CT Scans in Childhood Can Triple the Risk of Cancer

Children who underwent multiple CT scans had three times the risk of brain cancer and leukemia later on. Overall, however, researchers say the risk of cancer is still small, while getting a necessary scan can potentially save a child's life.

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Having multiple CT scans in childhood may triple the risk of certain cancers, according to a new study.

In the first study of its kind, researchers from the U.S., U.K. and Canada worked for nearly 20 years, tracking cancer rates among children who had had CT scans during their first 15 years of life and comparing them to children who did not have the same exposure to radiation from the scans. Researchers found that children who had had two to three CT scans in childhood had triple the risk of later developing brain tumors, and children who had had five to 10 scans also had three times the risk of developing leukemia, a blood cancer.

“We need to make sure that everyone understands that yes, we’ve shown a significantly increased risk of cancer,” says the study’s lead author, Mark Pearce, a senior researcher at the Institute of Health and Society at Newcastle University in the U.K., but adds that “the absolute risks are small.”

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Pearce and his colleagues relied on health data collected by hopsital radiology departments and the National Health Service in the U.K. They looked at 180,000 patients who had had at least one CT scan between 1985 and 2002, when they were younger than 22. Based on the number and types of scans that patients received, according to hospital records, the researchers estimated the dose of radiation — in milli-Grays (mGy) — absorbed by the brain and bone marrow of each patient. Then, the researchers tracked rates of cancer reported in the National Health Service registry between 1985 and 2008.

To ensure that pre-existing conditions did not confound the results, the researchers excluded patients who had been diagnosed with cancer before their scans were taken. That left a population of otherwise healthy children who received scans for other reasons, the most common being trauma to the head from a fall or accident and infections of the abdomen, including appendicitis, that needed to be diagnosed with imaging.

Among 176,587 patients, a total of 135 were diagnosed with brain cancer, and 74 out of 178,604 patients developed leukemia. Compared with people who had received a radiation dose of less than 5 mGy in childhood, patients who received average cumulative doses of 60 mGy to the head (two to three head CTs) were three times more likely to develop brain cancer. Those receiving a cumulative dose of 50 mGy on average (about five to 10 scans) had triple the risk of leukemia. The reason for the differences in number of scans relates to the fact that radiation doses differ depending on the organ scanned, and brain CTs typically involve more intense radiation than that required to image bone marrow, says Pearce.

(MORE: Use of CT Scans Soars in Children, Raising Concerns Over Radiation)

According to a recent survey by a medical imaging marketing research firm, 85.3 million CT scans were performed in the U.S. in 2011, 4% more than the previous year. CT scanning rates among children are also relatively high; in a study published in the Archives of Pediatric & Adolescent Medicine in May, researchers reported that among children enrolled in a large health care system between 2005 and 2007, CT scans accounted for 12% of all imaging that youngsters received.

But while the current study, published in the Lancet, raises concerns about the risks of CT scans, experts say parents should not assume that having their child scanned will put his or her health at risk. First of all, the doses of radiation absorbed from CT scans have dropped significantly in the last two decades since the study participants had their scans, says Dr. Donald Frush, chief of pediatric radiology at Duke Medical Center, who was not involved in the study. Already, most machines currently use up to 50% less ionizing radiation than devices from a decade ago. In addition, says Pearce, many of the hospitals where children were scanned in the 1980s may not have had machines that were capable of adjusting radiation doses for smaller children, as they do now.

Second, while CT scans, like all medical interventions, carry with them a certain risk, that risk must be weighed against the benefits of getting the scan. Most children receive CT scans after an accident or trauma, primarily to the head, to rule out potentially serious problems like internal bleeding that may require immediate surgery. In a previous study to track the benefit that CT scans conferred in treating children with head injury, 2% showed a positive result that required some type of additional treatment. That means that 1 in 50 children benefited from the head CT, compared with the 1 in 10,000 who might develop brain cancer or leukemia from the same scan, according to calculations in the latest study. “That’s what people need to realize, that there is a 200-fold difference between the benefit of a scan and the risk,” says Frush.

(MORE: Physicians Groups Call for Fewer Tests for Patients)

Still, there is growing concern that the proliferation of imaging machines, from CT scanners to MRI and ultrasound devices, is leading to their overuse, and that overuse of CT scans in particular, which involve ionizing radiation (MRI and ultrasound do not), could also be contributing to cases of avoidable cancers.

“I think the fear of litigation drives a lot of imaging that is done,” says Dr. George Bisset, president of the Radiological Society of North America and chief of pediatric radiology at Texas Children’s Hospital. When parents bring to the ER a young child who has hit his head in a fall, it’s hard for a doctor not to order a CT scan of the brain to make sure that nothing more serious than a bump has occurred.

The latest study highlights why doctors should reconsider that decision. The authors caution physicians to perform scans only when medically justified. “CT scans have relatively high doses, particularly compared to X-rays, of radiation — about 10 times the dose from an X-ray,” Pearce said during a press briefing. “And with increasing usage of CT scans around the world, it raised concerns about whether more needs to be done to assess the safety and particularly the justification of their use.”

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For parents worried about their children getting CT scans, Bisset says there are several things they can do minimize the risk. First, ask the doctor if there is an alternative to the CT scan that can answer whatever medical question he or she is trying to resolve; in potential cases of appendicitis, for example, some facilities can use ultrasound instead of a CT scan to image the abdomen. Second, ask whether the facility adjusts doses of radiation for children; smaller children don’t need to be exposed to as much radiation as adults to produce a high-quality image.

“What we definitely don’t want is parents saying, No, I don’t want my child to have a CT scan, when he absolutely needs it,” says Pearce. “The risks are small. Although there is a tripling of risk, we are talking about a tripling of something small, and three times something small is still something small. We just need to look into making sure the scans are justified.” In most cases, he says, the scans can save children’s lives.

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Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.