A new study offers the most solid evidence to date that radiation from CT scans increases children's risk of developing leukemia and head and neck cancer.

Children and adolescents who received two to three computed tomography scans of the head were three times as likely to develop brain cancer as those in the general population, according to the study of 176,587 children.

The study, published online Wednesday in the journal Lancet, also said the risk of contracting leukemia was three times as great for children who received five to 10 CT scans of the head.

The study's authors emphasized that the overall likelihood of getting the cancers remained very small and that the immediate benefits of the scans, such as in detecting head injuries, still outweigh the risks.

The report found cancer risk grew with each scan received, and there was an elevated risk even in those who received only one CT, formerly called a CAT scan.

Previous studies raised concerns about cancer risk from the scans; this report offers the first direct evidence of the magnitude of the link. It examined the U.K.'s National Health Service records of patients who had received a CT before age 22 and looked at their medical history for an average of 10 years afterward.

'This paper confirms that radiation, even in relatively low doses, does lead to risk' of certain cancers, said Alan Craft, emeritus chair at Newcastle University and an author of the paper. 'There is no safe dose.'

The increased risk corresponded to one additional case of leukemia in the 10 years after the first scan for every 10,000 patients younger than 10 who were scanned, or to an extra case of brain cancer for every 30,000 such people scanned.

CT scans combine a sequence of X-rays from different angles to offer a composite view of bone and soft tissue. The study focused on head scans since the brain is believed to have heightened sensitivity to radiation. It looked at young people since their tissue is more susceptible than adults' and cancer can take a long time to develop.

Patients are five times as likely to receive a CT scan in the U.S. as in the U.K., said Dr. Craft. In the U.S., use of CT scans quadrupled to 85.3 million in 2011 since the early 1990s.

Marta Schulman, chair of the American College of Radiology Pediatric Imaging Commission, said the study adds to the impetus that patients should be scanned only when necessary and with the lowest dose of radiation possible.

Efforts are under way in the U.S. to curb overuse of CT scanning in children, including the Image Gently initiative, launched in August 2009 and spearheaded by a coalition of health-care organizations dedicated to changing medical practice.

In Wednesday's study, patients who already had cancer diagnoses or who developed cancer within two years of a CT scan were excluded, to decrease the likelihood that risk calculations would be inflated by patients who would have developed cancer without getting a CT scan.

The study, a collaboration of researchers at institutions in five countries, was jointly funded by the U.K. Department of Health and the National Institutes of Health in the U.S.