Is all that scanning putting us at risk?

The cumulative radiation exposure from those scans, the researchers estimated, raised her lifetime risk of cancer by about 10 percent. Another patient had 20 chest, 35 head, and seven spine CTs over the two decades, raising her estimated cancer risk by 4 percent.

Partly because of these results - overall 5 percent of patients studied underwent at least 22 scans in the 22-year study period - the hospital plans to become one of the first in the US to notify doctors of their patients’ imaging histories, and resulting cancer risk. Unfortunately, said Dr. Michael Lauer, director of the Division of Cardiovascular Disease at the National Heart Lung and Blood Institute, these warning systems are rare.

The risk of medical imaging “is not as high on doctors’ radar screens as it should be,’’ he said.

While scans can be invaluable in helping doctors diagnose disease, the potential dangers of computed tomography, nuclear imaging, and other medical imaging procedures were made starkly clear last month, when researchers from across the US reported that very large numbers of Americans are being scanned.

The team looked at the health insurance records of nearly 1 million adults, ages 18 to 64, and found that nearly 70 percent had been exposed to radiation from at least one imaging procedure during a three-year period. Most of these adults received low doses of radiation, but 20 percent got moderate doses.

And 2 percent received high or very high doses, meaning they’d been exposed to more radiation than the annual amount allowed for workers in the health care and nuclear industries. Generalizing these findings to the entire adult population suggests that about 4 million Americans are being exposed to large amounts of radiation from medical care intended to heal them, according to the study published in the New England Journal of Medicine in August.

Particularly surprising, said lead author Dr. Reza Fazel, is that even among the youngest participants in the study, ages 18 to 34, about half underwent scans during the three-year period. Since cancers caused by radiation take years, even decades, to develop, excessive scanning in this age group is particularly worrisome, the authors said.

“Many of us in the medical profession had no idea how much radiation people are being exposed to in routine daily practice,’’ said Lauer, who wrote an accompanying editorial.

Since the 1980s, the use of medical scans has exploded. Since 1992, the number of CT scans alone has quadrupled. It’s not just because doctors push these fast, high-powered X-rays; patients demand them.

“These tests are so good now and so easy to do, a lot of times as physicians we use them in place of doing a thorough exam and talking to the patients,’’ Fazel said.

To be sure, many scans benefit patients, providing crystal-clear three-dimensional pictures in minutes. They help doctors detect appendicitis and bowel obstructions, internal injuries to organs and brain hemorrhages, and are used to monitor whether cancer patients are responding to their drugs and to check them for recurrence after treatment.

“For any single patient undergoing a single test [if medically needed] it’s far more likely that the benefits will outweigh the risks,’’ said Dr. Brahmajee Nallamothu, senior author on the New England Journal of Medicine study.

Mammograms, for example, are one of the most common imaging tests. Studies show that mammography catches cancer earlier and improves survival, and the test exposes patients to very small amounts of radiation - about 0.4 millisieverts, the unit commonly used to measure doses of radiation. By comparison, most Americans receive a dose of about 3 millisieverts per year from naturally occurring radioactive materials and cosmic radiation from space. Dental X-rays of a patient’s mouth emit about 0.15 millisieverts.

Cardiac scans are another story, some doctors said. Fast growing cardiac CT angiography is being used to detect whether fatty or calcium deposits are gunking up a patient’s arteries. Many hospitals have invested millions of dollars in ultra-fast cardiac scanners to diagnose patients with chest pain and some are screening patients without symptoms but with risk factors for heart disease.

But these cardiac scans expose patients to much higher doses of radiation - about 10 millisieverts - and there is little evidence they actually benefit patients when used as a screening test by detecting disease earlier and prolonging lives, several doctors said.

In cases like this, where the benefits are unclear, doctors may need to step back, Lauer said. Likewise, it may not be necessary to scan a patient with a history of kidney stones every time they come to the ER with abdominal pain. Instead, Sodickson said, for some patients doctors may be able to use ultrasound, which uses sound waves rather than radiation to see inside the body, as an initial screening test.

“We need to change our threshold for imaging,’’ said Sodickson, assistant director of emergency radiology at the Brigham.

A few doctors groups and hospitals are trying to do that. The American College of Radiology recently began an “Image Gently’’ campaign to encourage pediatricians to lower radiation doses when scanning children.

The Brigham plans to expand its electronic medical record system as soon as early 2010 to protect patients against unnecessary radiation from medical scanning. When a doctor orders an imaging test for a patient, the system will tell the physician how many scans the patient has had during the past 22 years, and by how much those scans have raised the person’s cancer risk above normal. The hope is that the system will help doctors - and patients - better weigh the risks and benefits of additional scans.

The system will have limits: It will capture only scans patients have had within the Brigham system, though the hospital hopes to expand the system to include all scans within Partners HealthCare, the parent organization of the Brigham that includes several other hospitals. And, because a patient’s radiation exposure depends on their size and other individual factors, estimates of increased cancer risk are just that, estimates.

Most hospitals and doctors don’t have sophisticated alert systems, but Lauer said patients should not hesitate to ask their physician for their scanning history and whether the benefit of another scan is worth the radiation exposure.

For example, Harvard Vanguard Medical Associates, a large physicians’ group, tracks patients’ scans in its electronic medical record system and since 2003 has included radiation doses for CT scans. The system does not automatically notify doctors, but they can look up the information, and patients can ask them to.

“The fact is indisputable that radiation exposure is going up and that it’s being driven primarily by CT scanning,’’ said Dr. Steven Seltzer, chairman of radiology at the Brigham. The risk “needs to be factored into the equation more aggressively.’’