Cancer risk in 680 000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians
Objective To assess the cancer risk in
children and adolescents following exposure to low dose ionising
radiation from diagnostic computed tomography (CT) scans.
Design Population based, cohort, data linkage study in Australia.
Cohort members
10.9 million people identified from Australian Medicare records, aged
0-19 years on 1 January 1985 or born between 1 January 1985 and 31
December 2005; all exposures to CT scans funded by Medicare during
1985-2005 were identified for this cohort. Cancers diagnosed in cohort
members up to 31 December 2007 were obtained through linkage to national
cancer records.
Main outcome Cancer
incidence rates in individuals exposed to a CT scan more than one year
before any cancer diagnosis, compared with cancer incidence rates in
unexposed individuals.
Results 60 674
cancers were recorded, including 3150 in 680 211 people exposed to a CT
scan at least one year before any cancer diagnosis. The mean duration of
follow-up after exposure was 9.5 years. Overall cancer incidence was
24% greater for exposed than for unexposed people, after accounting for
age, sex, and year of birth (incidence rate ratio (IRR) 1.24 (95%
confidence interval 1.20 to 1.29); P<0.001). We saw a dose-response
relation, and the IRR increased by 0.16 (0.13 to 0.19) for each
additional CT scan. The IRR was greater after exposure at younger ages
(P<0.001 for trend). At 1-4, 5-9, 10-14, and 15 or more years since
first exposure, IRRs were 1.35 (1.25 to 1.45), 1.25 (1.17 to 1.34), 1.14
(1.06 to 1.22), and 1.24 (1.14 to 1.34), respectively. The IRR
increased significantly for many types of solid cancer (digestive
organs, melanoma, soft tissue, female genital, urinary tract, brain, and
thyroid); leukaemia, myelodysplasia, and some other lymphoid cancers.
There was an excess of 608 cancers in people exposed to CT scans (147
brain, 356 other solid, 48 leukaemia or myelodysplasia, and 57 other
lymphoid). The absolute excess incidence rate for all cancers combined
was 9.38 per 100 000 person years at risk, as of 31 December 2007. The
average effective radiation dose per scan was estimated as 4.5 mSv.
Conclusions
The increased incidence of cancer after CT scan exposure in this cohort
was mostly due to irradiation. Because the cancer excess was still
continuing at the end of follow-up, the eventual lifetime risk from CT
scans cannot yet be determined. Radiation doses from contemporary CT
scans are likely to be lower than those in 1985-2005, but some increase
in cancer risk is still likely from current scans. Future CT scans
should be limited to situations where there is a definite clinical
indication, with every scan optimised to provide a diagnostic CT image
at the lowest possible radiation dose.
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