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Study Finds Small Increase in Cancer Risk after Childhood CT Scans

A new study shows that radiation exposure from computed tomography scans (CT scans) in childhood results in very small but increased risks of leukemia and brain tumors in the first decade after exposure. The study, led by researchers from NCI and Newcastle University in the United Kingdom, is the first analysis of CT scans that looked directly at exposed patients instead of using models based on atomic bomb survivors or other populations accidentally exposed to large doses of radiation.

The findings were published June 6, 2012, in The Lancet.

CT scans remain an invaluable diagnostic technique, and the absolute risks are small, the study authors emphasized. "If providing a CT scan is clinically justified, and performed using the appropriate dose, the benefits from the scan should far outweigh any of these small cancer risks," said Dr. Amy Berrington de González, a senior investigator in NCI's Radiation Epidemiology Branch and senior author of the new cohort study.

For the study, researchers led by Dr. Mark Pearce of Newcastle University's Institute of Health and Society used data collected between 1985 and 2002 from more than 175,000 patients in the United Kingdom who had at least one CT scan before the age of 22. The researchers reviewed records from radiology departments of hospitals within the U.K. National Health Service and cross-referenced them with the National Health Service Central Registry, which provides information on cancer incidence and mortality.

The researchers focused on leukemia and brain tumors because bone marrow and the brain are particularly sensitive to radiation. Children are also more sensitive to radiation than adults because of their rapidly dividing cells and growing bodies.

Because different types of CT scans (for example, CT scans of the head or abdomen) deliver different and nonuniform radiation doses to the organs of the body, the researchers assessed the risk of leukemia and brain tumors in relation to estimated doses absorbed by the bone marrow and brain for each type of CT scan.

Other factors can affect the estimated absorbed dose, including a patient's age and sex. The year the CT scan was performed can also affect the estimated absorbed dose, because the radiation doses used in CT have fallen since the procedure was introduced to the clinic in the 1980s.

Taking all of these factors into account, the analyses suggested that, using current CT scanner settings, the cumulative dose from two to three head CT scans could roughly triple the risk of brain cancer, and the cumulative dose from five to ten head CT scans before the age of 15 could roughly triple the risk of leukemia.

The researchers concluded that although these risks may seem large, the absolute risks remain very small: about one extra case of leukemia and one extra brain tumor would occur for every 10,000 head CT scans during the first decade after exposure, according to the estimates.

"This is a large public health problem but a very, very small individual problem," commented Dr. Thomas Slovis, editor of the journal Pediatric Radiology and chief of radiology at Children's Hospital of Michigan, who was not involved in the study. Though parents may worry about the potential harms to a child of radiation exposure, "there's [also] harm from not making a diagnosis," he explained. "If CT is the best test for an indication, it absolutely should be done.

"It's only when the use is inappropriate or unnecessary that there is a risk without benefit," he added. Over the last 10 years, the medical community has been reducing the unnecessary use of medical radiation in children and tailoring imaging protocols to require the absolute minimum amount of radiation possible, continued Dr. Slovis. 

One of the largest educational and advocacy efforts, the Alliance for Radiation Safety in Pediatric Imaging's Image Gently campaign, now has 70 medical organizations participating, explained Dr. Donald Frush, a member of the alliance's steering committee and professor of radiology and pediatrics at Duke University.

Whether they work directly in imaging or not, "people are much more aware of the issue of radiation dose now than they were 10 years ago," Dr. Frush said. "We're beginning to have a much greater and more enduring impact on our technologists and trainees—our residents and our fellows—about this as well."

In addition to education, he added, significant improvements in the CT machines and in scanning protocols have allowed for reductions in the radiation dose required for a scan.

Medical experts are working to ensure that CT is used only when it is the best test to answer a given clinical question—and to understand when tests that don't require radiation may be a better choice. For example, the recent Choosing Wisely campaign, led by the American Board of Internal Medicine Foundation, included a recommendation to consider ultrasound before CT to diagnose appendicitis in children.

"If you do an unnecessary CT examination," said Dr. Frush, "it's an unjustifiable radiation exposure."