Purpose or Case Report: To compare methods of estimating the fetal radiation dose delivered during computed tomography (CT) scanning. Further, to assess sources of variability in estimation methods.
Methods & Materials: A database of 40 CT scans of pregnant patients was collected. Mean gestational age was 159±63 days (7 first, 19 second, and 14 third trimester). Mean maternal weight was 78±18kg. All patients underwent abdomen (n=10) or abdomen/pelvis (n=30) scanning.

Four estimation methods were investigated: 1) Classic calculation (Felmlee, et al. AJR 154:1990), 2) updated calculation (Angel, et al. Radiology 249(1): 2008), 3) iPad app (Sahbaee, et al. Medical Physics 41: 2014), and 4) dose tracking software (Radimetrics, Bayer).

Two observers made measurements necessary for dose calculation. These included fetal depth, maternal circumference, and maternal diameter.

Agreement among methods was assessed using regression and Bland-Altman analysis. Causes of disagreement between methods and variability within methods was also investigated.
Results: Fetal dose ranged from <1mGy to 32mGy. Differences between methods were significant (p<0.05), but generally small (<10mGy). Bias between methods was <5mGy and 95% Confidence Intervals (CI) were less than ±15mGy for pairwise method comparison. Regression between methods demonstrated typical slopes between 0.8 and 1.2 and R2 values between 0.3 and 0.8. When datasets were limited to avoid violating method assumptions both bias and 95%CI were cut by 50% or more.

Complete patient circumference was not visible in the CT field of view for 30 of 40 patients (75%). This likely causes overestimation of fetal dose due to underestimation of the maternal size. Differences in measurement between the observers generally resulted in fetal dose estimate changes of <10mGy, though several outliers demonstrated larger changes.
Conclusions: Fetal dose estimation methods demonstrated good agreement; however, variability and outliers exist. A qualified medical physicist should review dose estimates to ensure proper calculation. Differences between methods could become important if combined with other dose data in a dose index tracking system as it could elevate estimated dose above threshold considered acceptable for fetal scanning.