Nov. 28, 2007 -- Radiation from the increasingly popular computed tomography scanning machines may be causing cancer, a new study finds, but experts say this shouldn't deter people who need the scans from getting them.
In a new study published in the New England Journal of Medicine, researchers at the Columbia University Medical Center in New York estimate that CT scans could cause as much as 2 percent of all cancers in the United States in the next 20 to 30 years.
Some experts point out that this is just a theoretical estimate based on data from Japanese victims of the atomic bomb. Still, the predictions have many worried about the possible increase in cancer risk that accompanies these tests.
"It has been pretty clearly demonstrated that [radiation] doses in CT scans have increased cancer risk," says study author David Brenner, a professor of radiation oncology at Columbia.
"Although the risks for any one person are not large, the increasing exposure to radiation in the population may be a public health issue in the future."
And while radiologists agree that there are cancer risks associated with CT scans, they say that no controlled epidemiological studies have been done to show how great this risk is.
"There is no definitive evidence to suggest the risk is as bad as they state," says Dr. Carl Schultz, professor of clinical emergency medicine at the University of California Irvine School of Medicine. "I'm not saying radiation is good. I am saying we have absolutely no idea what the real lifetime risk from CTs is."
CT imaging is used today to diagnose a wide array of diseases -- from hemorrhages in the brain to cancer in lungs to appendicitis. During the procedure, X-ray machines rotate around a section of the body, taking cross-sectional pictures that computers assemble into a 3-D image.
Although the image is a big improvement over conventional X-rays, the radiation dose that patients receive is also much higher. Researchers say an abdominal CT scan exposes patients to 50 times more radiation than an X-ray of the same region does.
"The important part of this article is that newer, more powerful imaging tools result in greater radiation exposure," says Robert Smith, director of cancer screening at the American Cancer Society. "In summary, concerns about excess exams and excess exposure, especially among pediatric patients, are appropriate."
Children at Special Risk
The risk is especially high for children. Researchers say that children are 10 times as sensitive to the radiation as adults are, putting them at an even greater cancer risk.
"Children's tissues are more sensitive to radiation, and they live longer than adults do, which means the cancer has more time to develop," says Cynthia McCollough, a radiological physicist at the Mayo Clinic in Rochester, Minn. "However, if you need a CT scan, parents shouldn't lose a minute of sleep over it."
The reason parents shouldn't be too concerned is that the individual risk of developing cancer is small.
"One in four Americans will develop cancer in their lifetime," explains McCollough. "That's 25 percent. Radiation from CT scans when you are a baby increases your risk of developing cancer to 25.1 percent."
Really, What's the Risk?
Although radiology experts agree there is a small risk of cancer, they disagree about exactly how high that risk is.
Researchers at Columbia based their estimates on a single study showing an excess risk of cancer death among survivors from Hiroshima and Nagasaki. They say that people several miles away from where the atomic bombs were dropped received radiation doses similar to those patients receive from CT scans.
However, experts do not think the actual risk of cancer can be estimated from the Japanese data because atomic bomb radiation and medical radiation are significantly different.
"This article makes the same mistakes as every other article on the subject to date -- they extrapolate atomic bomb data to medical radiation," says Schultz. "This is like comparing apples with oranges.
"The radiation from atomic bomb blasts is essentially gamma radiation and is received in a single exposure. Medical radiation is closer to beta radiation, as it can be completely attenuated with shielding and has lower energy."
But aside from the debate about how much CT scans increase the risk of cancer, researchers and radiologists alike agree that too many are being performed.
Because of its numerous applications and relative ease of use, the number of CT scans being performed in the United States today has increased dramatically from 3 million in 1980 to more than 68 million today.
Dr. Devra Davis, director of the Center for Environmental Oncology at the University of Pittsburgh Cancer Institute, explains there are two main reasons for this large increase.
"First, we are tempted to use technology simply because it is available even when it's not necessary," she says. "We tend to use this kind of technology without thinking about negative health impacts down the road.
"Second are issues of defensive medicine. The growth in lawsuits has put doctors on high alert to use all technologies at their availability to diagnosis patients."
And while CT scans have certainly made diagnosis easier, researchers say that about one-third of the scans could be replaced by alternative approaches or not done at all.
"If it is true that about one-third of all CT scans are not justified ... perhaps 20 million adults and, crucially, more than 1 million children per year in the United States are being irradiated unnecessarily," write the authors in the study.
Cutting Down on CTs
Performing CT scans only in emergencies and using other imaging techniques, such as MRIs, may be the solution.
"The number of scans could possibly be reduced by using more ultrasound and MRI," says Davis. "CT is an important and valuable tool for true emergencies, but it is used excessively for nonemergency situations when other technologies could be used instead."
Schultz agrees. "Many times doctors order CT scans because it's protocol instead of carefully assessing the patient."
And while this study shouldn't deter anyone who needs a CT scan from getting one, it does highlight the need for more education among both medical professionals and the patients themselves.
"The bottom line for both practitioners and the public should be essentially the same -- use diagnostic tools that have some potential for harm on a very judicious basis," says Dr. Tim Byers, professor in epidemiology and program leader for the Clinical Cancer Prevention and Control at the University of Colorado Cancer Center.
"Providers need to ask themselves if the test is really needed, or if an alternative test might be just as good. The public also needs to be prepared to ask the same questions as informed consumers."
Consumers may also be relieved to hear that CT technology itself is evolving in ways that allow technicians to garner the same images with less radiation.
"The [radiation] dose for CT exams has fallen by factors of two or three in the last two decades," McCollough says. "We in the radiology field have been working to reduce doses for a long time."