The first study to assess directly the risk of cancer after CT scans in childhood found a clear dose-response relationship for both leukemia and brain tumors: risk increased with increasing cumulative radiation dose. For a cumulative dose of between 50 and 60 milligray or mGy (mGy is a unit of estimated absorbed dose of ionizing radiation) to the head, the investigators reported a threefold increase in the risk of brain tumors; the same dose to bone marrow (the part of the body responsible for generating blood cells) resulted in a threefold increase in the risk of leukemia. For both findings, the comparison group consisted of individuals who had cumulative doses of less than 5 mGy to the relevant regions of the body.

The number of CT scans required to give a cumulative dose of 50-60mGy depends on the type of CT scan, the age of the patient, and the scanner settings. If typical current scanner settings are used for head CT in children, then two to three head CT scans would result in a dose of 50-60mGy to the brain. The same dose to re. bone marrow would be produced by five to 10 head CT scans, using current scanner settings for children under age 15.

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It is important to stress that the absolute cancer risks associated with CT scans are small. The lifetime risks of cancer due to CT scans, which have been estimated in the literature using projection models based on atomic bomb survivors, are about 1 case of cancer for every 1,000 people who are scanned, with a maximum incidence of about 1 case of cancer for every 500 people who are scanned.

In the breakdown of results, the study authors from Group Health Research Institute and University of California, Davis note that the risk of developing leukemia was highest from head scans for kids under age 5 with a rate of 1.9 cases per 10,000 CT scans. Younger children and girls seemed more susceptible to solid cancers than older kids and boys. Every 300 to 390 scans of a girl’s abdomen or pelvis was associated with the development of one solid cancer. The study estimates that 4,870 future cancers may be caused by the 4 million pediatric CT scans each year. Cutting radiation doses for the top quarter of exposures may prevent 43% of those cancer diagnoses.