RSNA News - March 2005

Medical X-Rays Added to Government's Carcinogen List

It would be a tragedy if someone did not have a medical imaging procedure that might save his or her life or alter the course of treatment solely because of concern over the effects of radiation.
—G. Donald Frey, Ph.D.

Courtesy of the Uranium Information Centre Ltd.

It came as no great surprise to radiologists, medical physicists and other members of the radiologic community when, in late January, ionizing radiation was added to the Department of Health and Human Service's (HHS) list of carcinogens.

The 11th Report on Carcinogens (RoC) added six substances under the category "known human carcinogens" and 11 under the category "reasonably anticipated to be human carcinogens." For the first time, the "known" list includes viruses—hepatitis B virus, hepatitis C virus and some human papillomaviruses. It also includes x-radiation, gamma-radiation and neutrons. Lead and lead compounds, compounds found in grilled meats, and a host of substances used in textile dyes, paints and inks were added to the "reasonably anticipated" list.

RSNA issued a press release within two hours of the report's release clarifying for the public the benefits of medical x-rays. The Society also sent an E-News Alert to members.

The RoC, a biennial report prepared for HHS by the National Toxicology Program (NTP) at the National Institute of Environmental Health Sciences, said x-radiation and gamma-radiation were listed "because human studies show that exposure to these kinds of radiation causes many types of cancer including leukemia and cancers of the thyroid, breast and lung."

The report added that exposure to x-radiation and gamma radiation has been shown to cause cancer of the salivary glands, stomach, colon, bladder, ovaries, central nervous system and skin. It also stated that 55 percent of worldwide exposure is from low-dose medical diagnosis such as bone, chest and dental x-rays.

NTP Associate Director Christopher Portier, Ph.D., said the decision to list ionizing radiation as a carcinogen follows valid research. "We no longer rely on atom bomb survivors for our data on the effects of ionizing radiation," he said. "There's a lot of mechanistic work that has been done. There have been follow-up studies on animals that match what we've seen in Hiroshima and Nagasaki. There have also been a number of studies since the early days of x-ray use for medical diagnosis that looked at radiologists and radiologic technologists and their cancer risk from radiation exposure."

While Dr. Portier wants the public to be aware that ionizing radiation carries some risk, he said that the benefits of medical care and intervention outweigh those risks.

For decades, physicians and medical physicists have been concerned about the risks of radiation exposure, said G. Donald Frey, Ph.D., a professor of radiology at the Medical University of South Carolina and immediate past-president of the American Association of Physicists in Medicine, "The crucial thing for people to understand is that it would be a tragedy if someone did not have a medical imaging procedure that might save his or her life or alter the course of treatment solely because of concern over the effects of radiation."

What Radiologists Can Do

"First," said Dr. Frey, "radiologists should be aware of the amount of radiation that is given for higher-dose procedures, such as CT, and then they should be prepared to explain to the public the value of the procedures and what protective measures they are taking."

Donald P. Frush, M.D., chief of pediatric radiology at Duke University Medical Center, said radiologists also need to be aware that the research data linking cancer to low-levels of ionizing radiation—levels produced by the majority of medical procedures—are debatable. "It really comes down to no risk versus potential risk," he said. "Researchers are trying to figure out if there's a linear dose response with no threshold for the development of cancer or if there's a threshold for the development of cancer. In my opinion, and most physicians and scientists who are familiar with this topic will agree, patient safety always comes first and we are vigilant to avoid unnecessary radiation."

Dr. Frush said it is important that radiologists, through organizations such as RSNA, do everything possible to inform the public about this topic and use only the necessary amount of radiation for medical imaging.

"Radiologists, by virtue of our training in physics, are really the gatekeepers with regard to optimal use of the technology," he said. "We also have the responsibility to educate other physicians—those who order the examinations—including emergency room physicians, pediatricians, surgeons, internists and cardiologists. They are not always aware of the technical side of examinations that utilize radiation and don't always understand the potential risks."

The National Council on Radiation Protection and Measurements (NCRP) is taking the RoC in stride. NCRP Executive Director David A. Schauer, Sc.D., said the report is consistent with the radiation protection programs already in place worldwide as represented by the work of NCRP and the International Commission on Radiological Protection.

NCRP has been active in the areas of radiation protection and measurements since its inception in 1929. "Overall, NCRP agrees with the assessment that ionizing radiation is a known carcinogen," said Dr. Schauer. "However, we feel that the medical uses of radiation must be put into a benefit versus risk context, while bearing in mind that the patient voluntarily submits to these medical procedures under conventional rules of informed consent."

Dr. Frey recommends that patients ask questions, seek out accredited sites and review the information on RadiologyInfo.org, the patient education Web site developed by RSNA and the American College of Radiology that offers quick access to easy-to-understand information about radiology procedures and treatments. It describes what to expect, what the equipment is going to look like, what a patient will feel, what preparations would be appropriate, why the procedure is being done and what risks may be involved.