Much has appeared in the medical and lay press over the past 5 years about the potential for cancer associated with CT. A study used medical records from the United Kingdom in a retrospective cohort approach to evaluate the development of leukemia and brain tumors in 170,000 children who received CT between 1985 and 2002. Two thirds of the CT scans were of the head, and all children were younger than 22 years when initially exposed.

The investigators tried to account for the degree of radiation exposure in multiple ways. First, they estimated exposure associated with each scan on the basis of body areas scanned, and they estimated radiation exposure of the bone marrow and brain. They also took into consideration the year of the scan, given that newer scanners generally use less radiation. They used appropriate statistical analyses to account for the fact that cancer diagnoses are relatively rare in children, even when they have been exposed to CT.

Overall, 74 cases of leukemia were found in 1.7 million person-years of follow-up and 135 cases of brain tumors in 1.1 million person-years of follow-up. The risk for either type of cancer was positively associated with the estimated radiation dose from CT.

The investigators estimated that 5-10 head CTs would result in a cumulative radiation exposure of about 50 mGy to the bone marrow, whereas 2-3 CTs of the head would result in a brain radiation exposure of approximately 60 mGy. The level of radiation exposure from 5-10 head CTs was associated with a relative risk for leukemia of 3.18 (95% confidence interval, 1.46-6.94). Similarly, 2-3 head CTs resulted in a relative risk for brain tumor greater than 3.0.

The study concluded that CT exposure in children is associated with a significantly increased risk for later cancer, with 2-3 head CTs tripling the relative risk for future brain tumor and 5-10 head CTs tripling the relative risk for leukemia.