The addition of x-rays and gamma rays to a national list of carcinogens has prompted some concern among radiology professionals who worry that the inclusion could unnecessarily deter patients from undergoing diagnostic tests.
Three types of ionizing radiation—x-rays, gamma rays, and neutrons—were labeled as known carcinogens in the National Toxicology Program's “11th Report on Carcinogens.”
“This is certainly not a surprise to anyone in this field,” said Richard L. Morin, Ph.D., chairman of the American College of Radiology's (ACR) commission on medical physics.
The potential health effects of ionizing radiation have been acknowledged for more than 50 years. A number of agencies in the United States and worldwide—the Environmental Protection Agency, the Nuclear Regulatory Commission, and the World Health Organization—already recognize ionizing radiation as carcinogenic.
Yet the potential for this latest document to be sensationalized is of concern, particularly because it's not clear how much ionizing radiation can potentially lead to cancer. Some researchers contend that there is a risk associated with x-ray or gamma ray exposure at any level. However, this is “somewhat controversial at the low levels that we're talking about” in the medical setting, Dr. Morin noted.
It's well established that at very high levels, x-rays and gamma rays are carcinogens. But when x-rays and gamma rays are used for diagnosis, “the levels are very significantly less than in any studies in which cancers have been produced,” Dr. Morin added.
“The report could lead patients to mistakenly believe that they are being placed at undue risk by undergoing a [radiologic] procedure, and cause many, who may desperately need care, to avoid seeking appropriate medical attention,” James Borgstede, M.D., chairman of the ACR Board of Chancellors, said in a statement.
When radiation exposure is performed appropriately, its benefits outweigh any accompanying risk. In addition, the total exposure is optimized to be as low as is reasonably achievable, David A. Schauer, Ph.D., executive director of the National Council on Radiation Protection and Measurements, wrote in an e-mail.
Advising a patient who has concerns about the cancer risk from x-ray or gamma ray procedures should include a discussion of the genuine need for such a diagnostic test and the real risks of not correctly diagnosing a condition, Dr. Morin said.
Without the diagnostic information provided by x-rays and other imaging tests, “there are only two other options,” Dr. Morin said. “One is to do nothing and wait and see if the patient gets worse.” The other is to do an exploratory surgery. “Clearly the risk associated with exploratory surgery is greater than the risk of diagnostic imaging,” he said.
The National Toxicology Program's latest report emphasizes that the listing identifies potential cancer hazards but does not establish that a substance presents a cancer risk to an individual in daily life. The report also does not attempt to weigh the potential benefits of exposure to certain carcinogenic substances in special situations, such as diagnostic testing. Nor does the report address acceptable dose ranges for diagnostic procedures.
The annual limit on public exposure from a single source of ionizing radiation is 100 mrem (1 mSv), both in the United States and internationally.
Medical applications are excluded from this limit in the United States. With the exception of mammography, there are no nationally set limits on radiation exposure. Mammography has an established maximum exposure limit of 300 mrem (3 mSv).
In perspective, the average person in the United States is exposed to about 360 mrem/yr (3.6 mSv/yr) from all sources of radiation, including cosmic and natural background radiation.
Radiation exposure from a medical procedure is generally minimal in terms of the biologic risk of developing cancer, Dr. Morin said. “Of all the risks there are in life to the patient, this is a very low one.”
Patient information about radiology topics is available at www.radiologyinfo.org
Viruses Make List For First Time; Hepatitis Added
For the first time, a national list of known carcinogens includes several viruses.
Hepatitis B and C viruses and certain human papillomaviruses are listed as carcinogens in the National Toxicology Program's recently released “11th Report on Carcinogens.” The report identifies agents that are known—or are reasonably expected—to cause cancer. The report is published every other year.
The report identifies only potential cancer hazards. It does not establish that a substance presents a substantial cancer risk to an individual in daily life.
The inclusion of the hepatitis B and C viruses was based on epidemiologic studies that have demonstrated that infections with either of these viruses can lead to liver cancer.