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As previously highlighted in a Dynamed Weekly Update in 2010 (volume 5, issue 2), radiation exposure from computed tomography (CT) scans in adults has been associated with increased lifetime cancer risk (Arch Intern Med 2009 Dec 14;169(22):2071). A new retrospective cohort study has evaluated the risks of leukemia and brain tumors associated with childhood CT scans. Patients in Great Britain who had at least 1 CT scan before the age of 22 years and had no previous cancer diagnosis were assessed for absorbed radiation doses in red bone marrow and brain tissue. A cohort of 178,604 patients was analyzed for incidence of leukemia and a cohort of 176,587 patients was analyzed for brain tumors. Patients were excluded for diagnosis of leukemia within 2 years or brain tumor with 5 years of CT scan to reduce the risk of counting preexisting cancers.
Absorbed radiation dose, measured in milliGrays (mGy) varies with age and scan location. For a 5-year old child receiving a head CT, the typical brain exposure would be about 28 mGy and the estimated red bone marrow dose would be about 9 mGy. A chest CT gives lower doses to both brain tissue and bone marrow. A 15-year old adolescent would typically receive a brain dose of about 44 mGy and a bone marrow dose of 5 mGy from a head CT.
During follow-up of up to 23 years, leukemia was diagnosed in 74 patients (0.04% of cohort) and brain tumors were diagnosed in 135 patients (0.08% of cohort). Increasing radiation dose was associated with increased cancer risk. For each 1 mGy increase in dose the excess relative risk for diagnosis of any leukemia was 0.036 (95% CI 0.005-0.12), and the excess relative risk for any brain tumor was 0.023 (95% CI 0.01-0.049). Compared to cumulative doses< 5 mGy, a cumulative dose of ≥ 30 mGy was associated with increased risk of leukemia (relative risk 3.18, 95% CI 1.46-6.94) and a cumulative dose of 50-74 mGy was associated with increased risk of brain tumor (relative risk 2.82, 95% CI 1.33-6.03).
In terms of absolute risk, these results correspond to an estimate of 1 additional case of leukemia and 1 additional brain tumor over 10 years per 10,000 children having a head CT scan at ≤ 10 years old. Though low, these risks should be considered when weighing the clinical value of a CT scan in a child, especially when other options may exist (Lancet 2012 Jun 7 early online).
For additional information, see the Acute lymphoblastic leukemia (ALL), Myelodysplastic syndrome, and Brain tumor topics in DynaMed.