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Timely reminder on CT scans

Wednesday 22 May, 2013

CT scans are often necessary for accurate diagnosis and monitoring of disease – but a recent study has reinforced the fact they should only be used when there is a clear clinical need.

A study of almost 11 million Australians, published in the British Medical Journal today, suggests people are at slightly greater risk of cancer after having a CT scan in childhood or adolescence.

The researchers say that in a group of 10,000 young people they would expect 39 cancers to occur during the next 10 years, but if they all had one CT scan up to six extra cancers would occur. However, the absolute excess for all cancers combined was low, at 9.38 for every 100,000 person years of follow-up.

The study involved researchers at several centres in Australia, including the Cancer Council's Cancer Epidemiology Centre.

Cancer Epidemiology Centre Director, and a co-author of the study, Professor Graham Giles, said the small increase in cancer risk of CT scans must be considered alongside proven benefits.

"This research is a reminder to the wider community that there are risks associated with the use of CT scans," Professor Giles said.

"Patients, families and health professionals must continue to work closely together to ensure CT scans are only used in situations where there is a definite clinical need. At every scan the radiation dose must also be kept at the lowest level possible."

Cancer Council Victoria's Senior Clinical Consultant Associate Professor Michael Jefford added: "We mustn't forget that CT scans are an important medical tool and, with improvements in CT technology, some of the issues raised by this study are already being addressed."

Study leader, Professor John Mathews, of the University of Melbourne, also confirmed that this small increase in cancer risk must be weighed against the undoubted benefits from CT scans.

"Our new findings will remind doctors to order CT scans only when there is a definite medical reason, and to insist that CT scans use the lowest possible X-ray dose," Professor Mathews said.

It is well known that large doses of radiation can damage DNA and increase the risk of a later cancer. However, the radiation doses from CT scans are very small, and there has been uncertainty about whether such small doses would cause cancer, and whether any small increase in risk could be measured reliably.

This study answered such questions by linking de-identified Medicare records of CT exposures for the entire population of young Australians, aged 0-19 years between 1985 and 2005, to cancers diagnosed up to the end of 2007. The cohort included 10.9 million people, 680,211 of whom were CT-exposed at least 12 months before any cancer diagnosis.

The risk of cancer increased with the number of CT scans, and the proportional increase in risk was greater for those exposed at younger ages. Nevertheless, as CT usage increased with age, the majority of cancers in this study followed exposures in the teenage years. Although this study did not directly assess the effects of CT exposures after the age of 19, the results suggest that cancer risk would increase following CT scans in adult life.

With improvements in CT technology, the average radiation dose per scan is expected to fall, although CT scan numbers have continued to increase in many countries.

If you have a cancer question and would like to speak to an experienced cancer nurse in confidence, please call the Cancer Council Helpline on 13 11 20.