CT scans are known to be resorted the world over more frequently than is required. Now a study in the US warns that its overuse might cause as many as 3 million excess cancers in the USA over the next two to three decades.
"About one-third of all CT scans that are done right now are medically unnecessary," says David Brenner of Columbia University, lead author of the study reported in today's New England Journal of Medicine.
Researchers say they're not trying to discourage all use of CT scans -- CT stands for computed tomography -- which superimpose multiple X-ray images to make 3-D pictures. Rather, they say, CT scanning is an invaluable tool in many cases. The problem is that doctors too often overlook its risks.
CT scans offer an unparalleled window into the human body, and their use has grown dramatically in recent decades as doctors use them to identify ailments in the head, abdomen and heart.
Today, about 62 million CT scans are performed nationwide every year, up from 3 million in 1980, the authors say. Medical exposure to radiation, mainly through CT scans, has replaced environmental radon as the dominant source of radiation exposure for the U.S. population, the doctors say.
"On average, we now get double the radiation exposure we got in 1980 because of increased CT scans," Brenner says. "Virtually anyone who presents in the emergency room with pain in the belly or a chronic headache will automatically get a CT scan. Is that justified?"
University of New Mexico radiologist Fred Mettler, who was not part of the study, agrees that CT scans are overused. "We're always behind on CT scans because of demand from clinicians," he says.
As many as 5 million scans are now done in children, who are 10 times more sensitive to radiation than adults. The increase was driven by technical advances that allow doctors to capture images in less than a second, eliminating the need for anesthesia to keep a child from moving.
And the use of the scans continues to grow, Brenner says. Doctors are scanning smokers and ex-smokers for early-stage lung cancer, a highly controversial practice; they're using non-invasive "virtual" colonoscopies to check for colon cancer; and CT angiography is now being tested as a possible complement to ordinary angiography as a way to diagnose blockages in arteries leading to the heart.
In critiquing a study on CT angiography at an American Heart Association meeting in Orlando last month, Michael Lauer of the National Heart, Lung and Blood Institute called that practice into question. He said there is no evidence of benefit from the technology, and a real concern for harm.
New machines being developed by Philips and Toshiba for CT angiograms, however, may be safer because they emit 80% less radiation than standard CT scanners, Brenner says.
Brenner and his co-author, Eric Hall, also of Columbia, say many doctors don't realize that just a scan or two can bathe a patient in roughly the same amount of radiation as the atomic bomb delivered to the Japanese survivors of Hiroshima and Nagasaki standing a mile or two from ground zero. And many people receive multiple scans over a lifetime.
The amount of radiation delivered during a single CT scan can range from 1,000 to 10,000 millirems, depending on the machine and the protocol. Japanese survivors a mile or two from ground zero received about 3,000 millirems on average.
The cancer rates in the new study were drawn directly from a joint $1 billion study of the bomb survivors financed by the United States and Japan.