The second article in our new Recent Highlights Series discusses the recent article “Fetal Radiation Dose from 18F-FDG in Pregnant Patients Imaged with PET, PET/CT, and PET/MR” by Zanotti-Fregonara et al. published in the Journal of Nuclear Medicine (volume 56, issue 8, pages 1218–1222; doi: 10.2967/jnumed.115.157032). The purpose of the study was to estimate the fetal radiation dose from 18F-FDG in a group of pregnant females who underwent a PET scan during a clinical workup for suspected malignancies.
One of the most common reasons for PET imaging using 18F-FDG is cancer staging, and the annual number of scans performed in the United States increased from ~250,000 in 2001 to >1.7 million in 2010. However, this means that an increasing number of pregnant women are being injected with radiopharmaceuticals. Fetal radiation exposure can either be accidental (if the pregnancy is unknown at the time of scanning) or be the result of a cancer workup in the mother. In both cases it is important that the fetal radiation dose is assessed accurately to allow the fetal risk and clinical benefit to the mother to be weighed accurately. However, the fetal dose can only be determined using in vivo studies because 18F-FDG crosses the placental barrier and accumulates in fetal tissues. Although some data have been reported in women with different stage pregnancies, the results are conflicting. Therefore, the authors planned to expand the amount of available information by performing dose calculations in six women aged 23–39 years who were 5 weeks to 7 months pregnant and were imaged using PET only, PET/CT, or PET/MR.
All patients received a Foley catheter to promote the drainage of radioactive urine and reduce fetal photon exposure. The mean dose of injected radioactivity was 328 MBq (range, 296–385). Two patients underwent PET alone, two received MET/MR, and two received PET/CT. The fetal volume and concentration of radioactivity were calculated manually. The full experimental methods can be found in the manuscript.
The key results reported by the authors are as follows. Fetal uptake was generally slightly higher than or similar to the mean background activity in the mothers’ organs. In second and third trimester pregnancies the highest uptake was seen in the abdominal and myocardial regions, but there was no obvious brain-specific uptake. The fetal detail achieved with PET/MR was very good, and the fetal contours could be defined precisely. Importantly, these scans revealed several areas of low 18F-FDG uptake. Higher 18F-FDG doses were uptaken in the three fetuses in the first trimester, although the doses were consistent with published standards and previous reports. The absorbed doses were also lower than the accepted standards in the later stages of pregnancy. Importantly, bladder voiding had a significant effect on the radiation dose in the fetus, particularly in patients in the early stages of pregnancy. Finally, the placenta retained and concentrated 0.27% of the injected 18F-FDG activity.
Overall, this patient series revealed that fetal radiation exposure is generally low with 18F-FDG, particularly during the later stages of pregnancy. The authors concluded that the relatively higher doses of exposure observed during the early stages of pregnancy could likely be explained by the smaller fetal volume, as well as the high proportion of glucose-utilizing rapidly proliferating and undifferentiated cells.
Finding ways to calculate the likely fetal exposure during early exposure is particularly important and challenging, in part because the fetus cannot be seen on CT, the fetal contours are not always clear on PET, and the fetal skeleton is not yet formed. Furthermore, uncertainties regarding the fetal volume and mean radiation concentration might cause variations in the final dose estimates. Based on the current study and previous reported results, the authors recommend suggested doses of 1.5E−02 to 4E−02 mGy/MBq during early pregnancy, which is consistent with the standard values proposed previously. The dose estimates are lower for the later stages of pregnancy, and range from 0.4E−02 to 1.2E−02 mGy/MBq. However, this is significantly lower than the proposed standard value of 1.7E−0.2 mGy/MBq.
Additional recommendations made by the authors are as follows. First, a full dynamic scan should be considered in pregnant females having an 18F-FDG PET scan because it would not increase the overall radiation dose but would benefit the patient and allow the fetal dose to be calculated. PET/MR can be beneficial compared with PET/CT because it does not require ionizing radiation to correct attenuation. Fetal exposure should be minimized as much as possible, and improvements in PET instrumentation might allow comparable image quality with lower radiation doses. This could be enhanced further by facilitating voiding using hydration and a catheter wherever possible, particularly during the early stages of pregnancy.
We would be interested to hear your thoughts on this study. Does it advance the field? Will any results change the choices you make in your practice?