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Do You Really Need That CT Scan?

Even for people who long ago discarded the notion that “the doctor always knows best,” it’s hard to raise questions when doctors order diagnostic imaging tests, such as CT scans. The array of tests is bewildering, the technology intimidating, and the circumstances and possible outcomes (is it cancer? a stroke? a herniated disc?) often terrifying. The ability to peer inside the body is nothing short of miraculous and has saved countless lives since X-rays were discovered more than a century ago. But experts have become increasingly concerned about the overuse of many types of scans in the U.S., not only because of the huge and growing expense, but even more because of the potential risk posed by the cumulative exposure to radiation.

Americans, on average, are exposed to about six times more radiation from medical imaging than they were three decades ago. Such scans now account for half of our total radiation exposure (“background” radiation from soil, rocks, air, water, and the cosmos accounts for most of the rest). They are used to diagnose disease, trauma, and abnormalities, as well as to guide and monitor treatment.

Because they’re so commonly used today, CT (computed tomography) scans are the biggest source of radiation of all imaging technologies. A special type of X-ray that produces cross-sectional images, or “slices,” of the body, CT scans are used to diagnose everything from heart disease and cancer to brain tumors, kidney stones, and injuries. Abdominal CT scans tend to produce the most radiation—averaging about 500 times more radiation than a simple chest X-ray, and 1,000 times more than a dental X-ray or bone mineral density test (DEXA). Other types of imaging that use radiation include nuclear diagnostic tests, such as PET scans, as well as fluoroscopy. MRIs and ultrasound, in contrast, do not use radiation.

Each year about 10% of Americans undergo CT scans, sometimes multiple scans, and the number is rising every year. The risk from a single CT scan, when appropriately done, is minuscule, but radiation exposures add up over a lifetime. A study in the Archives of Internal Medicine last year estimated that 72 million CT scans were done in the U.S. in 2007. After excluding scans done following a diagnosis of cancer and those performed during the last five years of a person’s life, the researchers projected that CT scans would cause about 29,000 extra cases of cancer in the future and about 14,500 deaths. That’s about 2% of all annual cancers.

While there’s debate about such estimates and the magnitude of risk from medical imaging, everyone agrees that unnecessary radiation exposure should be eliminated. Who could be against that? It’s estimated that at least one-third of all scans may be unnecessary. The hard part is to define “unnecessary” and minimize the risks posed by appropriate testing.

More isn’t always better

Americans get far more medical scans, on average, than people in other industrialized countries, without necessarily getting better health outcomes or lower mortality rates. Why do we do so many imaging tests?

• Doctors may order tests out of habit, wanting to “leave no stone unturned,” and may not focus on radiation doses or the cancer risks of medical imaging. They may also order scans because of fear of failing to diagnose something and being sued for malpractice.

• Patients may pressure doctors for scans, feeling that high-tech testing is optimal care, or “just to be safe.” They may be influenced by ads and websites that promote the benefits of diagnostic imaging but underplay the risks and costs.

• There’s an increased supply of machines, and doctors who own them or have a financial stake in the facilities have an incentive to order more scans.

• As scanning machines become faster and more powerful, providing greater resolution (and often using more radiation), more “incidental findings” are detected. These findings are often clinically insignificant, but lead to more testing and possibly over treatment.

• Certain conditions, such as cancer and some cardiac problems, may need periodic monitoring.

• CT scans are also being used more often for screening of healthy people, without evidence of benefit. Some examples: CT scans of smokers and former smokers for lung cancer; and “full-body” scans that are promoted as a way to detect a variety of cancers, cardiovascular disease, and other disorders.

Little oversight, big variations

There are no federal rules on radiation doses from medical scans—except for mammograms, which use low-to-moderate doses of radiation and contribute much less than 1% of total radiation from medical imaging, according to a paper in the Journal of the American Medical Association in July. Earlier this year, however, the FDA announced a broad initiative to reduce unnecessary radiation exposure from medical imaging by, for instance, regulating the devices, recommending appropriate justifications for various procedures, and setting optimal radiation doses.

Several studies have found that radiation doses from CT scans tend to be higher than the amounts generally cited and, for a given procedure, vary by as much as 13-fold within and among facilities. Moreover, the radiation dose from most CT scans could be reduced by half without affecting diagnostic accuracy, according to Dr. Rebecca Smith-Bindman, of the University of California, San Francisco, writing recently in the New England Journal of Medicine.

What to do

The American College of Radiology and other expert groups advise that imaging tests be done only if there is a clear benefit that outweighs the risks—and that the minimal level of radiation be used. Among the questions you should ask before undergoing a diagnostic scan:

• Is this test really necessary, and will it really improve my health care?

• Is there a nonradiation alternative, such as ultrasound or MRI, that’s equally good? Keep in mind, though, that CT is better for certain things (such as broken bones, damaged vertebrae, cancer, pneumonia, bleeding in the brain, organ injuries), while MRIs are better for others (tendons, ligaments, spinal cord, brain tumors, most strokes); sometimes both are used. MRIs are more expensive.

• Is the facility accredited by the American College of Radiology?

• Will the test use the lowest amount of radiation and fewest scans for adequate imaging? For children, it’s important to make sure the radiation level is appropriate for their smaller size.

• Will the scan be limited to the indicated area, and will nearby areas be properly shielded?

Bottom line: Medical imaging tests are invaluable in the diagnosis and treatment of many disorders. You shouldn’t avoid necessary exams and procedures out of concern about radiation. And if a test doesn’t find anything, that doesn’t mean it was unnecessary. But, unless your doctor suggests it, don’t go to those widely advertised clinics offering full-body or coronary screening CT scans for the “worried well.” The best advice: Before having any imaging scan, discuss the pros and cons with your doctor.

UC Berkeley Wellness Letter, October 2010