CT Scans: Too Much of a Good Thing?
Tuesday, January 15, 2008
The number of Americans undergoing CT scans has increased dramatically in recent years, driving a significant increase in the amount of radiation that many people are being exposed to from medical procedures.
That trend is raising concern that the high-tech exams could cause a small but significant excess of cancers in coming decades and spurring debate about whether the increasingly ubiquitous tests are being overused, exposing millions of Americans to needless risk. "The radiation doses from CT have been pretty clearly demonstrated to increase cancer risk," said David J. Brenner, a professor of radiation oncology at Columbia University who co-authored the most recent of a series of reports in the past year warning about the potential risks of CT scans. "On an individual basis there's probably not a big risk, but a small cancer risk applied to an increasingly large population spells trouble down the road. That's the concern."
Although some prominent medical groups have issued warnings against overusing the exams -- especially for children -- some experts worry about frightening patients away from the invaluable tests.
"You don't want people to avoid getting a potentially lifesaving diagnosis or therapy because they are afraid to get a CT scan," said Arl Van Moore Jr., a Charlotte radiologist who chairs the American College of Radiology's board of chancellors. "When a scan is done the right way for the right reasons, the benefits clearly outweigh the risks of doing it."
CT (computed tomography), sometimes called CAT (computed axial tomography), is performed by computerized machines that scan patients with X-rays to create three-dimensional cross-sectional images of organs and other parts of the body, providing far greater detail than standard single X-rays.
The number of scans performed each year in the United States has increased sharply, rising from about 3 million in 1980 to at least 67 million in 2006. There are many reasons for the increase, including improvements in the technology that have speeded the procedure, a growing list of uses and a proliferation of the machines.
"CT has become ubiquitous," said Jason Launders, a senior project officer at the ECRI Institute, an independent nonprofit health-care research organization outside Philadelphia that released a report last year warning about the potential risks. "Now if you go into an emergency room you can be scanned in a few minutes. You can't get that kind of information any other way. So physicians tend to use CT as sort of a first line in working out a diagnosis."
The use of CT scans is expected to continue to rise as possible new applications, such as for lung and colon cancer screening, become more widespread.
While the value of CT scans for screening remains the focus of debate, no one questions the value of the tests for allowing doctors to quickly diagnose a wide variety of health problems, including head injuries, heart problems, cancer, appendicitis, fractures and gallstones.
"CT is probably the biggest advance in diagnostic radiology that has ever occurred," said Thomas Ohlhaber, deputy director of the Division of Mammography Quality and Radiation Programs at the Food and Drug Administration. "Exploratory surgery that was common 10 or 15 years ago has virtually disappeared and all the risks associated with that have gone away because of CT. But nothing comes free of risks."
The concern comes from the fact that CT scans expose patients to much more radiation than standard X-rays. A CT chest scan, for example, exposes a patient to more than 150 times more radiation than a standard chest X-ray. That's more than twice as much as a typical person receives from the environment in an entire year. (See graphic.)
The upsurge in CT use has fueled a big jump in the annual amount of radiation that each American is receiving from medical procedures of all kinds. The average level of that exposure has increased about 600-fold since 1980, according to a federal report being published this year.
"At the moment, about half of medical radiation exposure is from CT scans," said Fred A. Mettler Jr., a radiation health expert at the University of New Mexico who helped prepare the report. CT scans represented just a few percent of the average American's exposure to radiation from medical tests in 1980, he noted.
Because it would take decades to follow a large number of patients who have undergone CT scans to determine the exact risks posed by the exams, there are no direct data demonstrating the danger. But researchers have estimated the risk based on what is considered the best information available about the health effects of radiation -- data collected from survivors of the atomic bombings of Hiroshima and Nagasaki, as well as exposure of nuclear plant and medical workers.
Using that information, experts have estimated that every 1,000 to 2,000 CT scans may produce one fatal cancer that would not have occurred otherwise.
Although that overall risk is relatively small, the large number of scans being performed could translate into a significant number of cancers. Brenner and his colleagues estimate that CT scans may be responsible for perhaps 2 percent of all cancers in the United States. The ECRI Institute estimated the scans may be causing 6,000 extra cancers each year, half of them fatal.
"Twenty years from now we could see a huge bonus of cancer coming through because of indiscriminate use of CT today," Launders said. "That's the real issue people are worried about."
The individual risk varies depending on the age of the patient and the type of scan. Younger people face more risk than older people because they have more time for a cancer to develop. Children are especially vulnerable because their rapidly developing bodies are at least four times as sensitive to the damaging effects of radiation. A child's risk may run as high as one case of cancer for every 500 scans, experts say.
Among adults, "young women have the highest risk," said Andrew Einstein of Columbia University, who evaluated the risks from cardiac CT scans in a paper published in July in the Journal of the American Medical Association. A 20-year-old woman faced the greatest risk -- one additional cancer for every 143 scans, he reported.
Some question the estimates, saying the risk of exposure from radiation from multiple CT scans may not be the same as from radiation from an atomic bomb.
"It's very difficult, in our opinion to equate the data from Hiroshima, where you had an instantaneous entire-body dose of radiation, compared to a cumulative dose for multiple CT scans," said Moore, the radiologist. "It's apples and oranges. It's not really the same."
Einstein, Mettler and others, however, disagree. "Radiation is radiation. Those are X-rays that the folks at Hiroshima and Nagasaki got. These are X-rays from a CT scan. They're really the same thing," said Brenner, whose report was published in November in the New England Journal of Medicine. "If the dose is the same, then the risk is going to be the same."
Although the companies that make the CT machines have been developing new protocols designed to minimize radiation exposure, particularly for children, everyone agrees that the most prudent thing to do is to eliminate unnecessary use.
While there are scant hard data about how often CT scans are done needlessly, several experts estimated that perhaps one-third could be eliminated. There are many reasons CTs may be overused. Some CTs are full-body "virtual physicals" performed on people who appear perfectly healthy -- a practice widely opposed by medical authorities.
Some say part of the blame lies with physicians who have financial interests in imaging facilities. "There is statistical evidence that indicates that more scans are ordered when a physician has a financial interest than when he doesn't," said Moore.
Defensive medicine also plays a role, with some doctors ordering the tests to stave off accusations that they withheld the most cutting-edge technology from their patients.
In other cases, overuse results from worried patients and from parents demanding CT scans that may not be needed. Other times doctors may not realize how many tests involving radiation patients have already undergone -- information that could influence a doctor's decision -- or may order a CT scan when other tests that do not involve radiation, such as a blood test, an MRI or an ultrasound, would suffice. "There are alternatives that are arguably just as good," Brenner said.
And patients also can take more responsibility for reducing the number of unnecessary scans.
"If a patient walks in and says, 'Tell me what the dose is from this and tell me what the risk is,' they are going to make the doctor start thinking about it," Mettler said.