HealthDay News — Children and young adults who had computer tomography (CT) scans of the head had a small but increased risk of later leukemia and brain cancer, data indicate.

However, the clinical benefit of CT scans — diagnostic accuracy and speed without  the need for anaesthesia or sedation — continue to outweigh this small increased risk, Mark S. Pearce, PhD, from Newcastle University in the United Kingdom, and colleagues reported in Lancet.

“Further refinements to allow reduction in CT doses should be a priority, not only for the radiology community, but also for manufacturers,” Pearce said in a press release. “Of utmost importance is that where CT is used, it is only used where fully justified from a clinical perspective.”

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Although it has long been known that radiation can cause cancer, debate about the magnitude of risk that relatively low doses like those received from CT scans pose has been ongoing.

“Ours is the first study to provide direct evidence of a link between exposure to radiation from CT in childhood and cancer risk and we were also able to quantify that risk,” Amy Berrington de González, DPhil, said in the press release.

The researchers conducted a retrospective cohort study involving more than 178,000 patients aged younger than 22 years who were first examined with CT between 1985 and 2002. To avoid possible confounding, they also excluded patients whose leukemia was diagnosed within two years of their last CT scan and those whose brain tumors appeared within five years of their last scan.

A total of 74 of 178,604 patients were diagnosed with leukemia and 135 of 176,587 with brain tumors during follow-up through 2008.

There was a significant positive correlation between radiation dose from CT scans and the risk for leukemia (excess relative risk per mGy, 0.036;  P=0.0097) and brain tumors (RR per mGy, 0.023; P<0.0001).

The relative risk of leukemia for patients who received a cumulative dose of at least 30 mGy was 3.18 compared with those who received a dose of less than 5 mGy. Similarly, for those who received a cumulative dose of 50 to 74 mGy, the relative risk of brain cancer was 2.82.

Despite these findings, the cumulative absolute risks remain small because both cancers are rare, the researchers concluded. Among patients younger than 10 years, there would be one excess case of leukemia and one excess case of brain cancer for every 10,000 head CT scans.

“Although clinical benefits should outweigh the small absolute risks, radiation doses from CT scans ought to be kept as low as possible and alternative procedures, which do not involve ionizing radiation, should be considered if appropriate,” the researchers wrote.

In an accompanying editorial, Andrew J Einstein, MD, from New York Presbyterian Hospital and Columbia University Medical Center in New York City, wrote, “Pearce and colleagues confirm that CT scans almost certainly produce a small cancer risk. Use of CT scans continues to rise, generally with good clinical reasons, so we must redouble our efforts to justify and optimise every CT scan.”

Pearce MS et al. Lancet. 2012; doi:10.1016/S0140-6736(12)60815-0.

Einstein AJ. Lancet. 2012; doi:10.1016/S0140-6736(12)60897-6.