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Kids who get multiple CT scans appear at higher risk for cancer

Children who get four or more computerized tomography (CT) scans before the age of 18 face more than double the risk of cancer compared to those who don’t receive this imaging, a new study has found.

The study out of Taiwan, published Monday in the Canadian Medical Association Journal (CMAJ), found children who undergo a single CT scan are not at an increased risk of developing brain tumours, leukemia, or lymphoma. However, the study found that exposure to multiple scans significantly increases a child’s risk of developing these types of cancers.

“The associated risk of cancer we observed was highest among children who had received four or more CT scans at or before six years of age, followed by those aged seven to 12 years and adolescents aged 13–18 years, suggesting that younger children are more vulnerable to radiation than older children,” the researchers stated in the study.

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Children are generally more sensitive to the harmful effects of radiation than adults, as their developing cells are more vulnerable to damage, according to the U.S. National Cancer Institute. As a result, the risk of developing radiation-related cancer can be several times higher for a young child than an adult.

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Because CT scans are a common way to diagnose cancer or examine a head injury in children, the researchers said they wanted to evaluate whether it caused a risk of developing intracranial tumours, leukemia, non-Hodgkin lymphoma and Hodgkin lymphoma among children, adolescents and young adults.

To do this, the researchers examined national health records from more than 7,000 Taiwanese patients diagnosed with these types of cancers between 2000 and 2013. They then compared tumour rates for those who had had CT scans versus those who had not.

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For one CT scan, there was no increased risk of any of the cancers compared with no exposure.

Children who received two to three CT scans had an increased risk of intracranial tumours; and those who received four or more had a more than twofold risk of intracranial tumours, leukemia and non-Hodgkin lymphoma, the study found.

Why use CT scans on kids?

In Canada, there has been a continuous increase in the number of CT scanners and examinations performed over the past 25 years, according to Health Canada.

This is because they are a very effective way of imaging organs, said Dr. James Whitlock, division head of haematology and oncology at SickKids hospital in Toronto. Whether it’s used to identify congenital malformations in a child’s organ, scan for damages after a physical trauma, or diagnose and stage cancer.

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“CT scans have really been one of the miracles of modern medicine, they can lead to a rapid diagnosis, and in many cases, save the life of a child,” he said. “But like any test, they do have their own risks.”

During a scan, a patient is briefly exposed to ionizing radiation, leading to a small but increased risk of development of cancers, Whitlock explained. Because of this risk, health providers must determine if the potential benefits of obtaining that CT scan is worth the risk.

Past medical literature has pointed out that a single or small amount of CT scans has no clear increased risk of developing subsequent childhood cancer.

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When human cells are exposed to low-dose radiation, small DNA breaks are generated and mended by an internal repair process, the authors state. Therefore, low-level CT exposure appears not to be carcinogenic.

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However, when there is cumulative DNA damage caused by radiation exposure, it “exceeds DNA repair abilities, and the risk of carcinogenesis rises,” the researchers state in the study.

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“There is a growing body of literature, to which this study adds yet another piece of evidence, that the more CT scans you get, the more that risk can increase,” Whitlock said.

The study out of Taiwan is “interesting”, he said, as the researchers identified four CT scans as a “threshold”, which has not been seen in previous research.

“Certainly, there is a risk with less than four, four should not be interpreted as being the magic number. And if your child gets five, they’re not going to necessarily get cancer,” he explained. “The risk remains very low, but there is an incremental risk.”

CT scans safer than a decade ago

It is the responsibility of the medical community to minimize the number of CT scans and only use them when necessary and there aren’t any other alternatives, such as ultrasound or Magnetic Resonance Imaging (MRI), explained Dr. John Donnellan, a paediatric radiologist at McMaster Children’s Hospital in Hamilton, Ont.

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Unfortunately, radiation is sometimes a “necessary evil,” he said, adding that if a child does require multiple scans, “there are ways that we have to reduce the radiation dose that those patients will receive.”

Over the past 10 to 20 years, Whitlock said there have been a lot of advances in CT technology, meaning a scan today is likely to provide much less radiation exposure than one that was done ten years ago.

“The study did begin enrolling patients in 2010, but there’s likely been a lot of progress since that time,” he said.

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Despite the risks, parents should not be afraid to get a CT scan for their children, as in certain cases it can save the child’s life, Whitlock stressed.

But if there are concerns, parents should discuss them with their healthcare provider and ask if there are alternatives.

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“One other situation that I’ve seen occur is many parents may take comfort if their child has a complaint or a medical situation and push very hard to get a scan to be sure that, ‘this isn’t a brain tumour’,” he said.

“So these parents also have to be aware that CT scans carry risks and maybe if the child doesn’t meet the criteria from the health care professional, maybe they shouldn’t push for it now.”