CT scans and other nuclear medicine diagnostic tests during a woman's pregnancy do not increase her baby's risk of cancer, new data suggest.

In about one out of every 160 pregnancies, a CT scan or other nuclear medicine diagnostic test is done to diagnose serious problems like lung clots, appendicitis or bleeding in the brain in the pregnant woman.

But little is known about the potential effects on the fetus and how the radiation might affect the child as he or she grows older.

In about one out of every 160 pregnancies, a CT scan or other nuclear medicine diagnostic test is done on the woman to diagnose serious problems like lung clots, appendicitis or bleeding in the brain. (CBC)

Now, a team of scientists, most of them associated with the Institute for Clinical Evaluative Sciences in Toronto, has scrutinized the medical records of 1.8 million mother-child pairs in Ontario and identified 5,590 women who had major radiodiagnostic tests during pregnancy.

The rate of diagnosed childhood cancer was actually lower in the children who were exposed to a major radiodiagnostic test, said co-author Dr. Joel Ray, a specialist in obstetrical medicine at St. Michael's Hospital in Toronto.

On average, children in the study, published Tuesday by the Open Access journal PLoS Medicine, were followed for nine years after birth.

There were four cases of childhood cancer among offspring of the exposed group of mothers.

"We found that the rate of childhood cancer in the [children of]women who were not exposed was 1.56 cancers for every 10,000 child years years of followup," the researchers said. "In the exposed group, the women who had had a CT scan or nuclear medicine test in pregnancy, the rate of cancer in their children was 1.13 for 10,000 child years."

Person years, or child years, refers to the total sum of the number of years that each member of a study group has been under observation.

The critical finding is that the scientists didn't see a higher number of childhood cancers among the children of women who had been exposed to the scans or nuclear medicine tests. But Ray said there's still some uncertainty around the risk estimates.

"It's always possible that the rate of childhood cancer is actually still higher in statistical terms, but it can't be particularly much higher," he said. "And the absolute risk is still phenomenally low for childhood cancer, anyway."

More radiation to pelvis could alter findings

In a perspective piece also published by PLoS Medicine, Eduardo Franco and Guy-Anne Turgeon of McGill University wrote that the innovative study adds to our understanding of cancer risk following prenatal exposure to radiodiagnostic imaging.

They noted that because of the low number of childhood malignancies, the researchers couldn't examine whether risk effects were different depending on which part of a woman's body was scanned — limbs, for example, versus pelvis or abdomen, which would leave the fetus in the field of view.

Franco and Turgeon wrote that diagnostic CT imaging radiation involving the pelvis and abdomen exposes the fetus to a high dose of radiation, and theoretically might increase the risk of childhood and even adult malignancies relative to scans in which the fetus is outside the field of view.

"Future studies should focus on accurately stratifying risk on the basis of this premise," they wrote.