer life expectancies and because their cells divide more rapidly, making their DNA more vulnerable to damage. A child's risk of developing a fatal cancer from one CT scan is as high as 1 in 500. Although newer machines can be adjusted to deliver up to 50% less radiation for children and small adults, a 2001 study published in the American Journal of Radiation showed that radiologic technologists (RT) rarely make those adjustments. "Changing technical factors is very easy. It just requires a little thought and a few extra seconds," says Michele Scoglietti, a spokesperson for the American Society of Radiologic Technologists. "But I think there are many RTs who are either not trained to vary the technique, don't know how, are in a hurry or are just lazy."

When doctors first ordered a CT scan for Jen Houck's six-month-old daughter in 2003, the new mom was more worried about the risks of anesthesia (used to keep children from squirming in the machine) than of radiation exposure. In 2006 and 2007, her daughter, now 5, had two additional CT scans, 6 months apart, for what doctors initially thought was a growth abnormality. They've since determined the child was perfectly healthy. "All that, just to find out her head is bigger than normal," says the 27-year-old mother of two in Boone, North Carolina. In hindsight, Houck wishes she had done things a bit differently. "I would have asked more questions about the necessity for a third scan so soon after the second." She also says no one mentioned the option of a low-dose scan, and she has no idea how much radiation her daughter received. "I wish I'd known to ask the question."

So what should you ask, if a doctor recommends a CT scan for you or your child? Brenner suggests the following: is a CT scan really necessary? Might a test that doesn't involve radiation, such as an ultrasound or MRI, do the job? In some cases, a CT scan is absolutely required — for example, for diagnosing severe head trauma or internal injuries, for acute abdominal pain, or to diagnose an existing cancer. If your doctor schedules you for a scan, call ahead to see if you can bring a flash drive. That way you can take an electronic copy of your CT scan to go, and may be able to avoid another scan later, should you move or change doctors. (A hard copy of the scan is bulkier, but may also be an option.) If your child needs a CT scan, ask the technologist to put the machine on pediatric-appropriate settings. For more information on kids and CT scan safety, visit the Alliance for Radiation Safety in Pediatric Imaging.