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#50 CT scans and other forms of low-dose ionizing radiation – What is the risk of cancer?

Is there an increased incidence of cancer in patients who undergo CT scans?

Individual risk estimates related to CT use are real but small, and depend on a number of factors including CT type, age, sex and number of CT scans. Alternative imaging methods with lower or no radiation dose should be used when possible, particularly in children and young adults.

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Modeling studies extrapolating from observations of atomic bomb survivors.1-4 
  • BIER VII.4 
    • Ten mSv confers a lifetime risk of cancer of one in 1,000. 
    • Example one exposure would increase lifetime risk from 420 (baseline risk) to 421 per 1,000. 
  • Applied to retrospective cross-sectional study (1,119 pts) to estimate radiation doses:5 
    • Number Needed to Scan (NNS) to cause one additional cancer: 
      • Ex: 40 year old female. 
        • Routine head = 2 mSv, NNS=8,100.   
        • Routine CT chest = 8 mSv, NNS=720. 
        • Multiphase abdomen and pelvis CT = 31 mSv, NNS=460. 
      • Risks for those 20 years of age are ~doubled. 
      • Risk for those 60 years of age are ~halved. 
Retrospective cohort studiesDose dependent increase in the risk of cancer with radiation exposure. 
  • Cardiac imaging in patients with recent myocardial infarction (82,861 patients):6 
    • Hazard Ratio (HR) per 10 mSv increase in radiation = 2.8% [1.028 (1.018–1.039)]. 
  • Head CT in pediatric patients (24,418 patients).7 
    • Increased risk of brain tumor. HR = 2.56 (CI 1.44-5.54). 
  • Many modeling studies but very few cohort studies evaluating the risk of cancers with radiation exposure. 
  • Population wide modeling studies have to be analyzed with care given that the risk of death from the underlying morbidity is often much higher than death from radiation induced cancer.8 
  • Dose of radiation varies widely depending on area scanned, institution, protocol, age and sex – healthy children at higher risk due to their size and life expectancy.9  
  • CT imaging has increased more than 25 fold in the US in the last 30 years.10 Risk models estimated that 29,000 future cancers could be related to CT scans in the US in 2007.11

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  • Christina Korownyk MD CCFP
  • Edward Wiebe MD FRCPC

1. Preston DL, Ron E, Tokuoka S, et al. Radiat Res. 2007 Jul; 168(1):1-64.

2. Preston DL, Pierce DA, Shimizu Y, et al. Health Phys. 2003 Jul; 85(1):43-6.

3. Pierce DA, Preston DL. Radiat Res. 2000 Aug; 154(2):178-86.

4. Board of Radiation Effects Research Division on Earth and Life Sciences, National Research Council of the National Academies. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2. Washington, DC: National Academies Press; 2006.

5. Smith-Bindman R, Lipson J, Marcus R, et al. Arch Intern Med. 2009; 169:2078-86.

6. Eisenberg MJ, Afilalo J, Lawler PR, et al. CMAJ. 2011; 183:430-6

7. Huang WY, Muo CH, Lin CY, et al. Br J Cancer. 2014; 110:2354-60.

8. Zondervan RL, Hahn PF, Sadow CA, et al. Radiology. 2013; 267:460-9

9. Radiation Risks and Pediatric Computed Tomography (CT): A Guide for Health Care Poviders. Rockville, MD: National Cancer Institute. Available from: Accessed June 30, 2011.

10. IMV CT Products Overview. Available from: Accessed June 30, 2011.

11. Berrington de González A, Mahesh M, Kim KP, et al. Arch Intern Med. 2009; 169(22):2071-7.

Authors do not have any conflicts of interest to declare.

Les auteurs n’ont aucun conflit d’intérêts à déclarer.