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

BMJ 2013; 346 doi: (Published 21 May 2013) Cite this as: BMJ 2013;346:f2360
  1. John D Mathews, epidemiologist1,
  2. Anna V Forsythe, research officer1,
  3. Zoe Brady, medical physicist12,
  4. Martin W Butler, data analyst3,
  5. Stacy K Goergen, radiologist4,
  6. Graham B Byrnes, statistician5,
  7. Graham G Giles, epidemiologist6,
  8. Anthony B Wallace, medical physicist7,
  9. Philip R Anderson, epidemiologist89,
  10. Tenniel A Guiver, data analyst8,
  11. Paul McGale, statistician10,
  12. Timothy M Cain, radiologist11,
  13. James G Dowty, research fellow1,
  14. Adrian C Bickerstaffe, computer scientist1,
  15. Sarah C Darby, statistician10
  1. 1School of Population and Global Health, University of Melbourne, Carlton, Vic 3053, Australia
  2. 2Department of Radiology, Alfred Health, Prahran, Vic, Australia
  3. 3Medical Benefits Scheme Analytics Section, Department of Health and Ageing, Canberra, ACT, Australia
  4. 4Department of Diagnostic Imaging, Southern Health, and Monash University Southern Clinical School, Clayton, Vic, Australia
  5. 5Biostatistics Group, International Agency for Research on Cancer, Lyon, France
  6. 6Cancer Epidemiology Centre, Cancer Council Victoria, Carlton, Vic, Australia
  7. 7Diagnostic Imaging and Nuclear Medicine Section, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Vic, Australia
  8. 8Data Linkage Unit, Australian Institute of Health and Welfare, Canberra, Australia
  9. 9Faculty of Health, University of Canberra, Canberra, Australia
  10. 10Clinical Trial Service Unit and Epidemiological Studies Unit, University of Oxford, Oxford, UK
  11. 11Medical Imaging, Royal Children’s Hospital Melbourne, Parkville, Vic, Australia
  1. Correspondence to J Mathews mathewsj{at}


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.


  • We thank Bruce Armstrong, D’Arcy Holman, and Richard Peto for epidemiological advice; and John Heggie, Luke Wilkinson, and Paul Einsiedel for advice on dosimetry. Officials of the Department of Health and Ageing, Medicare, the Australian Institute of Health and Welfare, Chief Health Officers, and registrars of state and territory cancer registries provided access to data.

  • Contributors: JDM conceived, designed, and managed the study; obtained funding; carried out most statistical analyses; drafted and finalised the manuscript; and was responsible for data integrity and guarantor of the study. AVF extracted data for analysis, assisted with data presentation, maintained ethical approvals, and reviewed the manuscript. ZB advised on CT scan classifications, use, and dosimetry; and revised and reviewed the manuscript. MWB extracted de-identified data, advised on data integrity and interpretation, and reviewed the manuscript. SKG advised on CT procedures and coding in paediatric practice, and reviewed the manuscript. GBB advised on the study design, statistical methods, data analysis, and interpretation; and reviewed the manuscript. GGG advised on the cancer nomenclature and epidemiology, study design, data analysis and interpretation; and reviewed the manuscript. ABW advised on medical radiation and radiation physics, provided guidance on dose estimation, and reviewed the manuscript. PRA and TAG were involved in the linkage of Medicare records with cancer and mortality data, and reviewed manuscript. PM provided a revised person years Stata program and reviewed the manuscript. TMC provided guidance regarding radiation dose estimation, Medicare descriptors, and CT classification; and reviewed the manuscript. JGD advised on study design and analysis, and reviewed the manuscript. ACB designed and implemented the database and data extraction procedures, and reviewed the manuscript. SCD advised on radiation epidemiology, statistical methods, study design, data analysis and interpretation, and data presentation (tables, figures, and formatting), and made substantial contributions to the manuscript.

  • Funding: This research was funded by the Australian government via the National Health and Medical Research Council, and supported by in-kind contributions of people funded by the Cancer Research Campaign UK or employed by other agencies. The funding bodies had no role in study design, data analysis, or reporting; government agencies contributed to data collection and data linkage, but did not control data analysis, interpretation, or writing of the report. The corresponding author had full access to all data in the study, and had ultimate responsibility for data integrity, statistical analysis, and the decision to submit for publication.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at and declare: support from the Australian government (via the National Health and Medical Research Council, salary support from the Cancer Research Campaign UK and other agencies) for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: This study was approved by the human research ethics committee of the University of Melbourne (ethics ID 0723066), and by ethics committees, data custodians, and cancer registrars of the Australian Government Department of Health and Ageing, Medicare, Australian Institute of Health and Welfare, and all states and territories of Australia.

  • Data sharing: The data used for this study were provided on the basis that they would be used in accordance with the study protocol, and the approvals granted by the University of Melbourne and by ethics committees and data custodians of the Australian government and all state and territory governments.

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