CT scans and cancer

CT scans and cancer

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A recent study (Mathews et al, 2013) of 180 000 young people exposed to CT scans in the United Kingdom found an increasing risk of leukaemia and brain cancer with increasing radiation dose.

 

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 doi: 10.1136/bmj.f2360 (Published 22 May 2013)

The article is available here,

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I’m posting this study because it is Australian and has high quality data from linkage studies.

This study showed that overall cancer incidence was 24% (95% confidence interval 20% to 29%) greater for children (<19years) who had a CT scan compared to those who had not.

There was a clear dose response effect.

24% sounds like a big increase but of course the low background rate means absolute effect size is small but not insignificant – an excess of 600 cancers in the 680,000 cases who had a CT scan. These rates are an underestimate because most individuals have not yet been followed for their whole lifespan. In fact the average follow-up period is only 9.5 years so many more cancers may develop in this group.

There is a clear case for limiting CT scans to situations where imaging is genuinely needed and will be highly likely to influence management and where alternative imaging is unsuitable/impractical. 

View 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. 

DFTB table on imaging and levels of radiation.

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Prof South is a paediatrician and intensivist based at Royal Children's Hospital, Melbourne. He regularly blogs on line Mike's webpage
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