CT scans may increase your risk of cancer, US study suggests

Computed tomography scans – also known as CT, or Cat, scans – may be causing one in 20 cancers, a study has found.

About five million of the scans, which use X-rays to create detailed internal pictures of the body, are carried out each year on the NHS. But new research from the University of California (UC) suggests they could lead to lung, breast and other cancers, with a tenfold increased risk for babies.

Modelling showed that nearly 103,000 cancers were expected from the 93 million CT scans that were performed in 2023 in the US – about three to four times more than previous assessments.

If the same rates were extrapolated to Britain, it would suggest that hundreds of cancers were being triggered by the scans.

“CT can save lives, but its potential harms are often overlooked,” said Dr Rebecca Smith-Bindman, a radiologist and professor of epidemiology at UC San Francisco.

“Our estimates put CT on par with other significant risk factors, such as alcohol consumption and excess body weight. Reducing the number of scans and reducing doses per scan would save lives.”

CT scans are used to diagnose a range of conditions including broken bones, tumours, internal bleeding and cardiovascular problems. But ionising radiation can damage DNA and trigger disease.

For the new study, researchers analysed 93 million exams from 61.5 million patients in the US to calculate how often cancer developed.

They found that in adults, scans of the abdomen and pelvis were likely to cause the most problems, while in children the biggest risk came from CTs of the head.

Cancers predicted to arise from the excess radiation included lung, colon, leukaemia, bladder, breast, and thyroid.

The risks were highest among those who underwent CT when they were less than one year old.

‘Focusing on risk alone is not helpful’

Researchers warned that medics should think twice before giving scans for more trivial conditions such as upper respiratory infections or headaches, and should make people aware of the dangers when discussing treatment.

However, British experts said that the benefits from CT scans were likely to outweigh the risks.

Lynda Johnson, professional officer for clinical imaging and radiation protection at the Society of Radiographers, said: “Accurate communication around the benefits and risks of CT is essential to protect the public from harm.

“Focusing on risk alone is not helpful and, in some cases, might prevent a person from attending a scan that could provide early diagnosis of cancer.

“Anyone undergoing a CT scan must be provided with balanced, accurate and relevant information to enable them to understand what it means to them as an individual in terms of their diagnosis, treatment and potential long-term care.”

Experts also pointed out that CT scan rates are much higher in the US than in the UK, where imaging is used more conservatively and with stricter clinical justification.

‘Use the lowest dose possible’

Dr Doreen Lau, a lecturer in inflammation, ageing and cancer biology at Brunel University of London, said: “The findings don’t mean that people should avoid CT scans when recommended by a doctor.

“In most cases, the benefit of detecting or ruling out serious illness far outweighs the very small risk of harm. What this research highlights is the need to minimise unnecessary imaging and use the lowest dose possible, particularly in settings where CT usage is high.

“Where appropriate, clinicians may also consider alternative imaging methods that do not involve ionising radiation, such as MRI or ultrasound – especially for younger patients, or when repeat imaging is anticipated.”

Stephen Duffy, emeritus professor of cancer screening at Queen Mary University of London, added: “I would say to patients that if you are recommended to have a CT scan, it would be wise to do so.”

The research was published in the journal Jama Internal Medicine.