Carcinogen Labels

X rays, gamma radiation, and neutrons were officially stamped as human carcinogens by the National Toxicology Program this year. The US Food and Drug Administration’s alarm over an increase in questionable whole-body computer tomography scans triggered the action, says Christopher Portier, associate director of the NTP. “The concern over CT scans led us to raise the priority for considering a review of x rays.”

Some medical physicists and radiologists worry that the listing will scare patients away from using ionizing radiation for medical diagnosis. In late January, the American Association of Physicists in Medicine and the American College of Radiology each issued statements about their concerns over the NTP report, which was released that same month.

“We’ve worked for many years to maximize the benefit of medical radiation by minimizing the risk,” says G. Donald Frey, chair of the AAPM board of directors. “We hope that the physicians are aware of this, that if they have calls from patients or colleagues, they’ll be ready to explain the situation to people.” NTP officials acknowledge that some carcinogens can be medically beneficial under specific circumstances.

The NTP, located in North Carolina at the National Institute of Environmental Health Sciences, began listing human carcinogens in 1978. The listing now contains more than 200 individual substances, mixtures of chemicals, and exposure circumstances that are known or “reasonably anticipated” human carcinogens. For the first time, the listing also includes viruses—hepatitis B and C and human papilloma