CT scans and later risk of leukaemia or brain tumour
Until now, estimates of cancer risk from CT scanning have been projections from data obtained from studies of the survivors of the atomic bombs in Japan. Such estimates might not be relevant to the current use of CT scans. Now the first direct study of CT scans and cancer risk has been reported (Mark S Pearce and colleagues. Lancet 2012;380:499–505; see also Comment, ibid: 455–7).
The study concentrated on the risk of leukaemia or brain tumour after CT scanning of people up to 21 years of age and included data from 178 604 people for leukaemia risk and 176 587 for brain tumour risk. All patients had CT scans in National Health Service (NHS) centres in England, Scotland, or Wales in 1985–2002 and data about cancer incidence, mortality, and loss to follow-up were obtained from the NHS Central Registry for the years 1985–2008. Average follow-up was for about 10 years. Only leukaemias diagnosed after the first 2 years of follow-up and brain tumours after the first 5 years were included. There were 74 cases of leukaemia in 178 604 patients and 135 cases of brain tumour in 176 587 patients. For both leukaemia and brain tumours there was a significant positive association with radiation dose received from CT scanning. For leukaemia the excess relative risk per mGy (mSv) of exposure was 0.036 and for brain tumours 0.023. Patients receiving a cumulative dose of 30 mGy or greater (mean 51.13 mGy) to the bone marrow had a relative risk of leukaemia of 3.18 compared with patients receiving a dose of <5 mGy. For patients receiving a cumulative dose of 50–74 mGy (mean 60.42 mGy) to the brain the relative risk for brain tumour was 2.82.
CT scanning of children to a cumulative dose of around 50 mGy to the bone marrow might triple the risk of leukaemia and a cumulative dose of around 60 mGy to the brain might almost triple the risk of brain tumour. The absolute risks are small: within 10 years of the first CT scan there might be one extra case of leukaemia and one extra case of brain tumour per 10 000 head scans of children <10 years old. It is concluded that the clinical benefit usually outweighs the small risk but radiation dosage should always be kept as low as possible and the appropriateness of using imaging procedures that do not involve ionising radiation should be considered in each case.
Competing interests None.
Provenance and peer review Commissioned; internally peer reviewed.