Substantial effort is now under way to identify and follow up patients who have received a computed tomography (CT) scan, to determine whether any increased risk of cancer resulting from exposure to ionising radiation during a scan can be detected. CT scans are becoming an increasingly popular and effective diagnostic tool, and their usage has risen dramatically in economically developed countries (UNSCEAR 2010). Each CT scan delivers an effective dose of between several mSv and a few tens of mSv, depending on the type of scan (UNSCEAR 2010). The radiation risk models that have been developed from the epidemiological study of groups receiving moderate and high doses (such as the Japanese survivors of the atomic-bombings of Hiroshima and Nagasaki) imply that the excess risk of cancer resulting from the low doses received during a CT scan is small, so that large and carefully designed and conducted studies are necessary to discern this predicted small additional risk. Such studies are important because of the direct evidence that they can potentially provide on the levels of risk resulting from low doses of radiation. The findings of large studies of patients who have experienced a CT scan at a young age are starting to become available—studies of infants, children and adolescents are a sensible starting point because the risk of radiation-induced cancer is generally greater at younger ages at exposure (UNSCEAR 2013).