(HealthDay News) — Irradiation from diagnostic computed tomography (CT) scans is associated with an increased risk of cancer among children and adolescents exposed to CT, according to a large study published online May 21 in BMJ.

John D. Mathews, M.D., from the University of Melbourne in Australia, and colleagues analyzed data from Australian Medicare records for 10.9 million people (aged 0 to 19 years) who had had CT scans (1985 to 2005). National cancer records were used to link cancers diagnosed through 2007 in this cohort.

The researchers found that there were 60,674 cancers over a mean of 9.5 years of follow-up after CT exposure. There were 3,150 in 680,211 people exposed to a CT scan at least one year before any cancer diagnosis. For those exposed, cancer incidence was 24 percent greater than for unexposed people, even after adjusting for age, sex, and year of birth (incidence rate ratio [IRR], 1.24). There was a dose-response association, with the IRR increasing by 0.16 for each additional CT scan. The IRR was also significantly greater for exposure at younger ages. For people exposed to CT, there was an excess of 608 cancers (147 brain, 356 other solid, 48 leukemia or myelodysplasia, and 57 other lymphoid). For all cancers combined, the absolute excess incidence rate was 9.38 per 100,000 person years at risk. It was estimated that the average effective radiation dose per scan was 4.5 mSv.

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“Radiation doses from contemporary CT scans are likely to be lower than those in 1985 to 2005, but some increase in cancer risk is still likely from current scans,” the authors write.

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