Children and young adults who undergo multiple computed tomography (CT) scans have an increased risk of leukemia and brain tumors in the decade following their first scan, according to the results of a study published in The Lancet.
CT scans are a common diagnostic tool used for imaging body tissues and organs. A CT scan takes a series of detailed pictures of areas inside the body taken from different angles. The pictures are created by a computer linked to an X-ray machine. A CT scan can reveal some soft-tissue and other structures that cannot even be seen in conventional X-rays.
Children who suffer head injuries or who have been diagnosed with a life-threatening illness might undergo one or more CT scans during the diagnostic and treatment process. The risk with CT scans is that they deliver a dose of ionizing radiation to the area of the body being scanned. Even at low doses, ionizing radiation can cause damage that may increase the risk of cancer. Children typically face a higher risk of cancer from ionizing radiation exposure than do adults exposed to similar doses.
The use of CT scans in children has become more common since their introduction 30 years ago. Scans performed in 2012 can have approximately 50 percent lower doses of radiation compared to scans from the 1980s and 1990s; however, the amount of radiation delivered during a single CT scan still varies greatly and is often nearly 10 times higher than the radiation delivered with a conventional X-ray.
To evaluate the risks and benefits of CT scanning in childhood, researchers at the National Cancer Institute (NCI), part of the National Institutes of Health, and at the Institute of Health and Society, Newcastle University, England conducted a retrospective cohort study of more than 175,000 people under age 22 with no previous cancer diagnoses. All subjects had undergone CT scans at British National Health Service hospitals between 1985 and 2002.
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Investigators used data from radiology departments across Britain as well as data for cancer incidence, mortality, and loss to follow-up from the NHS Central Registry from 1985 to 2008. Approximately 60 percent of CT scans were of the head.
Using the data, the researchers estimated the cumulative dose of radiation received by each patient and then assessed the subsequent risk of cancer for 10 years after the first scan. They found a clear relationship between increase in cumulative dose of radiation and increase in cancer risk. Radiation dose was measured in milligray (mGy), a unit of estimated absorbed dose of ionizing radiation.
The results indicated that children and young adults who received a cumulative absorbed dose to the head of 50 to 60 mGy had triple the risk of brain tumors compared to those from a control group who received a dose of less than 5 mGy. Those who received the same dose to the bone marrow had triple the risk of leukemia compared to the control. As a point of reference—using current scan settings, two or three CT scans of the head would yield a dose of 50 to 60 mGy to the brain and five to ten CT scans of the head would yield that same dose to the bone marrow in children under age 15.
Both of these cancers are relatively rare and the actual number of additional cases caused by radiation exposure from CT scans is small. In fact, the researchers estimate that for every 10,000 CT scans of the head performed on children age 10 or younger, there would be one case of leukemia and one brain tumor linked to the radiation exposure in the decade following the first scan. In other words, the risk is quite small and the benefits of CT scans may outweigh the risks. CT scans can be useful diagnostic tools; however, it’s important to justify their use and strive to keep radiation doses as low as possible.
Pearce MS, Salotti JA, Little MP, et al. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. The Lancet, Early online publication: June 7, 2012. Available at: doi:10.1016/S0140-6736(12)60815-0.