Purpose: The radiation dose delivered to pregnant patients during radiological imaging procedures raises health concerns because the developing embryo and fetus are considered to be highly radiosensitive. To appropriately weigh the diagnostic benefits against the radiation risks, the radiologist needs reasonably accurate and detailed estimates of the fetal dose. Expanding our previously developed series of computational phantoms for pregnant women, we here describe a personalized model for twin pregnancy, based on an actual clinical scan.Methods:The model is based on a standardized hybrid pregnant female and fetus phantom and on a clinical case of a patient who underwent an18F-fluoro-deoxyglucose (18F-FDG) positron emission tomography (PET) / computed tomography (CT) scan while expecting twins at 25-weeks' gestation. This model enabled us to produce a realistic physical representation of the pregnant patient and to estimate the maternal and fetal organ doses from the18F-FDG and CT components. The N-Particle eXtended (MCNPX) general purpose Monte Carlo code was used for radiation transport simulation.Results:The fetal18F-FDG doses for the two fetuses were 3.78 and 3.99 mGy, whereas the CT doses were 0.76 and 0.70 mGy, respectively. Therefore, the relative contribution of18F-FDG and CT to the total dose to the fetuses was about 84% and 16%, respectively. Meanwhile, for18F-FDG, the calculated personalized absorbed dose is about 40% - 50% higher than the values reported by other dosimetry computer software tools.Conclusion:Our approach for constructing personalized computational models allows the estimation of patient-specific radiation dose, even in cases with unusual anatomical features, such as a twin pregnancy. Our results also show that, even in twins, the fetal organ doses from both18F-FDG and CT present a certain variability linked to the anatomic characteristics. The CT fetal dose is smaller than the18F-FDG PET dose.