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

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

Average followup for the typical child in the study, published Tuesday by the Open Access journal PLoS Medicine, was up to about nine years of age, he said.

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 women who were not exposed was 1.56 cancers for every 10,000 child years of followup. 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."

The critical finding is that they didn't see a higher number of childhood cancers among the youngsters of women who had been exposed to the scans or nuclear medicine tests. But he 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. And the absolute risk is still phenomenally low for childhood cancer, anyway."

The information should provide a greater degree of comfort to physicians ordering a test when it's needed in a pregnant woman, rather than frightening her into thinking she's putting her child at higher risk of cancer, he said.

"There isn't a single practising physician or nurse who wouldn't want to know about this type of information, whether they work in radiology or whether they work in pediatrics or obstetrics or family medicine, surgery, trauma — all of them need to use these tests along the way," Ray said.

Plain X-rays only have limited capacity to look at some of the deeper tissues, so they wouldn't always be an option, he noted. Magnetic resonance imaging isn't available in all centres, and even if it is, it might not be possible to get one in off-hours.

That leaves CT scans, which are much more readily available. About three-quarters of the 5,590 cases had had these kinds of scans.

"If you want to look inside a person, it's better to look with a CT scan than with a scalpel," Ray said.

He gave the example of a pregnant woman from another country who got off a plane with breathing difficulty, and he sent her for a CT scan of her chest and neck because she had a big thyroid gland.

But as she got into the scanner, another doctor spoke to her about childhood malignancy risk and frightened her, so she got up off the table and was crying and upset.

Ray had to be called in to talk with her and reassure her.

"She had such a massive thyroid gland — goiter — compressing her airways that she really was at very high risk of losing her main airway," he said.

"She got the CT scan, she had a lead apron shield, the CT scan was extremely informative and that night she had a portion of the large thyroid gland removed. And it was life-saving in the most true sense."

The woman went on to have a "beautiful baby, no problems to date," he said.

The researchers received funding from the Pediatric Oncology Group of Ontario.

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.

"The rarity of the outcome among the exposed children prevented the authors from making more insightful analyses that could have revealed dose-response trends or permitted probing specifically for leukemia risk," they wrote.

They also noted that with the low number of childhood malignancies, the authors couldn't examine exposure risk effects by anatomical site — in other words, whether risk effects were different for scans of the woman's extremities, versus pelvis or abdomen when the fetus would be in the field of view.

Franco and Turgeon wrote that diagnostic CT imaging radiation involving the pelvis and abdomen yields a high dose to the fetus, and theoretically may increase the risk of childhood and even adult malignancies relative to imaging procedures with the fetus outside the field of view.

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

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