F-FDG Fetal Dosimetry Calculated with PET/MRI.

The fetal absorbed dose from F-FDG administration to the mother is an essential 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 F-FDG for cancer staging during the first 2 trimesters of pregnancy and is, to our knowledge, the first to be entirely acquired with PET/MRI. Eleven pregnant women (12 scans) with cervical cancer were imaged with F-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 from the ICRP publication 106. The fetal absorbed doses were calculated with OLINDA/EXM 1.1 and a dynamic bladder model. All fetuses after early pregnancy could be accurately delineated because of the coregistered MRI scans. F-FDG activity was unevenly distributed in the fetal body: the hearts and urinary bladders were generally visible, whereas the brain showed lower uptake. The estimated fetal doses were 2.21E-02 mGy/MBq for 1 woman imaged in early pregnancy, 7.38 ± 0.25 E-03 mGy/MBq for 3 women imaged at the end of the first trimester, and 4.92 ± 1.53 E-03 mGy/MBq for 8 women imaged during the second trimester. PET/MR images of pregnant women injected with F-FDG confirm that the fetal F-FDG dose is very low. Therefore, clinically appropriate F-FDG scans in women with cancer should not be withheld because of pregnancy.