Children and young adults scanned multiple times by computed tomography (CT) have a small increased risk of leukaemia and brain tumours in the decade following their first scan.

The findings, published online today in The Lancet, from a study of more than 175,000 children and young adults was led by researchers at Newcastle University and at the National Cancer Institute, part of the National Institutes of Health, USA.

The researchers emphasise that when a child suffers a major head injury or develops a life-threatening illness, the benefits of clinically appropriate CT scans should outweigh future cancer risks.

Lead author Dr Mark Pearce (pictured), Reader in Lifecourse Epidemiology at Newcastle University said: “CT scans are accurate and fast so they should be used when their immediate benefits outweigh the long-term risks. 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.”

The study represents the culmination of almost two decades of research in this area at Newcastle University, and is jointly funded by the UK Department of Health and NCI/NIH.

CT imaging is a vital and commonly used diagnostic technique and it is used more frequently in countries such as the USA and Japan. However, CT scans deliver a dose of ionising radiation to the body part being scanned and to nearby tissues. Even at relatively low doses, ionising radiation can break the chemical bonds in DNA, causing damage to genes that may increase a person’s risk of developing cancer. Children typically face a higher risk of cancer from ionising radiation exposure than do adults exposed to similar doses.

In the study, the researchers estimate that for every 10,000 head CT scans performed on children 10 years old or younger, one more case of leukaemia and one more brain tumour would occur than would normally be expected.

The investigators obtained CT examination records from radiology departments in hospitals across Britain and linked them to data on cancer diagnoses and deaths. The study included people who underwent CT scans at British National Health Service hospitals from birth to 22 years of age between 1985 and 2002. Information on cancer incidence and mortality from 1985 through 2008 was obtained from the National Health Service Central Registry, a national database of cancer registrations, deaths and emigrations.

Sixty percent of the CT scans were of the head, with similar proportions in males and females.  The investigators estimated cumulative doses from the CT scans received by each patient, and assessed the subsequent cancer risk for an average of 10 years after the first CT. The researchers found a clear relationship between the increase in cancer risk and increasing cumulative dose of radiation. A three-fold increase in the risk of brain tumours appeared following a cumulative absorbed dose to the head of 50 to 60 milligray (abbreviated mGy, which is a unit of estimated absorbed dose of ionising radiation). Similarly, a three-fold increase in the risk of leukaemia appeared after the same dose to bone marrow - the part of the body responsible for generating blood cells.

The absorbed dose from a CT scan depends on factors including age, sex, examination type, and year of scan. Broadly speaking,  two or three CT scans of the head using current settings would be required to yield a dose of 50 to 60 mGy to the brain. The same dose to bone marrow would be produced by five to 10 head CT scans, using current settings for children under age 15.

The researchers are calling for increased follow-up and analysis of other cancer types to identify the total excess risk for all cancers associated with CT scans.

In the UK, the Ionising Radiation (Medical Exposure) Regulations mean that a CT scan should only be done when clinically justified. This might explain the relatively low levels of CT use in the UK compared with other countries that do not have such regulations. The amount of radiation delivered during a single CT scan can vary greatly and can be up to 10 times higher than that delivered in a conventional X-ray procedure.

Dr Pearce adds: “This is the first direct evidence of a link between exposure to radiation from CT and cancer risk in children.

“While the risks are small, the medical community needs to consider carefully its use of CT scans and refine their use as a priority. We need to look at worldwide regulation.”

Responding to the publication Dr Hilary Cass, President of the Royal College of Paediatrics and Child Health said: "This research further confirms the need to keep doses as low as possible and only scan when absolutely necessary."  (link to full statement)

The Newcastle University team including Professor Sir Alan Craft who features in a profile in The Lancet, want to continue their work with a larger study to examine potential associations with rarer cancers.

Reference: Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Mark S Pearce, Jane A Salotti, Mark P Little, Kieran McHugh, Choonsik Lee, Kwang Pyo Kim, Nicola L Howe, Cecile M Ronckers, Preetha Rajaraman, Sir Alan W Craft, Louise Parker, Amy Berrington de González. The Lancet.

published on: 7 June 2012