Research into radiation exposure from CT scans

CT imaging is a vital diagnostic technique, and new applications continue to be identified. However, potential cancer risks exist due to the ionising radiation used in CT scans, especially in children who are more radiosensitive than adults.Research led by Newcastle University, and involving Great Ormond Street Hospital has studied the increase in risk associated with CT scanning in childhood.

While the research finds an increase in relative risk of brain cancer and leukaemia, the absolute risk of these cancers occurring after CT remains small.

The authors say that, of every 10,000 people between the ages of 0-20 years receiving 10 mGy from a CT scan, there would be about one expected excess leukaemia case, whereas there were would be one excess case of brain cancer for every 30,000 people. Applying the dose estimates for one head CT scan before the age of 10 years, this would translate into approximately one excess case of leukaemia and one excess brain tumour per 10,000 patients in the decade after first exposure.
The article, published online first in The Lancet, concludes that radiation doses from CT scans should be kept as low as possible and alternative procedures that do not use ionising radiation should be considered if appropriate.
These are steps radiologists at Great Ormond Street Hospital are already taking. 
Dr Kieran McHugh, a consultant radiologist at Great Ormond Street Hospital who was involved in the research explains: "We have always been very aware of the small potential risks of cancer in childhood from CT scans and as a result have led the field in the UK and beyond in reducing radiation exposure.
"We have the lowest published radiation doses for many of our X-ray techniques and our CT scanner is a modern scanner which is especially set up for scanning children with the lowest possible dose. It is important to note that during much, if not all, of the study period mentioned in the Lancet paper (1985-2002), the CT examinations were performed on older generation CT scanners which would have imparted a higher patient radiation dose than would be the case with more modern CT machines."
At Great Ormond Street Hospital the radiology team are constantly trying to maximise the image quality while at the same time attempting to minimise the radiation dose to the patient with their specific low dose paediatric CT scanning protocols.
The radiology team at the hospital recently won a major award for an image of tetralogy of fallot, a type of congenital heart defect. The image was chosen for its image quality and because it was obtained using the lowest radiation dose.
Doctors at Great Ormond Street Hospital carefully vet, and justify clinically, all CT examinations performed on the hospital’s patients. The hospital has four MRI scanners and the vast majority of young patients who need a scan have an MRI scan (with no radiation) rather than a CT scan. A CT scan is only used when it confers extra specific benefits over an MRI scan for that individual patient.

View an abstract of the paper on the Lancet website
This is a landmark paper establishing and quantifying the potential risk of cancer from having a CT scan in childhood. The extra risk of developing cancer from having a CT scan remains very small.
In this retrospective study, the authors studied close to 180,000 patients who underwent a CT scan between 1985 and 2002 according to records from the radiology departments of some 70% of the UK’s hospitals. They extracted the number and types of CT scan from the records and estimated the dose absorbed in milli-Grays (mGy) by the brain and bone marrow in patients for each scan.

These data were then linked to cancer incidence and mortality reports in the UK National Health Service Registry between 1985 and 2008. From this, they calculated excess incidence of leukaemia and brain tumours. The dose of radiation received by the brain and bone marrow varies by age, and body part scanned. 
A total of 74 from 178,604 patients were diagnosed with leukaemia and 135 of 176,587 were diagnosed with brain cancer. The authors calculated that the relative risk of leukaemia increased by 0.036 per extra mGy received, whilst for brain tumours this increased risk was 0.023. Compared with patients who received a dose of less than 5 mGy, patients who received a cumulative dose of at least 30 mGy (mean 50 mGy) had around three times the risk of leukaemia; and patients receiving a cumulative dose of 50-74 mGy (mean 60 mGy) had triple the risk of developing primary brain tumours.

The authors say that, of every 10,000 people between the ages of 0-20 years receiving 10 mGy from a CT scan, there would be about one expected excess leukaemia case, whereas there were would be one excess case of brain cancer for every 30,000 people. Applying the dose estimates for one head CT scan before the age of 10 years, this would translate into approximately one excess case of leukaemia and one excess brain tumour per 10,000 patients in the decade after first exposure.
The authors note that increased follow-up and analysis of other cancer types is needed to identify the total excess risk for all cancers associated with CT scans.
The study represents the culmination of almost two decades of research in this area, and is jointly funded by the UK Department of Health and NCI/NIH.