18F-FDG Fetal Dosimetry Calculated with PET/MRI

Introduction: The fetal absorbed dose from ¹⁸F-FDG administration to the mother is the most important piece of information when considering the use of PET to stage cancers during pregnancy. However, the few existing human case reports were obtained using either PET-only or PET/CT machines, which may not accurately identify the soft tissues of the fetus for dosimetric calculations. This study presents data from 11 women injected with ¹⁸F-FDG for cancer staging during the first two trimesters of pregnancy. This series significantly expands the pool of available human dosimetric data and is the first to be entirely acquired with PET/MRI.

Methods: Eleven pregnant women (12 scans) with cervical cancer were imaged with ¹⁸F-FDG PET/MRI, and their images were retrospectively analyzed for this study. The fraction of injected activity concentrated by the fetus was derived from manually drawing regions of interest on the MRI slices. From the activity fraction, the fetal time-integrated coefficients were derived and combined with the standard coefficients of the mothers' organs. The absorbed doses were calculated with OLINDA/EXM 1.1 and a dynamic bladder model.

Results: All fetuses after early pregnancy could be accurately delineated due to the coregistered MRI scans. ¹⁸F-FDG activity was unevenly distributed in the fetal body. The hearts and the bladders were generally visible, while the brain showed lower uptake. The estimated doses were 2.21E-02 mGy/MBq for one woman imaged in early pregnancy, 7.38 ± 0.25 E-03 mGy/MBq for three women imaged at the end of the first trimester, and 4.92 ± 1.53 E-03 mGy/MBq for eight women imaged during the second trimester.

Conclusions: PET/MRI allows precise delineation of fetal bodies in women injected with ¹⁸F-FDG during pregnancy. The resulting dosimetric values, which are likely more accurate than those obtained with PET only or PET/CT, confirm that the fetal ¹⁸F-FDG dose is very low. Therefore, clinically appropriate ¹⁸F-FDG scans in women with cancer should not be withheld because of pregnancy.

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