PLoS: Childhood cancer risk low from imaging during pregnancy

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The offspring of women exposed to major radiodiagnostic testing in pregnancy do not appear to be at higher risk of childhood malignancy than the children of unexposed mothers and the overall prevalence of childhood cancer following exposure to CT or radionuclide imaging in pregnancy was under 0.07 percent, giving an incidence rate of 1.13 per 10,000 person-years, according to a population-based study, published Sept. 7 in PLoS Medicine.

Major radiodiagnostic testing is now performed in about 1 in 160 pregnancies in Ontario, Canada. A population-based study of 1.8 million maternal-child pairs in the province of Ontario, from 1991 to 2008, was conducted by Joel G. Ray, MD, associate professor at St. Michael's Hospital, and associate professor at University of Toronto, and colleagues.

The researchers used Ontario's universal healthcare linked administrative databases to identify all term obstetrical deliveries and newborn records, inpatient and outpatient major radiodiagnostic services, as well as all children with a malignancy after birth.

There were 5,590 mothers exposed to major radiodiagnostic testing in pregnancy (3.0 per 1,000) and 1.83 million mothers not exposed. The rate of radiodiagnostic testing increased from 1.1 to 6.3 per 1,000 pregnancies over the study period; about 73 percent of tests were CT scans. Radionuclide imaging included ventilation/perfusion lung scan (89 percent) and thyroid scan (10.6 percent).

After a median duration of follow-up of 8.9 years, four childhood cancers arose in the exposed group (1.13 per 10,000 person-years) and 2,539 cancers in the unexposed group (1.56 per 10,000 person-years), a crude hazard ratio of 0.69, according to Ray and colleagues.

“Since the upper confidence limit of the relative risk of malignancy may be as high as 1.8 times that of an unexposed pregnancy, we cannot exclude the possibility that fetal exposure to CT or radionuclide imaging is carcinogenic,” noted Ray and colleagues.

Because this finding means that a very slight risk may exist, beta hCG testing should continue to be done in all potentially pregnant women before undergoing major radiodiagnostic testing, and lead apron shielding used in all women of reproductive age, whether or not known to be pregnant, suggested the authors. In addition, nonradiation-emitting imaging, such as MRI and ultrasonography, should be considered first, when clinically appropriate, added Ray and colleagues.

However, some pregnant women will still be faced with the decision to undergo CT or nuclear imaging because the test is clinically warranted. “The findings of this study suggest that when clinically indicated, major radiodiagnostic testing in pregnancy should be performed, along with brief counseling to help lessen the anxiety experienced by an expectant mother before and after the birth of her child,” wrote Ray and colleagues.

“As with most cohort studies, the Ontario investigation did not produce statistical evidence for an increased risk of childhood malignancies, but it provided important baseline data,” wrote Eduardo L. Franco, PhD, and Guy-Anne Turgeon, MESc, from McGill University in Montreal, in an accompanying editorial.

The Ontario investigation raised the bar substantially for future large-scale studies by showing the ways in which clever record linkage of multiple administrative and health care utilization databases can be used for cost-effective disease risk surveillance in a given setting, added Franco and Turgeon.