Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians

Abstract

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.

Fig 1 Schematic diagram showing how study members contributed to unexposed and exposed groups. All study members were classified as unexposed on entry to the study. Those who were exposed to a CT scan remained in the unexposed group for the duration of the lag period (one year in most analyses, but five or 10 years in some). They were then transferred to the exposed group, provided that their date of transfer was before their date of exit from the study. Study members who had no CT scan remained in the unexposed group for the duration of the study

Fig 2 Incidence rate ratios (IRR) for all types of cancers in exposed versus unexposed individuals based on a one year lag period, by the number of CT scans. The IRR increased by 0.16 (95% confidence interval 0.13 to 0.19) for each additional CT scan, calculated after stratification for age, sex, and year of birth (χ²=131.4 and P<0.001 for trend). If unexposed people were excluded, the trend remained significant (χ²=5.79 and P=0.02 for trend). The average number of scans among individuals exposed to three or more scans was 3.5. (Web figure A shows corresponding results based on lag periods of five and 10 years)

Fig 3 Incidence rate ratios (IRR) for exposed versus unexposed by site of CT scan and type of cancer, based on a one year lag period. IRRs were calculated after stratification for age, sex, and year of birth. Heterogeneity between cancer types, by site of CT scan exposure: all sites, χ²=23.58 (6 df), P=0.001; brain, χ²=104.1 (6 df), P<0.001; abdomen or pelvis, χ²=15.7 (6 df), P=0.02. Heterogeneity between sites of CT scan exposure, by cancer type: all cancers, χ²=111.1 (6 df), P<0.001; brain, χ²=13.9 (6 df), P=0.03; leukaemia, χ²=24.81 (6 df), P<0.001