The researchers used data from Australian Medicare records and national cancer records to compare cancer rates among patients who had a CT scan by age 19 to those who had never undergone the test.
The study involved nearly 11 million young people born between 1985 and 2005. The average length of follow-up for those who underwent a CT scan was 9.5 years, and about 17 years for those who did not have a CT scan.
Children exposed before age 5 had the greatest risk. The older a patient is at first exposure to a CT scan, the lower their risk, the researchers said. But even among the oldest participants - between 15 and 19 - the study revealed that risk for all cancers combined was still higher.
The US National Institutes of Health has more about CT scans and the risks involved.