Federal agency adds x-rays to carcinogen warning list

Ionizing radiation has been listed for the first time as a known human carcinogen in a report prepared by the National Toxicology Program, an interagency group coordinated by the U.S. Department of Health and Human Services. The report has been published every two years for more than two decades.

According to the "Report on Carcinogens, Eleventh Edition" released on Jan. 28, studies show that exposure to x-rays and gamma rays causes many types of cancer. Childhood exposure is linked to an increased risk for leukemia and thyroid cancer, while exposure during reproductive years increases the risk for breast cancer. Exposure later in life increases risk for lung cancer.

The report cites evidence that exposure to ionizing radiation is linked to cancer of the salivary glands, stomach, colon, bladder, ovaries, central nervous system, and skin.

The American College of Radiology will petition the NTP to have ionizing radiation removed from the list. The ACR fears that patients will be inappropriately alarmed.

"We tried to caution them not to include diagnostic imaging on a report like this," said Dr. James Borgstede, chair of the ACR board of chancellors.

X-rays and gamma rays are not substances that the general public has access or exposure to, and they do not belong on a list of substances that pose a risk to people in the course of their normal daily lives, Borgstede said.

"This report could lead patients to mistakenly believe that they are being placed at undue risk by undergoing a radiological procedure and cause many who may desperately need care to avoid seeking appropriate medical attention," he said.

Radiologists were concerned that inclusion of ionizing radiation on the list without a strong statement of the benefits would be misleading, said Christopher Portier, Ph.D., associate director of the NTP. The report's introduction states that the purpose of the information is only to identify hazards and not to address potential benefits.

"We tell people to talk to their physicians if they have concerns about medical x-rays. We point them to the FDA Web site, which does a good job of explaining the risks versus benefits in more detail than we could," Portier said.

A formal process exists for individuals or organizations such as the ACR to petition the NTP for removal of an agent. In the case of ionizing radiation, the evidence - both human and animal - is strong, according to Portier.

"The ACR can make the petition and we will seriously review it. But my gut feeling is that it would not happen, certainly not in the next review, unless there was a dramatic change in the evidence," he said.

Dr. David Brenner, a radiobiologist at Columbia University and coauthor of the seminal 2001 American Journal of Roentgenology study that exposed the dangers of routine pediatric CT scans, said it's really an issue of dose.

"I'd be surprised that anyone would argue against x-rays and gamma rays being carcinogens at high doses. At extremely low doses, it's still a controversial issue. The question that we are talking about is somewhere in between," Brenner said.

The report does not specify a dose range. Setting standards of safe exposure is the job of other government agencies, such as the FDA and the U.S. Environmental Protection Agency, Portier said.

Brenner said that making a statement of carcinogenic risk without reference to dose is vague and misleading.

Virtually every group worldwide lists ionizing radiation as a known human carcinogen, Portier said. In 1999, the World Health Organization's International Agency for Research on Cancer published a monograph detailing the carcinogenic evidence of x-rays and gamma rays. The International Commission on Radiological Protection long ago established the three principles of dose: justification, optimization, and limitation. The FDA in the last several years has cautioned against receiving medically unnecessary x-ray procedures.

Ionizing radiation had been an obvious omission from the toxicology report for several years, Portier said. This was more as a matter of cost than evidence. The NTP can add only six to eight new agents per year, and each agent is subject to a rigorous two-and-a-half-year review process.

"We believed that ionizing radiation was so well known as a carcinogen that we decided early on to concentrate on other less well-known agents," he said. "But the time had come to finally include it."

The NTP is located at the National Institute of Environmental Health Sciences (NIEHS), which is part of the National Institutes of Health. Most nominations for agents to be added to the list come from within the various governmental bodies associated with the NTP, including the Centers for Disease Control and Prevention, FDA, EPA, and Occupational and Safety Health Administration. Nominations can also come from individuals.

The nomination for x-rays came from within the NIEHS, Portier said. Panels that discuss the nominations consist of scientists who are toxicologists, exposure experts, medical physicians, public health experts, epidemiologists, statisticians, and physicists. The panel that deliberated the final report on carcinogens was composed mostly of epidemiologists.

"The human evidence is very important," Portier said.

Four other agents were also added. For the first time, viruses made the list of known human carcinogens: hepatitis B and C (liver cancer), and some human papillomaviruses that cause common sexually transmitted diseases (cervical cancer in women). Neutron radiation causes genetic damage similar to that of x- and gamma radiation and thus can cause the same cancers, according to the report.

Eleven substances that can be reasonably anticipated to cause cancer have been added as well. They include compounds found in grilled meats and a host of substances used in textile dyes, paints, and inks. The NTP report now contains 58 known human carcinogens and 188 "reasonably anticipated" carcinogens.

The full report is available at the NTP Web site National Toxicology Program.

For more information from the Diagnostic Imaging archives:

Radiation dose challenges PE diagnosis in women

Dose concerns limit trauma CT in pediatrics

Italian researchers advocate putting the brakes on x-ray dose

Comparative global study quantifies x-ray cancer risk

Are radiologists guilty of killing their patients?

Study challenges eye safety guidelines for IRs

Infant radiation dose poses threat to cognitive growth

Wide dose variation in CT scanners needs assessment