Will an mGy (milligray) a day keep the doctor away?

When it comes to radiation dose, the news media, and the general public by extension, zero in one aspect only: risk, risk, and more risk. But what about any potential positive effects of exposure to ionizing radiation? In a talk at the Stanford MDCT symposium in Las Vegas, Cynthia McCollough, Ph.D., took on the task of scientifically demonstrating that some level of radiation exposure may be a good thing.

"It's not news that radiation is a carcinogen," said McCollough, who is director of the CT Clinical Innovation Center and an associate professor of radiologic physics at the Mayo Clinic College of Medicine in Rochester, MN. "But the effect of radiation is based on the dose, the dose rate, the quality of the dose, the gender [of the person exposed], and the age at exposure."

Most of the data on the effects of radiation exposure are based on studies done on survivors of the atomic bomb. These people were exposed to much higher levels of radiation than the doses applied in medical imaging, McCollough said.

McCollough outlined three possible scenarios in which radiation exposure comes out a winner, starting with hormesis. Hormesis refers to a favorable biological response from low-level exposure to a toxin that would be bad at high-level exposures, she said. Some research has suggested that low-level exposure to radiation makes some cells more resistant to additional doses.

"This is like the theory that having a cold now and then challenges your immune systems and makes it stronger," she said. "But hormesis data have not been widely reproduced. Not every cell has this capacity for adaptive response, and the adaptive response doesn't appear to last long. So I don't think we can pursue [the idea] that radiation is like a vitamin pill, where a little bit is good for you."

While radiation exposure may not directly improve health, can it be a tool for verifying that people are in good health? Yes, McCollough said, as shown by successful screening programs in asymptomatic individuals.

McCollough cited coronary artery calcium screening and CT colonography as two instances in which limited radiation exposure is considered acceptable. For calcium scoring, imaging serves as a good predictor of coronary events, she said. And the American Cancer Society recently endorsed CT colonography for colon cancer screening.

On the other hand, the jury is still out on whether CT lung cancer screening will be worthwhile. The same holds for screening the coronary arteries with CT, as the imaging community is debating when the test is appropriate and whether it has any outcome benefits.

"This debate is reminiscent of the debate over mammography that went on for decades. 'Could mammography cause more cancers than it finds?'," McCollough said.

She noted that at about the time that breast cancer screening became widely embraced, mortality from breast cancer started to decrease.

"[Breast cancer screening] is a case where radiation has been proven by outcome, with mortality as the end point, that it is beneficial to the right population," she said.

Finally, there are the tangible and measureable health benefits derived from exposing symptomatic individuals to radiation. Studies have shown that CT in suspected appendicitis can reduce cost, shorten hospital stay, and make a differential diagnosis, thereby potentially averting surgery, McCollough said.

McCollough stressed that many recent studies on CT have linked radiation dose with imaging overuse, but that the connection is not always accurate. Unfortunately, it's those inaccuracies that the media latch on to. For example, in a 2007 New England Journal of Medicine paper, the researchers stated that, based on anecdotal evidence, some CT scans may have been performed unnecessarily, McCollough said (2007;357[22]:2277-2284).

But in subsequent media reports, that information became a statement of fact: that one-third of all CT scans were unnecessary, she said.

"Unfortunately, when science plays out in the media like this, it's a gross oversimplification...and the patient loses out," McCollough said. "We don't have hard outcome data about what is necessary and unnecessary. But it's dangerous to take the overuse discussion and then discuss radiation, because there is a risk associated with failure to perform medically needed CT."