Discussion
In this retrospective cohort study, we show significant associations between the estimated radiation doses provided by CT scans to red bone marrow and brain and subsequent incidence of leukaemia and brain tumours. Assuming typical doses for scans done after 2001 in children aged younger than 15 years, cumulative ionising radiation doses from 2–3 head CTs (ie, ∼60 mGy) could almost triple the risk of brain tumours and 5–10 head CTs (∼50 mGy) could triple the risk of leukaemia.