|Home | About | Journals | Submit | Contact Us | Français|
In 2008, as part of the BJR Hounsfield review series, we published “Cancer risks from diagnostic radiology” , which included an estimate of the lifetime cancer risk as a function of age at exposure, from the radiation associated with a head CT scan and an abdominal CT scan. These risk estimates were based on age-specific, gender-specific and organ-specific cancer risks derived primarily from the Japanese A-bomb survivors [2,3]. We estimated that these risk estimates were probably good to within a factor of three, in both directions.
Now, Pearce et al  have published in The Lancet the first epidemiological study to show an excess incidence of leukaemia and brain cancer in children and adolescents who had received CT scans. The authors studied a cohort of 178 604 children who underwent a CT scan between 1985 and 2002 in various hospitals in the UK, estimated organ doses involved, and then identified subsequent malignancies via linkage to the National Health Service Central Registry. The average follow-up time was a little under 10 years. This study was possible in the UK because of the available electronic records, but would be incredibly difficult to repeat in many other countries, including the USA. It is a large, well-conducted, record-based cohort study, and so deserves attention.
We know from the ongoing study of the Japanese A-bomb survivors that to obtain lifetime risk estimates the irradiated population needs to be studied for at least 50–60 years! To focus on leukaemia (and to an extent brain cancer) in children is effectively a shortcut because these malignancies have a short latent period in children. For example, the great majority of radiation-induced leukaemias appear within 10 years of exposure.
Pearce et al  reported that the risk of leukaemia was positively associated with estimated doses delivered by CT scans to the red bone marrow, as was the risk of brain cancers associated with estimated doses delivered by CT scans to the brain. Quantitatively, having two to three head scans triples the estimated risk of brain cancer, while five to ten head scans triples the estimated risk of leukaemia. Of course, these very small radiation risks must be viewed in the context of the extraordinary clinical usefulness of CT in a great variety of clinical settings.
Human epidemiology trumps all calculations and estimates, of course. So how do these new data impact our review  of what was known in 2008?
Meanwhile, for most CT scans, we have estimates based only on organ doses and the Japanese A-bomb data. These estimates have proved to be in reasonable agreement with the epidemiological data in the few cases where it is possible to check them (i.e. for leukaemia and brain cancer from a head CT scan). Based on the Japanese A-bomb data, the risk estimates for abdominal CT scans are about 10 times larger than for head CT scans because of all of the radiogenic organs located in the trunk of the body, and, as pointed out above, there are no epidemiological data in sight to check these numbers. Estimates are all we will have for the foreseeable future, but even if they are only approximate, they are important at a time of increasing concern about the major increase in the collective dose from medical radiation over the past two decades, with much of it due to the burgeoning use of CT. The current estimate is almost 5 million CT scans per year in the UK, and about 85 million per year in the United States. The Pearce et al study  is an important piece of additional evidence to show that low doses of radiation do carry some (admittedly small) risk, which must be taken into account.