We describe a rare case of a woman who underwent 18 F-FDG PET/CT during early pregnancy (fetus age, 10 wk). The fetal absorbed dose was calculated by taking into account the 18 F-FDG fetal self-dose, photon dose coming from the maternal tissues, and CT dose received by both mother and fetus. Methods: The patient (weight, 71 kg) had received 296 MBq of 18 F-FDG. Imaging started at 1 h, with unenhanced CT acquisition, followed by PET acquisition. From the standardized uptake value measured in

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... l tissues, we calculated the total number of disintegrations per unit of injected activity. Monte Carlo analysis was then used to derive the fetal 18 F-FDG self-dose, including positrons and self-absorbed photons. Photon dose from maternal tissues and CT dose were added to obtain the final dose. Results: The maximum standardized uptake value in fetal tissues was 4.5. Monte Carlo simulation showed that the fetal self-dose was 3.0 · 10 22 mGy/MBq (2.7 · 10 22 mGy/MBq from positrons and 0.3 · 10 22 mGy/MBq from photons). The estimated photon dose to the fetus from maternal tissues was 1.04 · 10 22 mGy/MBq. Accordingly, the specific 18 F-FDG dose to the fetus was about 4.0 · 10 22 mGy/MBq (11.8 mGy in this patient). The CT scan added a further 10 mGy. Conclusion: The dose to the fetus during early pregnancy can be as high as 4.0 · 10 22 mGy/MBq of 18 F-FDG. Current dosimetric standards in early pregnancy may need to be revised.