Two easy-to-obtain parameters can help physicists estimate the radiation dose to a fetus when pregnant patients undergo a CT scan, according to a study presented at the American Association of Physicists in Medicine (AAPM) Annual Meeting in Anaheim, Calif.

Michael McNitt-Gray, PhD, professor of radiology at the UCLA David Geffen School of Medicine, presented the study, which was conducted by a mix of his current and former students. Medical physicists are often asked to estimate the radiation dose to a fetus, McNitt-Gray said. They’re asked both before and after scans, depending on the situation, and it’s not a simple calculation.

“We get these questions reasonably routinely, and it’s a difficult and challenging task for the diagnostic medical physicist,” McNitt-Gray said.

The authors focused on abdominal and pelvic CT exams, because those are the scans that go right through the area where the fetus would be. Using Monte Carlo simulations, they went over the data and discovered a connection between the dose to the fetus and the radiation dose output metric, CDTlvol, which is reported on the CT scanner and easy to find. They also found a connection to a newer patient size metric known as the water-equivalent diameter.

“We found a nice relationship between the dose to the fetus and those other two parameters,” McNitt-Gray said. “And that means that a reasonably accurate dose to the fetus can be obtained by knowing simply the scanner output and the size of the patient expressed in this water-equivalent diameter.”

McNitt-Gray said the authors’ findings could be a valuable tool for medical physicists who are asked to make these challenging estimates on a regular basis.