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Children, CT Scans, and Cancer Risk

By: 
Renee Despres
Monday, February 20, 2017 - 05:24CT scanner

On June 6, 2012, a cross-Atlantic research team reported results of a large, retrospective cohort study, which suggest that children who have several computed tomography (CT) scans during childhood may be at greater risk of leukemia and brain cancer. However, the study authors observed, the influence of CT scans on the absolute risk of developing either condition is extremely small.

This is an important, well-designed study. Considering the widespread and increasing use of CT scans, any indication of risk associated with the scans is an important caution. Study results were published in the respected peer-reviewed journal The Lancet. Funding for the study came from the U.S.-based National  Cancer Institute (NCI) and the U.K. Department of Health.

It deserves the wide media attention it received. But that media attention, in most instances, could have been far more accurate.

News outlets broke the story with typical lack of nuance. “Children who are exposed to radiation from CT scans are at risk of developing brain cancers, according to a new study,” Medical News Today opened its article. “CT Scans Can Triple Risk of Brain Cancer,” trumpeted Fox News. “CT Scans in Kids Linked to Leukemia, Brain Cancer Risk,” offered ABC News. Even the usually judicious National Public Radio (NPR) ran a feature titled “CT Scans Boost Cancer Risk for Kids.”

Most media focused on the potential risk – real or imagined – of CT scans. For instance, ABC opened with the story of a 31-year-old female nurse who’s anxious about the long-term effects of a CT scan on her 1-year-old son. In most stories, the extremely small increase in absolute risk was mentioned only at the very end, often with a single quote from a risk assessment expert.

What the Researchers Did

The research team, which was led by Mark Pearce, M.D., Reader in Lifecourse Epidemiology at Newcastle University, conducted a retrospective cohort study – meaning they looked at medical records from a group of people with shared characteristics, then compared for particular outcomes. In this case, the “shared characteristic” was a history of having one or more CT scans during childhood or young adulthood; the outcomes were brain cancer or leukemia.

Pearce’s team examined the medical histories of nearly 180,000 people in the United Kingdom. All of the people had had CT scans between 1985 and 2002, when they were younger than 22 years of age. None had cancer before the CT scan. The CT scans had taken place in National Health Service (NHS) centers in England, Wales, or Scotland (Great Britain).

The research team determined how many CT scans each child had, then estimated the amount of radiation they had been exposed to via the scans. They estimated the amount of radiation dose absorbed by the bone marrow and brain by using a unit called the milliGray (mGy). A “Gray” is the absorption of one joule of ionizing radiation by one kilogram of matter (in this case, the human brain or bone). A milliGray is one-thousandth of that unit.

The researchers then compared their data on radiation exposure via CT scans with cancer and mortality data from the UK National Health Services Central Registry during the years 1985 to 2008.

What the Researchers Found

Out of 178,604 people, 74 had been diagnosed with leukemia, and 135 of 176,587 people with brain cancer. Investigators calculated that for every mGy received, the relative risk of brain tumor increased by 0.023 and that of leukemia by 0.036.

That relative risk translates into some impressive numbers: “Use of CT scans in children to deliver cumulative doses of about 50 mGy might almost triple the risk of leukemia and doses of about 60 mGy might triple the risk of brain cancer,” Pearce’s team wrote.

 Nice to Know:  Absolute Risk versus Relative Risk

Most media reports use relative risk to assess the impact of a change – positive or negative – on a person. But absolute risk is actually a better predictor of whether something will occur. When reporters cite only relative risk, they end up misportraying the real effects of an intervention.

Here's how to understand the difference between absolute and relative risk:

  • Absolute risk. Absolute risk is simply the probability of something occurring. In this study, Pearce's team found that exposure to significant amounts of radiation via CT scans increased the risk of leukemia and brain tumors. But even the threefold increase doesn’t make that much difference, because the incidence of both cancers is extremely low.
  • Relative risk. Relative risk is a way to compare the risks in two groups – in this case, people who had CT scans before age 22 years versus those who had not had scans. So, yes, relative risk means that having multiple CT scans before the age of 22 increases your risk of brain cancer from 6 per 100,000 to 10 per 100,000. But relative risk does not translate into actual risk.

 

While those numbers may sound frightening, they do not mean that every child who has had a CT scan is going to get caners. Because these cancers are relatively rare, the cumulative absolute risks are small: The researchers estimated that among children younger than 10 years who had CT scans, one excess case of leukemia and one excess case of brain tumor will occur per 10,000 CT scans of the head during the 10 years after the first scan.

Concern About Ionizing Radiation Exposure

Still, any increase in risk of a deadly disease – especially as the result of a screening test that’s supposed to help people – is disturbing. And while rates of brain cancer and leukemia are low, CT scans are ubiquitous. In the United States alone, about 5 million CT scans are performed on children each year.  About 65 million scans are performed on adults.  

The use of imaging tests has decreased since 2008, according to the American College of Radiology (ACR). But overall exposure to ionizing radiation is rising. In March 2009, the National Council on Radiation Protection & Measurements issued a report that suggests more people are being exposed to higher levels of ionizing radiation. One of the conclusions of the report, Ionizing Radiation Exposure of the Population of the United States, could be considered a wake-up call: The number of Americans exposed to ionizing radiation has nearly doubled over the past 20 years.

Children are especially vulnerable to the effects of radiation, and they are often exposed to relatively more radiation during scans. Because of children’s increased vulnerability, the U.S. Food and Drug Administration (FDA) is also acting to reduce children’s exposure to ionizing radiation through medical devices. Pearce’s study appeared only two weeks after the FDA published a draft guidance document with recommendations on how manufacturers should address their medical devices’ use on children. The FDA will hold a July 16 public workshop on the issue.

 “The cancer risk per unit dose of ionizing radiation is generally higher for younger patients than adults, and younger patients have a longer lifetime for the effects of radiation exposure to manifest,” FDA officials said in a press release.

Use CT Scans Judiciously

Not surprisingly, Pearce’s research, combined with the FDA emphasis on reducing ionizing radiation exposure in children, has many parents wondering just how risky CT scans are – and whether they should shun them for their children. But all three camps – the researchers, FDA, and the American College of Radiology, agree: CT scans play an important role in health care but need to be used judiciously.

"CT scans are accurate and fast so they should be used when their immediate benefits outweigh the long-term risks,” Pearce said in a press release. “However, now we have shown that CT scans increase the risk of cancer, we must ensure that when they are used they are fully justified from a clinical perspective."

 Likewise, FDA officials contextualize their concerns about increased exposure to ionizing radiation in terms of its potential benefits. “The risk from a medically necessary imaging exam is quite small when compared to the benefit of accurate diagnosis or intervention. There is no reason for patients who need these exams to avoid them,” said Jeffrey Shuren, M.D., director of the FDA’s Center for Devices and Radiological Health. “Parents should engage in a discussion with their child's physician about benefits and risks of X-ray, CT, and fluoroscopy exams.”

"Alternative diagnostic procedures that do not involve ionizing radiation exposure, such as ultrasound and MRI (magnetic resonance imaging) might be appropriate in some clinical settings," he added.

The American College of Radiology (ACR) issued a press release on the date of the study's release, urging that results of Pearce and colleagues’ study “should not keep parents from getting needed medical imaging care for their children, but should be discussed with their physician and factored into their shared decision making before an imaging scan is performed.”

In addition, as Pearce’s team noted, CT scanners from the period studied (1989-2003) used radiation doses much higher than those of today. Current protocols also allow for use of much lower dose than the study period.  So the risk from a current scan may be much less than the researchers calculated.

 “Nevertheless, although clinical benefits should outweigh the small absolute risks, radiation doses from CT scans ought to be kept as low as possible and alternative procedures, which do not involve ionizing radiation, should be considered if appropriate,” said Pearce.

"Further refinements to allow reduction in CT doses should be a priority, not only for the radiology community, but also for manufacturers. Alternative diagnostic procedures that do not involve ionising radiation exposure, such as ultrasound and MRI, might be appropriate in some clinical settings,"  Pearce told reporters.

He said scientists should make it a priority to refine CT technology to reduce radiation doses even further.

What It Means to You

So if your child’s doctor recommends a CT scan, ask some pointed questions before you consent to the scan.

  • How will this CT scan improve my child’s health care? What are the potential negative outcomes of not having the scan? Consider the context. CT scans are not standard screening exams given to all children.  Children who get CT scans are doing so because of specific and potentially dangerous health problems. “CT scans are most often performed on children who have experienced trauma to the head, neck or spine, or may suffer from neurological disorders or injury, or may need fast and accurate evaluation for complex and life threatening clinical issues such as pneumonia complications or chest infections,” said Marta Schulman, MD, FACR, FAAP, chair of the American College of Radiology Pediatric Imaging Commission. “If an imaging scan is warranted, the immediate benefits outweigh what is still a very small long-term risk. Parents should certainly discuss risk with their provider, but not refuse care that may save and extend their child’s life.”
  • Are there any other imaging tests that might give you the same information without exposing my child to radiation? Investigate other types of imaging tests. Magnetic Resonance Imagine (MRI) may offer not only an alternative, but a far better option that gives more information, especially about soft tissues, than a CT scan. MRIs do not use radiation, and there are no known health risks. However, MRIs are generally more expensive than CT scans. The scan itself can be noisy, lengthy, and frightening for a child. CT scans, on the other hand, are much better at helping doctors detect problems in bones. Neurosurgeons often prefer to use CT scans after head injury, because they can see all the subtle bones in the head.
  • Are the risks greater for my child because of  his/her history of previous imaging tests? Track your child's exposure. Keep a record of any X-rays, CT scans, or other imaging tests your child has had. Make sure your child’s physician has access to this information.
  • Will my child receive a “kid-size” radiation dose? How will you work with the radiology group to ensure that she or he is exposed to the minimum amount of radiation necessary? Not all imaging machines are made alike – and not all facilities are familiar with the special needs of pediatric patients. Be sure that the facility and equipment will give your child the absolute minimum dose of radiation necessary.

The upshot? If your child needs a CT scan, weigh the risks and benefits. In all likelihood, the benefits will outweigh the risks. And if that scan does happen, don’t lose any sleep over it. You probably haven’t given him or her brain cancer or leukemia.

Additional Resources

Related Links on ehealthMD.com
  • CT Scan: Find extensive information about CT scans: What is a CT scan? How is it performed? How do you prepare?  
  • MRI: A comprehensive overview of magnetic resonance imaging
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