¹⁸F-FDG Fetal Dosimetry Calculated with PET/MRI

Abstract

The fetal absorbed dose from 18 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 18 F-FDG for cancer staging during the first two trimesters of pregnancy and is the first to be entirely acquired with PET/MRI. Methods: Eleven pregnant women (12 scans) with cervical cancer were imaged with 18 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. Results:All fetuses after early pregnancy could be accurately delineated due to the coregistered MRI scans. 18 F-FDG activity was unevenly distributed in the fetal body: the hearts and the urinary bladders were generally visible, while the brain showed lower uptake. The estimated fetal 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. Conclusion:PET/MRI images of pregnant women injected with 18 F-FDG confirm that the fetal 18 F-FDG dose is very low. Therefore, clinically appropriate 18 F-FDG scans in women with cancer should not be withheld because of pregnancy.

Estimated Fetal Radiation Dose From 18F-FDG PET/CT During the Second and Third Trimesters of Pregnancy

“…PET-MRI would be an optimal alternative for pregnant women as it can provide the anatomical information without additional radiation 6 . A recent study by Zanotti-Fregonara et al 14 on 18 F-FDG PET-MRI fetal dosimetry found lower fetal dose exposure (4.92 ± 1.53 E-03 mGy/MBq) than the fetal doses estimated herein for PET/CT during the second trimester. Unfortunately, PET-MRI is not currently easily available for routine practice.…”

“…Time-integrated activity fraction, defined as the cumulated activity in the fetus over time divided by the total activity delivered to the mother and expressed in Bq·s/Bq, was determined. For absorbed dose calculation, we used the hypotheses of Zanotti-Fregonara et al 4,6,14 : only the physical half-life of 18 F was considered for time-integrated activity calculation; fetal uptake at the time of imaging was considered to have occurred immediately after injection; the time-integrated activities for the mothers’ organs were calculated from the International Commission on Radiological Protection 106 15 ; and a dynamic bladder model was used with a first fraction of 0.075 with a half-life of 0.2 hours, a second fraction of 0.225 with a half-life of 1.5 hours, and a voiding time of 1 hour.…”

“…Time-integrated activity fraction, defined as the cumulated activity in the fetus over time divided by the total activity delivered to the mother and expressed in Bq•s/Bq, was determined. For absorbed dose calculation, we used the hypotheses of Zanotti-Fregonara et al 4,6,14 : only the physical half-life of 18 F was considered for time-integrated activity calculation; fetal uptake All the PET acquisitions were combined with a CT. *The term of pregnancy phantoms in OLINDA were 85 g for the first trimester, 1100 g for the second trimester, and 3500 g for the third trimester. When the estimated fetal weight had a difference of at least 20% with the term of pregnancy phantom, calculations were performed again using the closest weight phantom, regardless of pregnancy term.…”

When the unimaginable happens: Lung cancer diagnosis during pregnancy

“…21 Thus far, no long-term adverse effects related to chemotherapy exist but rather those related to premature birth, which also may occur in pregnant patients with cancer. 5,21,22 To evaluate cancer progression, although 18 F-fluorodeoxyglucose uptake as a result of positron emission tomography/magnetic resonance imaging (PET/MRI) has demonstrated fetal safety 23 and is considered effective in evaluating cancer progression in pregnancy, 24 doses as low as reasonably possible should still be applied. 25 Given how PET/MRI, the preferred imaging modality, may not be available worldwide, other ionizing radiation imaging modalities including computed tomography (CT) scans, PET/CT, and bone scintigraphy may be used as alternatives, with appropriate protective measures (fetal shielding and low-dose CT scan), on the basis of a review of imaging modalities in pregnant patients with cancer.…”

Medical visualization in pregnancy: indications, limitations and prospects

“…I триместр беременности характеризуется быст рым ростом и пролиферацией, что сопровождается более активным захватом глюкозы, соответственно большей поглощенной дозой. Так, по данным одного из исследований по оценке поглощенной плодом дозы при ПЭТ/МРТ с 18-ФДГ при проведении исследования в конце I триместра беременности поглощенная плодом доза была рассчитана как 7,38 ± 0,25Е-02 мГр/МБк, тогда как у пациенток во II триместре беременности это значение составило 4,92 ± 1,52Е-03 мГр/МБк [50]. В связи с этим во II и III триместрах допустимы более высокие дозы, чем в I, при этом поглощенная плодом доза остается ниже порогового значения для проявления детерминированных эффектов [50].…”