In an effort to quantify those risks, David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc., of Columbia University, cited studies indicating that 0.4% of all cancers in the U.S. might be attributed to radiation exposure. However, they stated in the Nov. 29 issue of the New England Journal of Medicine, those estimates were based on CT-scan volume in 1991 and 1996.
Extrapolating those results to current CT use, the authors said, the current proportion of cancers attributable to CT-associated radiation could be as great as 1.5% to 2%.
Of particular concern, they said, is increased use of CT in pediatric diagnosis, which has exposed children to adult radiation protocols that only recently were modified for pediatric applications.
The cancer risk posed by CT-associated radiation might not be fully appreciated by health care professionals or the public, they said.
They pointed out that "part of the issue is that physicians often view CT studies in the same light as other radiologic procedures, even though radiation doses are typically much higher with CT than with other radiologic procedures.
In a recent survey of radiologists and emergency room physicians, they said, "about 75% of the group significantly underestimated the radiation dose from a CT scan, and 53% of radiologists and 91% of emergency room physicians did not believe that CT scans increased the lifetime risk of cancer."
"Although the individual risk estimates â€¦ are small, the concern about the risks from CT is related to the rapid increase in its use -- small individual risks applied to an increasingly large population may create a public health issue some years in the future," Drs. Brenner and Hall stated.
Fueled by advances that have made the technology more accessible, the estimated number of CT scans in the United States has increased from three million in 1980 to 62 million, the authors noted. The total scan volume includes at least four million studies in children.
CT involves larger radiation doses than conventional X-ray imaging. For example, a conventional abdominal X-ray delivers a radiation dose of 0.25 mSv (or mGy) to the stomach; an abdominal CT exposes the stomach to a dose of 10 mSv. That dose increases to 20 mSv with neonatal abdominal CT.
As the radiation dose increases, so do the risks of radiation-induced DNA damage, the authors said.
"Most radiation-induced damage is rapidly repaired by various systems within the cell, but DNA double-strand breaks are less easily repaired, and occasional misrepair can lead to induction of point mutations, chromosomal translocations, and gene fusions, all of which are linked to the induction of cancer," the authors stated.
Epidemiologic studies have provided evidence suggesting an increased cancer risk from radiation doses corresponding to common CT protocols. Studies of atomic-bomb survivors in Japan revealed a significant increase in overall cancer risk in the subgroup of survivors exposed to low doses of radiation in the range of 5 to 150 mSv. The mean dose exposure in the subgroup was 40 mSv, similar to the relevant organ dose of a typical adult CT involving two or three scans.
A study of workers in the nuclear energy industry demonstrated an increased risk of cancer mortality in workers exposed to an average radiation dose of about 20 mSv. Consistent with the study of atomic-bomb survivors, the increased cancer mortality risk remained significant across the range of radiation exposures of 5 to 150 mSv.
"The situation is even clearer for children, who are at greater risk than adults from a given dose of radiation, both because they are inherently more radiosensitive and because they have more remaining years of life during which a radiation-induced cancer could develop," the authors noted.
"Despite the fact that most diagnostic CT scans are associated with very favorable ratios of benefit to risk, there is a strong case to be made that too many CT studies are being performed in the United States," the authors concluded. "There is considerable literature questioning the use of CT, or the use of multiple CT scans, in a variety of contexts, including management of blunt trauma, seizures, and chronic headaches, and particularly questioning its use as a primary diagnostic tool for acute appendicitis in children."
Beyond these clinical issues, other problems arise when CT scans are used to practice defensive medicine or when an initially justified CT scan is repeated as a patient passes through the medical system, often because of a lack of communication, they added. Still another problem pertains to widespread underestimation of CT-related radiation doses by physicians.
To minimize the CT-associated radiation dose and risk, Drs. Brenner and Hall offer three suggestions:
- Reduce the CT-related dose in individual patients by use of automatic exposure control and other technologic developments.
- Replace CT with other imaging modalities when appropriate.
- Decrease the number of prescribed CT studies.
|The authors reported no relevant conflicts. Dr. Brenner disclosed grant support from the National Cancer Institute and National Institute of Allergy and Infectious Diseases, and Dr. Hall disclosed grant support from the Department of Energy.|
Zalman S. Agus, MD Emeritus Professor
University of Pennsylvania School of Medicine
New England Journal of MedicineSource Reference: Brenner DJ, Hall EJ, "Computed tomography -- an increasing source of radiation exposure" N Engl J Med 2007; 357: 2277-2284.