Millions of Americans, especially children, are needlessly getting dangerous radiation from "super X-rays" that raise the risk of cancer and are increasingly used to diagnose medical problems, a new report warns.

In a few decades, as many as 2 percent of all cancers in the United States might be due to radiation from CT scans given now, according to the authors of the report.

Some experts say that estimate is overly alarming. But they agree with the need to curb these tests particularly in children, who are more susceptible to radiation and more likely to develop cancer from it.

"There are some serious concerns about the methodology used," but the authors "have brought to attention some real serious potential public health issues," said Dr. Arl Van Moore, head of the American College of Radiology's board of chancellors.

The risk from a single CT, or computed tomography, scan to an individual is small. But "we are very concerned about the built-up public health risk over a long period of time," said Eric J. Hall, who wrote the report with fellow Columbia University medical physicist David J. Brenner.

It was published in Thursday's New England Journal of Medicine and paid for by federal grants.

The average American's total radiation exposure has nearly doubled since 1980, largely because of CT scans. Medical radiation now accounts for more than half of the population's total exposure; it used to be just one-sixth, and the top source was the normal background rate in the environment, from things like radon in soil and cosmic energy from the sun.

A previous study by the same scientists in 2001 led the federal Food and Drug Administration to recommend ways to limit scans and risks in children.

But CT use continued to soar. About 62 million scans were done in the U.S. last year, up from 3 million in 1980. More than 4 million were in children.

Since previous studies suggest that a third of all diagnostic tests are unnecessary, that means that 20 million adults and more than 1 million children getting CT scans are needlessly being put at risk, Brenner and Hall write.

Ultrasound and MRI, or magnetic resonance imaging, scans often are safer options that do not expose people to radiation, they contend.

CT scans became popular because they offer a quick, relatively cheap and painless way to get 3D pictures so detailed they give an almost surgical view into the body. Doctors use them to evaluate trauma, belly pain, seizures, chronic headaches, kidney stones and other woes, especially in busy emergency rooms. In kids, they are used to diagnose or rule out appendicitis.

But they put out a lot of radiation. A CT scan of the chest involves 10 to 15 millisieverts (a measure of dose) versus 0.01 to 0.15 for a regular chest X-ray, 3 for a mammogram and a mere 0.005 for a dental X-ray.

The dose depends on the type of machine and the person -- obese people require more radiation than slim ones -- and the risk accumulates over a lifetime.

"Medical care in this country is naturally so fragmented. Any one doctor is not going to be aware of the fact that a particular patient has had three or four CT scans at some point in the past," said Dr. Michael Lauer, prevention chief at the National Heart, Lung and Blood Institute.

People with chronic problems like kidney stones are likely to get too many scans, said Dr. Fred Mettler, radiology chief in the New Mexico Veterans Administration health care system.

"I've seen people who are 30 years old who have had at least 18 scans done," he said.

That puts them at risk of developing radiation-induced cancer, Brenner and Hall said. They base this on studies of thousands of Japanese atomic bomb survivors who had excess cancer risk after exposures of 50 to 150 millisieverts -- the equivalent of several big CT scans.

"That's very controversial. There's a large portion of the medical physics community that would disagree with that" comparison, said Richard Morin, a medical physicist at the Mayo Clinic in Jacksonville, Fla. However, others defended the data, which has been widely cited in other radiation studies.

"It's the best evidence we've got" on cancer risks, Lauer said.

Dr. Robert Smith, the American Cancer Society's director of screening, said the authors' estimate that 2 percent of future cancers may be due to CT scans "seems high." But since cancers take 10 to 20 years to develop, "the ability to even observe that kind of an increase is going to be very difficult," he said.

The authors stressed that they were not trying to scare people who need CT scans away from having them. In most cases, the benefits exceed the risks, especially for diagnostic scans.

However, using the scans to screen people with no symptoms of illness -- like screening smokers for signs of lung cancer -- has not been shown to save lives and is not currently recommended.

Many groups also condemn whole-body scans, often peddled by private practitioners in shopping centers as peace of mind to the worried well. Many of these centers are not accredited by the College of Radiology; only a third of all places that do CT scans in the U.S. are, although insurers are starting to require it for reimbursement, Moore said.

Many CT centers also are set up for adults and rarely image children, who need adjustments to limit dose and radiation risk, said Dr. Alan Brody, a radiologist at Cincinnati Children's Hospital Medical Center who wrote a report on the topic. He said parents should seek a center that often handles children.

Both doctors and patients need to be more aware of radiation risks and discuss them openly, Brenner and Hall said.

"We were astonished to find, when we were researching materials for this paper, how many doctors, particularly emergency room physicians, really had no idea of the magnitude of the doses or the potential risks that were involved," Hall said.

Other studies found the opposite problem: Three out of 10 parents in one study insisted on CT scans instead of observing the child's condition for awhile even after they were told of the radiation risk, Brody said.

"This is what our patients want," and they expect fast answers from doctors, he said.

The pressure is greatest for ER doctors who "are in a bind ... they have all these patients stacked up" and need to make quick decisions, Mettler said.

Future generations of devices using less radiation should help alleviate the concern, but these mostly are directed at the emerging field of heart scans, Lauer said.

"When we order a CT scan it just doesn't seem like such a big deal" but it should be, he said. "The threshold for ordering these tests is low and it's getting lower and lower over time, which means that the risks become potentially all that more important."