(ChemotherapyAdvisor) – Children <15 years of age exposed to radiation from two to three CT scans to the head (about 60mGy) may have triple the risk of developing brain cancer, while five to ten scans (about 50mGy) can triple the risk of developing leukemia, a retrospective cohort study jointly funded by the UK Department of Health and the National Cancer Institute/National Institutes of Health concluded in The Lancet online June 7.

However, cumulative absolute risks are small.

“In the ten years after the first scan for patients younger than ten years, one excess case of leukemia and one excess case of brain tumor per 10,000 head CT scans is estimated to occur,” the investigators noted. “Nevertheless, 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.”

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Data for cancer incidence and mortality were obtained on patients without previous cancer diagnoses first examined with CT in National Health Service centers in England, Wales, or Scotland (70% of the UK’s hospitals) between 1985 and 2002. The dose absorbed in mGy by the brain and bone marrow in patients for each scan was estimated. To avoid inclusion of CT scans related to cancer diagnosis, follow-up for leukemia began two years after the first CT and five years after the first CT for brain tumors.

During follow-up, 74 of 178,604 patients were diagnosed with leukemia and 135 of 176,587 patients with brain tumors. A positive association between radiation dose from CT scans and leukemia (excess relative risk per mGy 0.036, 95% CI: 0.005–0.120; P=0.0097) and brain tumors (0.023, 0.010–0.049; P<0.0001) was observed.

Relative risk of leukemia increased by 0.036 per extra mGy received and, for brain tumors, by 0.023. Compared with patients who received <5mGy, relative risk of leukemia for those who received a cumulative dose of at least 30mGy (mean 51.13mGy) was 3.18 (95% CI: 1.46–6.94) and the relative risk of brain cancer for patients who received a cumulative dose of 50–74mGy (mean 60.42mGy) was 2.82 (1.33–6.03).

The study represents the culmination of almost two decades of research in this area. Increased follow-up and analysis of other cancer types is needed to identify the total excess risk for all cancers associated with CT scans, the authors noted.