X-Rays Tied to Increased Risk of Childhood Leukemia

For B-cell ALL, even one X-ray was enough to slightly increase the risk. The findings differed somewhat by the region of the body imaged, with a modest increase associated with chest radiographs.

The new findings, published in the October 2010 issue of the International Journal of Epidemiology, come from the Northern California Childhood Leukemia Study, a population-based case-control study that includes 35 counties in the northern and central regions of the state. Whereas the link between high doses of radiation and cancer is well known, significant debate still surrounds the health impacts from the low doses of radiation typical of traditional X-rays.

"The general clinical impression has been that the level of radiation a child would be exposed to today from a conventional X-ray would not confer an additional risk for cancer,” said Dr. Patricia Buffler, UC Berkeley professor of epidemiology and lead investigator of the Northern California Childhood Leukemia Study. "The results of our study were not what we expected.”

Almost all cases of childhood leukemia are acute, with 80 percent being acute lymphoid leukemia (ALL), characterized by the overproduction of abnormal B- or T-cell lymphocytes, and 20% being acute myeloid leukemia (AML), in which granulocytes are overproduced. The study included 827 children up to age 15 diagnosed with either ALL or AML. The children with leukemia were each compared with other children randomly selected from the California birth registry who were matched by factors such as age, gender, ethnicity, and maternal race.

Interviews were conducted with mothers within four months of the diagnosis of leukemia, and the mothers were asked to report on the number of radiographs received by the child at least 12 months or more before the leukemia diagnosis. Mothers were also asked about their exposures to X-rays during pregnancy and the year prior to pregnancy. The researchers noted that dental X-rays were not considered because they are so common and deliver such a low dose of radiation that exposure to those radiographs would not discriminate between individuals with high and low levels of radiation exposure.

The study's findings found an increased risk from X-rays for ALL, but not for AML or T-cell leukemia, and there was no association with age at first exposure. Moreover, there was no increased risk associated with prenatal exposure to X-rays or maternal X-rays occurring before pregnancy, although these exposures were uncommon in this study population.

Dr. Smith-Bindman is in the process of characterizing the ionizing radiation exposure to children from CT scans as part of a study funded by the National Cancer Institute. She noted that two-thirds of the imaging procedures children undergo are conventional X-rays, accounting for about 20% of their exposure to radiation from medical imaging. In contrast, CT scans comprise only 10% of the medical imaging tests children undergo, but they account for two-thirds of their ionizing radiation dose.

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