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Computed tomography in childhood and adolescence is associated with small increased risk of cancer

BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f3348 (Published 22 May 2013) Cite this as: BMJ 2013;346:f3348
  1. Zosia Kmietowicz
  1. 1BMJ

Computed tomography in children and young people has a small but significant effect on their risk of developing cancer, a large population based study has found.

Overall the study found that the chance of developing cancer was 24% higher among children and teenagers who had undergone computed tomography than among those who had not. But because the incidence of cancer in early life is very low this increased risk translated into only an extra six cancers over 10 years for every 10 000 young people who underwent computed tomography. Had they not had any scans, 39 cases of cancer would have been expected in this group.1

The researchers, from Australia and Europe, looked at the incidence of cancer in patients exposed to computed tomography and those who weren’t exposed to help reduce the uncertainty concerning the safety of the investigation.

They identified 10.9 million people from Australian Medicare records aged up to 19 years (born between 1985 and 2005), including 680 211 who had undergone computed tomography. They compared the incidence of cancer in those who had undergone computed tomography at least a year before their diagnosis with that in young people who had not been exposed. The mean length of follow-up was 9.5 years in the exposed group and 17.3 years in the unexposed group.

By the end of the follow-up period, cancer had been diagnosed in 3150 of the exposed group and 57 524 of the unexposed group. After adjustment for age, sex, and year of birth, the data showed cancer incidence to be 24% greater in the exposed group (incidence rate ratio 1.24 (1.2 to 1.29)).

The researchers calculated that the risk of cancer rose by 16% for each additional computed tomography scan. They also found that the risk was highest among the youngest children and fell with age. And although the increased risk declined with the number of years since the first scan, there was still an elevated risk 15 years after exposure.

Of 500 excess solid cancers in the exposed group, the most common were brain cancer (147 cases), thyroid cancer (71), and melanoma (87).

The researchers said that almost 60% of computed tomography scans were of the brain and recognised that “in some cases the brain cancer may have led to the scan rather than vice versa.” They added that they “cannot assume that all the excess cancers . . . were caused by CT scans” and that they “cannot rule out the possibility of some reverse causation, particularly for some cases of brain cancer.”

However, they concluded that the “increased incidence of many different types of cancer . . . is mostly due to irradiation.” They pointed out that because the excess number of cancer diagnoses was still continuing at the end of follow-up, the “eventual lifetime risk from CT scans cannot yet be determined.”

In an accompanying editorial Aaron Sodickson of the Harvard Medical School said that it was important to recognise that the incidence of cancer in children was extremely small, and so “a 24% increase makes this risk just slightly less small.”2

He added, “Special attention should be paid to patients undergoing recurrent imaging, because if frequently repeated scans are found to provide little clinical benefit, the cumulative risk-benefit balance may support a decision not to image again for the same clinical presentation.”

Notes

Cite this as: BMJ 2013;346:f3348

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