Investigators reported that CTs elevated the risk of brain cancer and leukemia. But because these cancers are relatively rare, the increase in the number of cases remains small, with no more than one excess brain tumor and one excess case of leukemia expected per 10,000 head CT scans for each 10 milligray.
EXPERTS urge judicious use of CT and recommend dose-reduction strategies.
CT scans cause “excess” cases of leukemia and brain tumors, according to an Aug. 4 study in The Lancet, but the risk detected is small, and recent reductions in radiation exposure have undoubtedly lowered the risk even more, experts say.
“Radiation doses today are much lower,” said James H. Thrall, MD, radiologist-in-chief at Massachusetts General Hospital and the Juan M. Taveras professor of radiology at Harvard Medical School, who was not involved with the study. “The children in this study were scanned as far back as the middle 1980s, so it wouldn't surprise me if they got twice as much radiation as they would get today. One of the shortcomings of the paper is there was no ability of investigators to go back and see how much radiation the children actually got,” he said.
The study, led by researchers at the National Cancer Institute and the Institute of Health and Society at Newcastle University in England, examined rates of brain cancer and leukemia in people younger than 22 who received CT scans between 1985 and 2002 in Great Britain. Seventy-four out of 178,604 of them developed leukemia, and 135 out of 176,587 developed a brain tumor.
The authors concluded that a dose of about 60 milligray (mGy) — about two or three CT scans — might triple the risk of brain cancer, while a dose of about 50 mGy might triple the risk of leukemia. However, because these cancers are relatively rare, the increase in the number of cases remains small, with no more than one excess brain tumor and one excess case of leukemia expected per 10,000 head CT scans for each 10 mGy. The US annual rates for brain tumor and leukemia through age 21 are 2.9 and 4.3 per 100,000 respectively, according the NIH.
The results for leukemia coincided with the Life Span Study, which estimates cancer risk from radiation exposure by extrapolating data from survivors of atomic bomb blasts in Hiroshima and Nagasaki. However, for brain tumors, The Lancet study risk estimate was about four times higher than the Life Span Study's estimate, with an excess relative risk of 0.023 per mGy vs. 0.0061. Some of the excess risk might be due to the inclusion of patients who had CT scans for symptoms of a brain tumor not yet detectable, the authors said. Also, X-radiation from CT scans can be more biologically active than the gamma rays produced by the atomic bombs.
In an accompanying editorial to The Lancet study, Andrew J. Einstein, Victoria and Esther Aboodi assistant professor of medicine (in radiology) at Columbia University, wrote that the cancer risks of low-dose medical radiation can be minimized with the help of new technology and techniques. He encourages clinicians to adopt the “three As” — awareness (of radiation risks vs. benefits), appropriateness of each procedure, and audit of the imaging process to make sure it meets the standards of good practice.
“Pearce and colleagues confirm that CT scans almost certainly produce a small cancer risk,” Dr. Einstein wrote in the editorial. “Use of CT scans continues to rise, generally with good clinical reasons, so we must redouble our efforts to justify and optimise every CT scan.”
An editorial in the August issue of Radiology argues that the risks from ionizing radiation are often “sensationalized in electronic and print public media, resulting in anxiety and fear about medical imaging among patients and parents.”
RADIATION DOSE COMPARISON
One of the editorial's authors, William R. Hendee, PhD, former dean of the Graduate School of Biomedical Sciences at the Medical College of Wisconsin, praised the authors of The Lancet study for not making the type of speculative estimates of cancer risks from radiation often found in the literature and in the lay press. “They simply looked at leukemia and brain cancer in a population of individuals exposed to radiation doses from CT procedures as children and compared them to a cohort population that did not receive CT procedures,” Dr. Hendee told Neurology Today. “There's no speculation there.”
Dr. Hendee believes the benefits of CT scans far outweigh the risks, especially when newer machines are used. And even though the use of CT scans is increasing, he believes the risk can be reduced by using the most up-to-date technology, adopting protocols that minimize exposure, and avoiding over-utilization.
“We discovered a few years ago that some hospitals were using protocols for CT procedures that were not suitable for children,” he said. “They were adult protocols. So a campaign called Image Gently was launched to right-size the dose, and it has dramatically reduced doses.”
The Image Gently campaign was launched in 2007 as an initiative of the Alliance for Radiation Safety in Pediatric Imaging with the goal of increasing awareness of radiation risks in order to promote radiation protection in the imaging of children. “We've had a tremendous response,” said Image Gently founder Marilyn Goske, MD, the Corning Benton Endowed Chair for Radiology Education and staff radiologist at Cincinnati Children's Hospital Medical Center. “Over 72 organizations, many international, have signed on and joined the Image Gently alliance to raise awareness for child-sizing radiation dose for children. People are not only hearing the message, but acting on it.”
DR. JAMES H. THRALL: “If the numbers reported in The Lancet study are right, and that's open to discussion, I believe we'd see an excess cancer from CT scans once every 50 years or so.”
The NIH National Institute of Biomedical Imaging and Bioengineering held a summit last year on managing radiation dose in CT. In a consensus statement published in August in Radiology, the authors list various dose-reduction strategies including more sensitive radiation detectors and better image reconstruction that allow satisfactory results with less radiation. “Together, these existing techniques can reduce dose by a factor of two to four,” the authors state. Other promising dose-reduction advances may soon lead to “effective doses of less than 1 mSv (millisievert), which is well below the average annual dose from naturally occurring sources of radiation.” [In the case of x-rays, 1 mSV is equivalent to 1 mGY.]
Dr. Thrall, who wrote an editorial in the August issue of Radiology, believes that dose-reduction strategies will indeed soon bring CT scans down to a submillisievert level for all but the very largest patients. “We are substantially there at Massachusetts General Hospital,” he said. “If the numbers reported in The Lancet study are right, and that's open to discussion, I believe we'd see an excess cancer from CT scans once every 50 years or so.”
CT scans and other exposure to radiation in children have been on the decline at Boston Children's Hospital, according to Tina Young Poussaint, MD, professor of radiology at Harvard Medical School and director of the PBTC Neuroimaging Center at Boston Children's. “In general there has been an increase in the number of medical images acquired,” she said, “but in recent years I've seen a decrease in the volume of CT scans of children because we are trying to minimize their exposure to radiation, and there will probably be a greater diminution now that this evidence (in The Lancet) has been brought out.”
DR. WILLIAM R. HENDEE: “We discovered a few years ago that some hospitals were using protocols for CT procedures that were not suitable for children. They were adult protocols. So a campaign called Image Gently was launched to right-size the dose, and it has dramatically reduced doses.”
The dangers of radiation reported in The Lancet echo results reported in the current issue of Cancer from what is believed to be the largest case-control study to date examining the correlation between dental x-rays and the risk of meningioma. The study compared 1,433 patients with intracranial meningioma diagnosed between the ages of 20 and 79 to 1,350 controls, and found that the cancer patients were more than twice as likely to report receiving dental X-rays. The results also show that patients who reported having annual “bitewing” X-rays have a 40–90 percent greater risk of meningioma. The risk was higher in people who received panoramic X-rays when they were younger than 10.
“Our findings suggest that dental x-rays, particularly when obtained frequently and at a young age, may be associated with an increased risk of intracranial meningioma,” the authors stated.
The authors of the Cancer study reaffirm the American Dental Association's statement discouraging the use of dental x-rays to search for occult disease in asymptomatic patients, or to obtain routine dental studies from all patients at pre-set intervals. (The American Dental Association recommends dental X-rays for teenagers and adults every 18 months to three years.) The lead researcher, Elizabeth B. Claus, MD, PhD, of Yale University School of Medicine and Boston's Brigham and Women's Hospital, also pointed out that the X-rays reported may have been made with higher levels of ionizing radiation than those used today.
In response to The Lancet study on CT radiation exposure, the Society for Pediatric Radiology issued a statement encouraging patients and parents of children facing a CT scan to question the necessity of the scan, to inquire about alternatives, and to make sure the techniques used will minimize radiation exposure.
James H. Thrall, MD, radiologist-in-chief at Massachusetts General Hospital and the Juan M. Taveras professor of radiology at Harvard Medical School, agrees with that advice. “I urge people to challenge their doctor,” he said. “Quite honestly, asking, ‘Does my child really need this scan?’ could lead to not doing the scan, or deferring the scan for a period of time.”
But Massachusetts General Hospital has pioneered another strategy that frequently causes doctors themselves to question the need for images that require radiation exposure.
MGH's clinical decision-support system is integrated with computerized physician order-entry software that requires physicians to provide reasons for each radiological procedure, and then provides a “utility score” of high, medium, or low.
From 2004 through 2007, with the software active, the number of CT scans dropped by 25 percent.
“When a pediatrician or neurologist orders a scan they must give a reason for the scan,” said Dr. Thrall. “That allows us to compare reasons with all evidence in medical literature about likely value of the desired information. We score on 9-point scale that gives attending physician what I call just-in-time knowledge delivery at the point of care. It's sort of a final filter on whether scan is necessary or not.”
Physicians who get a score indicating a low likelihood that a CT scan will provide useful information frequently cancel the request, according to Dr. Thrall.
“Knowing they're going to go through filter is itself an intervention,” he said. “They know their reasoning is going to be tested, and if it's shaky they don't want to put it in the system.”
The number of CT scans performed had been growing at about 12 percent a year at MGH before the system was introduced. “After we put in this system that flattened out completely,” Dr. Thrall said. “Actually, since the number of patients has gone up, the number of scans per patient has gone down.”
• Claus EB, Calvocoressi L, Wrensch M, et al. Dental x-rays and risk of meningioma. 2012; E-pub 2012 Apr 10.
• Einstein AJ. Beyond the bombs: Cancer risks of low-dose medical radiation. 2012; E-pub 2012 Jun 7.
• Thrall JH. Radiation exposure in CT scanning and risk: Where are we? 2012;264:325–328.