Radiation Absorbed Dose to the Embryo and Fetus from Radiopharmaceuticals

Abstract

Nuclear medicine procedures are generally avoided during pregnancy out of concern for the radiation dose to the fetus. However, for clinical reasons, radiopharmaceuticals must occasionally be administered to pregnant women. The procedures most likely to be performed voluntarily during pregnancy are lung scans to diagnose pulmonary embolism and ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) scans for the staging of cancers. This article focuses on the challenges of fetal dose calculation after administering radiopharmaceuticals to pregnant women. In particular, estimation of the fetal dose is hampered by the lack of fetal biokinetic data of good quality and is subject to the variability associated with methodological choices in dose calculations, such as the use of various anthropomorphic phantoms and modeling of the maternal bladder. Despite these sources of uncertainty, the fetal dose can be reasonably calculated within a range that is able to inform clinical decisions. Current dose estimates suggest that clinically justified nuclear medicine procedures should be performed even during pregnancy because the clinical benefits for the mother and the fetus outweigh the small and purely hypothetical radiation risk to the fetus. In addition, the fetal radiation dose should be minimized without compromising image quality, such as by encouraging bladder voiding and by using positron emission tomography (PET)/magnetic resonance imaging (MRI) devices or high-sensitivity PET scanners that generate images of good quality with a lower injected activity.

Figure 1:

Plotting the fetal time-integrated activity data points calculated in pregnant women injected with ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) shows that fetal uptake follows a sigmoid curve. A mathematical function fitted through the datapoints allows the fetal dose to be interpolated for the weeks of pregnancy for which data are not currently available. Before the 10^th week, when the fetus is not clearly visible in the images, the dose to the uterus was used as proxy. The white point represents an outlying point automatically removed during the fitting process. Figure adapted from.

Figure 2:

Phantoms for the third month (top row), sixth month (middle row), and ninth month (bottom row) of gestation of the stylized phantoms (left column) and the voxel-based phantoms (right column). Reprinted with permission from^, .

Figure 3:

Coronal CT (A) and ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) positron emission tomography (PET) (B) image of a 25 weeks-pregnant woman diagnosed with non-Hodgkin lymphoma while expecting twins. The white arrow points to a lymphoma mass in the right breast. The yellow arrow points to the heart of one of the fetuses, which displays high ¹⁸F-FDG uptake, while the brain (red arrow) shows much lower metabolic activity, especially compared to the brain of the mother. This low cerebral metabolic activity is a general finding in fetuses imaged with ¹⁸F-FDG. The dose for both fetuses was calculated by creating a patient-specific anthropomorphic phantom (C) based on the CT images which was used as input for Monte Carlo calculations. Details on the construction of the phantom can be found in. Images reproduced with permission from^, .

Figure 4:

Magnetic resonance (MR) (A) and ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) positron emission tomography (PET) (B) transaxial slices of a 19 weeks-pregnant woman with lymphoma scanned with a PET/MRI machine. The images clearly illustrate how PET/MRI allows superior delineation of the fetal body. Indeed, the region of interest around the fetus encompasses some cold areas that would not have been included if a coregistered MRI had not been available. In consequence, it can reasonably be assumed that dosimetry estimates obtained with PET/MRI are more accurate than those obtained with PET only or even with PET/CT machines. Reprinted with permission from.