To Scan or Not to Scan: Largest Study to Date Links Childhood CTs to Increased Cancer Risk

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CT scans can reveal a remarkable amount of useful medical information, but more studies underscore the need to balance the benefits of that knowledge with the risks of being exposed to the scans’ small amounts of radiation.

Being able to peer into the human body without making a single incision is undeniably a major advance of modern medicine, and doctors and patients are taking full advantage of the latest techniques to scan our internal organs with CT, MRI, ultrasound and more. But as more studies show, that ability comes with a price.

Between 1996 and 2010, according to a study published in 2012, the use of computed tomography (CT) scans nearly tripled, from 52 scans per 1,000 patients to 149 scans per 1,000 patients. And those images certainly provided valuable information to help doctors diagnose internal injuries, infections and other hidden conditions that wouldn’t have been possible without the added insight. But CT scans generate those detailed images using small amounts of radiation, and while the absolute risk of that exposure is small, on a population level, increased reliance on the scans could drive a slight bump in cancer risk from CT.

That risk is especially concerning among children, for several reasons. Not only are their still-developing tissues more vulnerable to radiation’s effects, but they have more years to live after their exposure, which increases their chances of developing cancers related to the radiation they received.

That’s exactly what the latest study, published in the BMJ, found. It’s the largest study to date  — involving 10.9 million people born between 1985 and 2005. A team of researchers from Australia and Europe studied the cancer rates of Australian patients from birth to 19 years old who had CT scans in early childhood, and compared them to those who did not receive the scans.

(MORE: Too Many Scans? Use of CT Scans Triples, Study Finds)

Most of the cases were followed for 10 to 17 years, and by the end of the study period, 3,150 of the 680,000 patients exposed to CT scans during childhood and young adulthood developed cancer and 57,542 of the remaining participants, who were not exposed, developed cancer. After matching the cases for age, sex and birth year, those who were scanned had a 24% greater risk of cancer than those who did not receive them, with the risk increasing by 16% for each additional CT.

The risk was highest for children who received scans before age five; they showed a 35% increased chance of developing a cancer during the study period than those who weren’t scanned at this age, and brain cancers were the most common among youngsters who had the scans. Because the researchers assumed that children receiving brain scans might have needed the CTs because their doctors suspected a brain tumor, they re-analyzed the data excluding those who received brain CTs. Even after removing this information, the increased risk of all cancers remained higher among those who were scanned compared to those who were not.

How necessary are these scans? While this analysis did not delve into the reasons that the doctors ordered the CTs, previous research showed that many of the scans that are ordered, particularly for young children, may be more precautionary than necessary. And that raises concerns about the risks that the latest study measured. Describing the study that found rates of CT use have tripled in recent years, TIME’s Alice Park wrote:

So why the soaring rates? Part of the reason is medical: advanced imaging tests are a powerful way to diagnose internal injuries following a major trauma or accident without having to do surgery. The scans can also identify hidden causes of illness, such as infections in the abdomen that cause patients vague pain that isn’t obviously triggered by anything the physician can see.

Another powerful reason: the fear of malpractice lawsuits. Because CT scanning technology is ubiquitous, and because it is so effective, many doctors order the tests as a fail-safe, to protect themselves in case they might have missed some potentially life-threatening condition. In fact, many patients come to the doctor expecting to be scanned, and feel they haven’t been provided adequate care if their doctors don’t order a CT.

“We’ve lowered our threshold, so now we use [CT] so frequently and for any reason that we probably tipped the balance toward using it more than is clinically indicated or necessary,” says [Dr. Rebecca] Smith-Bindman, [the study’s lead author and a professor of radiology and biomedical imaging at the University of California San Francisco]. It’s often easier for a doctor to order an imaging test than it is to spend more time with the patient discussing his or her symptoms to determine whether the test is really necessary, she says.

Most children, for example, get CT scans after a fall or to rule out abdominal infections, and experts say other tests, such as ultrasound, could provide the same information to make these diagnoses, but without the radiation exposure. A 2011 study published in the Archives of Pediatric & Adolescent Medicine reported that CT scans made up 12% of the imaging received among a population of kids enrolled in a large health care system between the years 2005 and 2007. But a UK study found that reducing such scans should be a priority for children, since those who had two to three scans in childhood tripled their risk of later developing brain tumors.
That study, however, also underscored the utility of CT in making critical diagnoses that could save children’s lives. As Park wrote:

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 [Dr. Donald] Frush, [chief of pediatric radiology at Duke Medical Center].

(MORE: CT Scans in Childhood Can Triple the Risk of Cancer)

It’s also worth noting that the amount of radiation emitted by CT machines has dropped since the participants in the current study were imaged. Manufacturers have been modifying their devices to produce the sharpest pictures possible using the lowest amount of radiation, so most scanners now use up to 50% less ionizing radiation than those from a decade ago, and many centers that scan children now adjust doses for their smaller body size.

Researchers are even working on ways to counteract the cancer-causing effects of CTs. Scientists at the University of Toronto recently reported that a cocktail of antioxidants that patients could take before getting scanned could reduce the radiation damage from CTs by as much as 50%.

That research is still on-going, however, and in the meantime, doctors and patients are left to find the delicate balance between how necessary scans are to diagnose disease, and their potential risk of causing cancer, which, most experts say, is still very small. Being aware of this risk, however, as confirmed by a large study such as this one from Australia, should help to inform that decision. “All parties, including patients and families, need to work together to ensure that CT scans are limited to situations where there is a definite clinical indication, and where every scan is optimised to provide a diagnostic CT image at the lowest possible radiation dose,” the authors write. Only then will the benefits, they say, outweigh the risks of exposure.