Wilhelm Conrad Roentgen’s discovery of x-rays in 1895 sparked great enthusiasm for the technology, and its ability to uncover mysteries of the human body that had been hidden for centuries. X-ray "portrait studios" and coin-operated "amusement machines" appeared at theme parks, and electricians offered "bone portraits" to the general public.
Now, more than 100 years later, the U.S. government has decided to investigate whether x-rays should be formally classified as carcinogens. To this end, it has asked the National Toxicology Program (NTP), a multiagency federal program centered at the National Institute of Environmental Health Sciences (NIEHS), to evaluate x-rays for its 11th Report on Carcinogens, to be published in the fall of 2004. The listing would formalize what has been known almost since the moment Roentgen made his discovery: In large doses, radiation is bad for the body.
The NTP has been investigating x-rays for more than a year, according to Chris Portier, Ph.D., associate director of the program. A panel of experts including 11 independent scientists, a cohort of NIEHS scientists, and scientists from more than eight government organizations (including the Environmental Protection Agency, the Food and Drug Administration, and the Occupational Safety and Health Administration) will convene this summer at a public meeting to vote on whether x-rays should be included in the report.
Why now? One theory is that recent interest in using whole-body CT scans as a screening tool has renewed concerns about x-ray exposure and cancer, prompting the government’s request. The FDA, for one, is known to be highly suspicious of CT screening. Other detractors of whole-body screening go so far as to compare screening centers to those early x-ray portrait studios that cropped up in the late 1800s.
"We’re not the first to review x-rays," Portier said. "Others have decided that they’re known human carcinogens. But controversy over the use of full-body CT scans for screening people without known illness has provoked this nomination."
Since radiation doses for CT procedures tend to be far higher than those received in conventional x-ray exams, such exposure could potentially contribute to the development of radiation-induced cancer later in life, according to the FDA. Effective doses from diagnostic CT procedures range from 1 to 10 millisieverts (mSv), not much less than the lowest doses of 5 to 20 mSv received by Japanese survivors of atomic bombs, who were shown to have a small but increased relative risk for cancer mortality due to radiation exposure.
A typical CT scan of the abdomen delivers 10 mSv of radiation, equivalent to 500 chest x-rays. It would take more than three years to absorb this amount of radiation from the natural background radiation people receive from the sun and the soil.
Radiologists and scientists alike agree that radiation in large doses causes cancer; the current controversy centers on what dose should be considered "acceptable" before cancer risk begins to increase.
Some experts ascribe to the linear no-threshold theory for radiation carcinogenesis, which holds that any radiation dose, no matter how small, can cause cancer, meaning that the risk is never zero. But others, including Bernard Cohen, Ph.D., from the department of physics at the University of Pittsburgh, have begun to question this theory, believing that the risk estimates it produces are overstated.
After reviewing numerous studies on the effects of low-level radiation, Cohen concluded that the risk of health effects with radiation doses below 10 rad are negligible; in fact, he presented studies indicating that low-level radiation may protect people against cancer (American Journal of Roentgenology, November 2002, Vol. 179:5, pp. 1137-1143).
When it comes to using radiation for diagnostic purposes, the key is to decide whether its use is justified, then optimize it, according to Dr. Fred Mettler, professor of radiology at the University of New Mexico in Albuquerque.
"For example, with mammography, most people think that it finds more cancers than it causes, so we say mammography is a good thing. But is it good for everyone? Probably not. Once you’ve generally justified (the use of diagnostic radiation), you have to justify it for the person in front of you. Then you try to just use enough radiation to get the images necessary," Mettler told AuntMinnie.com.
Others beg to differ. Classifying x-rays as carcinogens is not only misguided, but potentially harmful, according to medical physicist John Cameron, Ph.D., professor emeritus at the University of Wisconsin in Madison.
"The U.S. Department of Energy still promotes fear of radiation, even though data from the 100 years of British radiologists study published in the British Journal of Radiology in June 2001 strongly support the beneficial effects of moderate doses of ionizing radiation," Dr. Cameron told AuntMinnie.com (British Journal of Radiology, June 2001, Vol. 74:882, pp. 507-519).
It is certainly necessary to monitor and refine radiation doses to both patients and those who operate medical imaging devices, according to Kenneth Mossman, Ph.D., professor of health physics and director of the office of radiation safety at Arizona State University in Tempe. But he added that classifying x-rays as carcinogens will create unnecessary anxiety among patients.
"Public concerns and fears may be heightened by listing medical x-rays as a carcinogen," Mossman wrote in an e-mail to AuntMinnie.com. "One serious consequence is that patients may deny themselves important medical information by refusing x-ray tests that they think might cause cancer. If the NTP decides that a medical x-ray listing is appropriate, I hope they would list general types of procedures rather than lump everything together. I know of no evidence that would support listing mammography, chest, or dental x-rays, or other very low-dose procedures as carcinogenic to humans."
By Kate Madden Yee
AuntMinnie.com contributing writer
February 18, 2003
Related Reading
Studies cast doubt on low-level radiation dangers, January 30, 2003
Worrisome radiation dose seen in CT lung screening, follow-up, December 23, 2002
Limiting PET radiation exposure requires tools, techniques, October 25, 2002
Even moderate doses of radiation may cause nervous system tumors, October 16, 2002"
Whole-body CT: Radiology to the people?, October 2, 2002
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