Two pregnant patients in the second and third trimester with an indication for a clinical 18F-FDG PET/CT scan for staging of the disease were retrospectively included. Data was collected at the Amsterdam UMC, location VUmc from ongoing clinical investigations and the use of anonymized clinical data was waived by the VU Medical Centre ethics review board. Both scans were collected on the Siemens Vision Quadra PET/CT system (Siemens Healthineers, Knoxville, TN, USA) (LAFOV PET/CT system). The scans were acquired using one static bed position for 20 min. In one patient, the foetus was not included in the field-of-view (FOV) to further reduce foetal radiation exposure. The scan covered the skull vertex to the lower part of the kidneys with the head centred in the middle of the FOV. In the other patient, the foetus was included in the FOV and covered the skull vertex to mid-thigh (Fig. 1a, b).

18F-FDG PET/CT image including both the mother and the foetus with (a) a Maximum Intensity Projection (MIP) 18F-FDG PET image and (b) a slice of the ultra-low-dose CT scan

The patients received an intravenous bolus injection of 0.3 MBq/kg radioactive 18F-FDG and were fasting for 4–6 h before the scan. The scans were reconstructed with the European Association of Nuclear Medicine Research Ltd 2 (EARL-2) compliant reconstruction protocol [5, 7, 9]. Foetal radiation dose from radioactive 18F-FDG administration was estimated using dose conversion factors (mGy/MBq) from three published papers including second and third-trimester pregnancies [1, 3, 6]. All voiding models were included since the voiding pattern was unknown. Dose conversion factors from first-trimester pregnancies were excluded because foetuses in earlier stages absorb more radiation than foetuses in later stages of the pregnancy [3, 6, 8].

An ultra-low-dose CT (LDCT) scan was applied to correct the PET scan for attenuation. A kilovoltage peak of 100 kVp, tube current of 20 mAs, 0.5 s rotation time and a pitch factor of 1.45 was applied for both CT scans. Radiation exposure to the foetus from the LDCT scans was based on the computed tomography dose index (CTDI) measured in milligray (mGy) and estimated using CT-Expo (version 2.7).