CT scans in kids slightly raise leukaemia risk

Children who receive multiple computerised tomography (CT) scans of the head or bone marrow have triple the risk of developing leukaemia or brain cancer compared with children who had minor scans of other parts of the body. However, the heightened risk is still low, generating just one extra leukaemia or brain cancer case per 10,000 scans.

"Our study is the first to show directly that CT scans can cause cancer later," says Amy Berrington de Gonz?lez of the US National Cancer Institute in Bethesda, Maryland.

Researchers stress that CT scans are indispensible, lifesaving tools that can rapidly reveal the scale of head injuries after an accident, for example. But the latest results reinforce the message that every scan should be medically justifiable, and that other types of diagnosis that don't expose children to X-rays should be considered wherever possible.

"MRI, ultrasound, physical examination and ordinary X-rays are other options," says Mark Pearce of Newcastle University in the UK, co-leader of the research team. "The doses from ordinary X-rays are about 10 times lower."

To investigate whether the scans raise the risk of cancer, the researchers analysed the records of almost 180,000 UK patients who had received CT scans before their 22nd birthday. All of the scans were performed between 1985 and 2002, and all patients were then followed until 2008.

The "controls" in the group were those who had minor scans to their extremities, which restricted the absorbed radiation dose to the two organs of interest ? the brain and the bone marrow ? to less than 5 milligrays.

There were 74 cases of leukaemia and 135 cases of brain cancer. Those who had received cumulative doses of around 60 milligrays ? equivalent to two or three CT scans of the head ? had triple the risk of brain cancer compared with the "controls". The risk was also trebled for leukaemia.

Careful use

Pearce says that the results reinforce the need to do the scans only when they are absolutely necessary. He says that doctors in Europe do relatively few CT scans because by law there must be a medical justification for it.

This contrasts with the US, where scanning the "worried well" annually for early signs of disease has become increasingly popular.

Over the past decade, some studies have warned that the routine scans could raise the risk of cancers. Other studies have likened exposures from the routine scans to those from nuclear bombs dropped on Japan during the Second World War.

Pearce says the risks to children are known to be higher because sensitive tissues in the bone marrow and brain are still rapidly developing. "We need to study adults too, and although it's not something we're planning, it's something we'd like to do in the future," he says.

"Although our study is in children, it is supporting evidence that there's probably a risk associated with adult scans," says Berrington de Gonz?lez. "Most are offered by private companies, and there have been efforts by several health organisations to explain that there is no evidence of benefit."

Journal reference: The Lancet, DOI: 10.1016/S0140-6736(12)60815-0

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