Cancer risk — kids and CT scans

Published in Health News and Evidence

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Practice points | Radiation and kids — what’s the risk? | New Australian research |
Clinical actions — what needs to change? | References


A new Australian study has confirmed that CT scans in young people are associated with an increase in cancer risk . Although having a CT scan in childhood or adolesence was associated with an increased risk, the overall increase in incidence of cancer was relatively small. This again highlights the need for health professionals to take into account the risk of malignancy when deciding on the best diagnostic pathway to use with their young patients.

Practice points

  • CT scans in children and adolescents are associated with a small increase in future cancer incidence
  • The overall incidence of cancer is low and the increased risk is small. However, before referring a young person for a scan it is important to consider:
    • Is the CT scan really needed?
    • Will it change management?
    • Are there previous imaging results that provide diagnostic or management information?
    • Is CT the best choice of imaging technology?
    • Can the same information be obtained without exposure to ionising radiation?
  • Use a diagnostic decision tool such as the Diagnostic Imaging Pathways.
  • If a CT scan is chosen as the best option, inform the parents  of the potential risks. This may involve reassurance that the overall increase in cancer risk is small.

Radiation and kids: what’s the risk?

CT scans are an important diagnostic tool providing non-invasive imaging that can be essential in the diagnosis of many disorders. However, to what extent does the exposure to ionising radiation associated with CT scans increase the future risk of developing cancer?1

Calculating risk

Calculating the risk posed by the relatively small doses of radiation used in CT scans has been problematic. Most of the research about risk associated with ionising radiation has been based on cancer rates in Japanese survivors of the Hiroshima and Nagasaki nuclear bombs. However, these people were exposed to much greater amounts of ionising radiation than any imaging procedure.1 Whether these data can be extrapolated to populations exposed to fractions of this radiation has been debated.1,2

Exposure in kids carries increased risk

Radiation exposure in children is a concern for two particular reasons:

  • as children are still growing rapidly, more cells are actively dividing which provides a greater risk for DNA damage and disruption to cell development.
  • compared with an average 50 year old given a CT scan, a child has a much longer time for radiation damage to negatively affect their health.3
In Australia there are around 80,000 CT scans performed on people under the age of 20 each year.3 Quantifying the risk to these young people is an important public health issue.

Recent evidence of risk for young people

Recent evidence from the UK and US has demonstrated that CT scans in young people may increase the risk of cancer.2,4 In the UK, 180,000 young people who received head, abdominal and chest CT scans were assessed for cancer risk. A positive correlation between estimated radiation doses to the red bone marrow and brain and incidence of leukaemia and brain tumours was observed.4

The US study with nearly 5 million child–years of observation also demonstrated a correlation between dose of radiation from CT scans and cancer risk (see Table 1).  Each year in the US around 4.2 million CT scans are performed on young people. Based on the results of this study, this would cause an estimated 4870 additional future cancers per year in the US.2

New Australian research

New Australian research has investigated the risks posed from CT scans in childhood. This large study identified 10.9 million people aged under 19 years between 1985 and 2005.5 Medicare records identified 680,211 young people who had CT scans. Cancer incidence rates were compared between those who had received a scan and the remaining 10 million people who had not.

CT scans associated with increased incidence of cancer

Overall, more than 60,000 cancers were recorded during the mean 9.5 year follow-up — 3150 in the group exposed to CT scans.5 When the groups were compared, following stratification by age, sex and year of birth, the incidence of cancer was 24% greater in the exposed group. This equated to 608 excess cancers, representing one excess cancer for every 1800 CT scans performed in people aged under 19 years.5 The report estimated that 39 cancers per 10,000 would be expected in young people, but 45 cancers per 10,000 would be expected in young people given CT scans.6

There was also a dose–response relationship, with a 16% increase in the risk of cancer with each additional scan. The risk of cancer was also greater for those people who had been scanned at younger ages.5

Brain tumours represented the most common type of cancer seen following CT scans with 147 excess cancers, 122 of which occurred following a head CT scan. Lymphoid and haematopoietic cancers were the next most common followed by melanoma.5

Limitations — reversing the causality?

One of the main issues with these types of studies is controlling for reverse causality. In terms of CT scans, this could mean that an excess of cancers are seen in the scanned group because CT scans are used to diagnose cancer. In order to control for this the authors introduced a lag period of 5 and 10 years. This was to rule out cancers that were present at the time of scanning and therefore to control for those scans that were performed for cancer diagnosis.

This analysis produced a similar outcome — at 5 years there was a 21% excess in cancer, and at 10 years an 18% increase. Based on the 10-year lag period it would be expected that one excess cancer would be seen for every 2200 CT scans performed.5 While reverse causality cannot be ruled out, the results suggest that excess cancers are truly associated with CT scans.

This study was performed on a population scanned between 1985 and 2005. It is likely that the actual imaging doses currently used in Australia will be lower than those used in the study population.5

This new research supports previous findings, that CT scans in young people are associated with an increase in risk of cancer.2,4,5 The overall increase in risk is small, but important.

Clinical actions — what needs to change?

A CT scan for examination and diagnosis is justified in young people (or any other person) if there is a high likelihood that it will inform management or have a positive health benefit, such as early detection of cancer.1 However, it is estimated that a significant proportion of radiological examinations are inappropriate7 exposing young people to the potentially negative effects of radiation without a benefit to their health.1

Each CT scan delivers a dose of radiation well above the normal background dose; in some cases this can be equal to 10 years worth of background radiation (Table 1). This may be particularly significant for young people requiring multiple scans. It is thus imperative to ensure that the benefit of the scan outweighs the risk of radiation, and any risk/benefit assessment should also include the risk of malignancy.1,5,7

Table 1. Average effective dose by CT scan location2 compared with natural background radiation3



Average effective dose (mSv)

Equivalent to natural annual dose*

Head CT

< 5 years


2 years 4 months

5–9 years


1 year

10–14 years


9 months

Chest CT

< 5 years


3 years 6 months

5–9 years


5 years

10–14 years


4 years 4 months

Abdominal CT

< 5 years


7 years

5–9 years


7 years 5 months

10–14 years


10 years

mSv =  milliSievert
*Australian natural annual exposure is around 1.5 mSv — equivalent amounts of background calculated by dividing the average effective dose by the natural annual exposure in Australia3

Assess benefits and risks before you refer

Before deciding on whether to request a CT scan consider whether the CT scan is the best imaging option; other techniques that do not use ionising radiation, such as MRI and ultrasound, may provide sufficient diagnostic information.3

Confirm that the child actually needs imaging; will other methods of investigation be sufficient to inform management?3

Using the Diagnostic Pathway tools may help in decisions about how to manage a young person’s treatment and what may be the most appropriate choice and correct sequence of examinations.

There are guidelines for radiologists that help reduce the radiation dose in the CT. It is predominantly the responsibility of the imaging facility to implement these guidelines, but in the referral it can be  requested that the scan is only performed on the indicated region, only performed once and the dose of radiation reduced for the size of the child being scanned.1

  1. Brady Z, Cain TM, Johnston PN. Justifying referrals for paediatric CT. Med J Aust 2012;197:95–9. [PubMed]
  2. Miglioretti DL, Johnson E, Williams A, et al. The Use of Computed Tomography in Pediatrics and the Associated Radiation Exposure and Estimated Cancer Risk. JAMA Pediatr 2013:1–8. [PubMed]
  3. Australian Radiation and Nuclear Safety Agency. CT Scans for Children: Information for referrers. 2013.  (accessed  19 June 2013).
  4. Pearce MS, Salotti JA, Little MP, et al. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet 2012;380:499–505. [PubMed]
  5. Mathews JD, Forsythe AV, Brady Z, et al. 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:f2360. [PubMed]
  6. Small cancer risk following CT scans in childhood and adolescence confirmed.[Editorial.] 2013. (accessed 12 June 2013).
  7. Malone J, Guleria R, Craven C, et al. Justification of diagnostic medical exposures: some practical issues. Report of an International Atomic Energy Agency Consultation. Br J Radiol 2012;85:523–38. [PubMed]