In vivo quantification of 18f-fdg uptake in human placenta during early pregnancy

Fetal Dose from PET and CT in Pregnant Patients

When pregnancy is discovered during or after a diagnostic examination, the physician or the patient may request an estimate of the radiation dose received by the fetus as per guidelines and standard operating procedures. This study provided the imaging community with dose estimates to the fetus from PET/CT with protocols that are adapted to University of Michigan low-dose protocols for patients known to be pregnant. Methods: There were 9 patients analyzed with data for the first, second, and third trimesters, the availability of which is quite rare. These images were used to calculate the size-specific dose estimate (SSDE) from the CT scan portion and the SUV and ¹⁸F-FDG uptake dose from the PET scan portion using the MIRD formulation. The fetal dose estimates were tested for correlation with each of the following independent measures: gestational age, fetal volume, average water-equivalent diameter of the patient along the length of the fetus, SSDE, SUV, and percentage of dose from ¹⁸F-FDG. Stepwise multiple linear regression analysis was performed to assess the partial correlation of each variable. To our knowledge, this was the first study to determine fetal doses from CT and PET images. Results: Fetal self-doses from ¹⁸F for the first, second, and third trimesters were 2.18 mGy (single data point), 0.74-1.82 mGy, and 0.017-0.0017 mGy, respectively. The combined SSDE and fetal self-dose ranged from 1.2 to 8.2 mGy. These types of images from pregnant patients are rare. Conclusion: Our data indicate that the fetal radiation exposure from ¹⁸F-FDG PET and CT performed, when medically necessary, on pregnant women with cancer is low. All efforts should be made to minimize fetal radiation exposure by modifying the protocol.

Use of Positron Emission Tomography for Pregnancy-Associated Cancer Assessment: A Review

Background. Positron emission tomography (PET) has proven clinical utility both in the initial and relapse staging phase, but this technique is controversial during pregnancy. The objective of this review is to provide a compendium of available information on the use of PET during pregnancy. Materials and methods. A systematic literature review was conducted from 1 January 2004 until 20 May 2021. A total of 4 small series and 9 case reports consisting of 25 cases were selected. Results. During the first trimester, the fetus is most sensitive to ionization damage, so lower doses are recommended (2.6E-02 mGy/MBq). Fetal-effective doses are higher in this period and the average fetal dose (4.06 ± 3.22 mGy) remains significantly below the threshold for deterministic effects. During the second and third trimesters, recommended doses are higher (1.4E-02 mGy/MBq at 6 months, and 6.9E-03 mGy/MBq at 9 months of gestation). ¹⁸F-FDG activity was distributed to the whole fetus with a prevalence of myocardial tissue in seven cases. The use of special precautions, such as PET-magnetic resonance (MR) and urinary bladder catheterization, reduces the amount of radioactive tracer. Breastfeeding interruption is not recommended. Conclusions. ¹⁸F-FDG PET is not contraindicated in pregnancy, but multidisciplinary discussion is necessary and strict precautions are recommended.

Radiation Absorbed Dose to the Embryo and Fetus from Radiopharmaceuticals

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.

A Case of Recurrent Esophageal Cancer Treated with Concurrent Chemoradiation Therapy in Pregnancy

Esophageal cancer rarely coincides with pregnancy, and only five cases have been reported thus far. The management of esophageal cancer during pregnancy is extremely challenging due to its aggressive nature. We herein report a case of recurrent esophageal cancer in pregnancy. A 41-year-old multigravida with a history of esophageal squamous cell cancer treated with esophagectomy and perioperative chemotherapy was diagnosed with local recurrent carcinoma of the residual esophagus at 16 weeks of gestation. The patient strongly desired to continue the pregnancy, and concurrent chemoradiation therapy (CRT) consisting of 50.4 Gy of radiation, cisplatin, and 5-fluorouracil was carried out from 19 weeks of gestation. CRT was dramatically effective, and the recurrent lesion disappeared. At 38 weeks of gestation, she underwent cesarean section and delivered a healthy female baby. Both maternal and fetal courses were satisfactory, and the patient has been free of disease for 12 months. This is the first case of recurrent esophageal cancer in pregnancy in which CRT was completed without reducing treatment intensity and led to a complete response. Nevertheless, little is known regarding the safety and possible adverse effects of CRT on the fetus. Therefore, deliberate selection of patients and long-term follow-up of the child are necessary.

The use of PET/CT in pregnancy: A case report of malignant parathyroid carcinoma and a review of the literature

Positron emission tomography scanning is not commonly performed in pregnancy but can be done if required. Fetal doses of radiation can be minimized, and our case exemplifies the safe application of positron emission tomography/computed tomography in pregnancy. A 38-year-old woman in her first ongoing pregnancy presented at 28 weeks' gestation with symptomatic hypercalcemia. Given a history of parathyroid carcinoma, recurrence was suspected. Ultrasound and magnetic resonance imaging failed to locate the lesion. However, positron emission tomography/computed tomography identified a culprit supraclavicular lymph node. This was excised under local anesthesia resulting in normalization of parathyroid hormone and calcium levels. A term, healthy baby was delivered. The literature provides support that the use of positron emission tomography/computed tomography is acceptable when indicated, and there are modifications to protocols that can further limit risk.

Development of computational pregnant female and fetus models and assessment of radiation dose from positron-emitting tracers

PURPOSE

Molecular imaging using PET and hybrid (PET/CT and PET/MR) modalities nowadays plays a pivotal role in the clinical setting for diagnosis and staging, treatment response monitoring, and radiation therapy treatment planning of a wide range of oncologic malignancies. The developing embryo/fetus presents a high sensitivity to ionizing radiation. Therefore, estimation of the radiation dose delivered to the embryo/fetus and pregnant patients from PET examinations to assess potential radiation risks is highly praised.

METHODS

We constructed eight embryo/fetus models at various gestation periods with 25 identified tissues according to reference data recommended by the ICRP publication 89 representing the anatomy of the developing embryo/fetus. The developed embryo/fetus models were integrated into realistic anthropomorphic computational phantoms of the pregnant female and used for estimating, using Monte Carlo calculations, S-values of common positron-emitting radionuclides, organ absorbed dose, and effective dose of a number of positron-emitting labeled radiotracers.

RESULTS

The absorbed dose is nonuniformly distributed in the fetus. The absorbed dose of the kidney and liver of the 8-week-old fetus are about 47.45 % and 44.76 % higher than the average absorbed dose of the fetal total body for all investigated radiotracers. For ¹⁸F-FDG, the fetal effective doses are 2.90E-02, 3.09E-02, 1.79E-02, 1.59E-02, 1.47E-02, 1.40E-02, 1.37E-02, and 1.27E-02 mSv/MBq at the 8th, 10th, 15th, 20th, 25th, 30th, 35th, and 38th weeks of gestation, respectively.

CONCLUSION

The developed pregnant female/fetus models matching the ICRP reference data can be exploited by dedicated software packages for internal and external dose calculations. The generated S-values will be useful to produce new standardized dose estimates to pregnant patients and embryo/fetus from a variety of positron-emitting labeled radiotracers.

PET/CT imaging reveals unrivaled placental avidity for glucose compared to other tissues

INTRODUCTION

The goal of this study was to define the kinetics of glucose transport from maternal blood to placenta to fetus using real time imaging.

METHODS

Positron emission tomography (PET) imaging of the glucose-tracer [(18)F]fluorodeoxyglucose (FDG) was used to temporally and spatially define, in vivo, the kinetics of glucose transport from maternal blood into placentae and fetuses, in the late gestational gravid rat. Computed tomography (CT), with intravenous contrast, co-registered to the PET images allowed anatomic differentiation of placentae from fetal and maternal tissues.

RESULTS

FDG was rapidly taken up by placentae and subsequently appeared in fetuses with minimal temporal lag. FDG standardized uptake values in placentae and fetuses approached that of maternal brain. In both anesthetized and awake dams, one quarter of the administered FDG ultimately was accrued in the collective fetuses and placentae. Accordingly, kinetic modeling demonstrated that the placentae had very high avidity for FDG, 2-fold greater than that of the fetus and maternal brain, when accounting for the fact that fetal FDG necessarily must first be taken up by placentae. Consistent with this, placental expression of glucose transporter 1 exceeded that of all other tissues.

DISCUSSION

Fetal and placental tissues place a substantial glucose metabolic burden on the mother, owing to very high avidity of placentae for glucose coupled with the large relative mass of fetal and placental tissues.

CONCLUSIONS

The placenta has a tremendous capacity to uptake and transport glucose. PET/CT imaging is an ideal means to study metabolite transport kinetics in the fetoplacental unit.

Fetal and maternal absorbed dose estimates for positron-emitting molecular imaging probes

PET and hybrid (PET/CT and PET/MR) imaging currently play a pivotal role in clinical diagnosis, staging and restaging, treatment, and surveillance of several diseases. As such, limiting the radiation exposure of special patients, such as pregnant women, from PET procedures is an important challenge that needs to be appropriately addressed because of the high sensitivity of the developing embryo/fetus to ionizing radiation. Therefore, accurate radiation dose calculation for the embryo/fetus and pregnant patient from common positron-emitting radiotracers is highly desired.

METHODS

To obtain representative estimates of radiation dose to the human body, realistic biologic and physical models should be used. In this work, we evaluate the S values of 9 positron-emitting radionuclides ((11)C, (13)N, (15)O, (18)F, (64)Cu, (68)Ga, (82)Rb, (86)Y, and (124)I) and the absorbed and effective doses for 21 positron-emitting labeled radiotracers using realistic anthropomorphic computational phantoms of early pregnancy and at 3-, 6-, and 9-mo of gestation and the most recent biokinetic data available. The Monte Carlo N-Particle eXtended general-purpose Monte Carlo code was used for radiation transport simulation.

RESULTS

The absorbed dose to the pregnant model is less influenced by the gestation for most organs or tissues, but the anatomic changes of the maternal body increases the effective dose for some radiotracers. For (18)F-FDG, the estimated absorbed doses to the embryo/fetus are 3.05E-02, 2.27E-02, 1.50E-02, and 1.33E-02 mGy/MBq at early pregnancy and 3-, 6-, and 9-mo gestation, respectively. The absorbed dose is nonuniformly distributed in the fetus and would be 1.03-2 times higher in the fetal brain than in other fetal soft tissues.

CONCLUSION

The generated S values can be exploited to estimate the radiation dose delivered to pregnant patients and the embryo/fetus from various PET radiotracers used in clinical and research settings. The generated dosimetric database of radiotracers using new-generation computational models can be used for the assessment of radiation risks to pregnant women and the embryo/fetus undergoing PET/CT imaging procedures. This work also contributes to a better understanding of the absorbed dose distribution in the fetus.

Pelvis: normal variants and benign findings in FDG-PET/CT imaging

With the widespread use of whole-body fluorodeoxyglucose (FDG)-PET/computed tomography as a diagnostic tool in patients with cancer, incidental findings are of increasing importance. This is particularly true within the pelvis, where several benign findings might present with increased FDG uptake. In addition, physiologic excretion of radiotracer by way of the urinary tract can complicate image analysis. This article reviews potential incidental benign findings in the pelvis that one should be aware of when interpreting FDG-PET/computed tomography scans.

Radiation dose to the fetus from [(18)F]-FDG administration during the second trimester of pregnancy

The authors estimated the fetal radiation dose from [(18)F]-FDG in a rare case of a woman who underwent a PET/CT scan during the second trimester of pregnancy. The patient, a 27-y-old female with a paraganglioma, received 181.3 MBq [(18)F]-FDG. From the concentrations of radioactivity measured on the images, the time-integrated activity coefficients of the fetus and the placenta were derived. The time-integrated activity coefficients of the mother's organs were taken from the standard values of ICRP publication 106. The final fetal dose was calculated using the 6-mo pregnant model of the OLINDA/EXM software. The fetus showed an overall low and homogeneous [(18)F]-FDG uptake, with an average concentration of 2.41 kBq cm(-3). The uptake in the placenta was generally higher (average concentration = 3.69 kBq cm(-3)). The estimated time-integrated activity coefficients were 0.0130 and 0.0058 Bq h Bq(-1) for the fetus and the placenta, respectively. The final average dose to the fetus was 1.97 × 10(-2) mGy MBq(-1) (3.6 mGy in this patient who received 181.3 MBq). Therefore, the dose to the fetus from [(18)F]-FDG administration during the second trimester of pregnancy is low. When medically indicated, pregnancy should not be a categorical basis for withholding [(18)F]-FDG PET scans.

Effect of insulin and dexamethasone on fetal assimilation of maternal glucose

The growing fetus depends upon transfer of glucose from maternal blood to fetal tissues. Insulin and glucocorticoid impact maternal glucose metabolism, but the effects of these hormones on fetal glucose assimilation in vivo are understudied. We thus used positron emission tomography imaging to determine the disposition of [(18)F]fluorodeoxyglucose (FDG) in rats on gestational d 20, quantifying the kinetic competition of maternal tissues and fetus for glucose. Three fasting maternal states were studied: after 2-d dexamethasone (DEX), during euglycemic hyperinsulinemic clamp insulin receiving (INS), and control (CON). In CON and DEX mothers, FDG accumulation in fetuses and placentae was substantial, rivaling that of maternal brain. By contrast, FDG accumulation was reduced in INS fetuses, placentae, and maternal brain by approximately 2-fold, despite no diminution in FDG extraction kinetics from maternal blood into these structures. The reduced FDG accumulation was due to more rapid clearance of FDG from the circulation in INS mothers, related to increased FDG avidity in INS select maternal tissues, including skeletal muscle, brown adipose tissue, and heart. DEX treatment of mothers reduced fetal weight by nearly 10%. Nonetheless, the accumulation of FDG into placentae and fetuses was similar in DEX and CON mothers. In our rat model, fetal growth restriction induced by DEX does not involve diminished glucose transport to the fetus. Maternal insulin action has little effect on the inherent avidity of the fetal-placental unit for glucose but increases glucose utilization by maternal tissues, thus indirectly reducing the glucose available to the fetus.

Absorbed 18F-FDG dose to the fetus during early pregnancy

We describe a rare case of a woman who underwent (18)F-FDG PET/CT during early pregnancy (fetus age, 10 wk). The fetal absorbed dose was calculated by taking into account the (18)F-FDG fetal self-dose, photon dose coming from the maternal tissues, and CT dose received by both mother and fetus.

METHODS

The patient (weight, 71 kg) had received 296 MBq of (18)F-FDG. Imaging started at 1 h, with unenhanced CT acquisition, followed by PET acquisition. From the standardized uptake value measured in fetal tissues, we calculated the total number of disintegrations per unit of injected activity. Monte Carlo analysis was then used to derive the fetal (18)F-FDG self-dose, including positrons and self-absorbed photons. Photon dose from maternal tissues and CT dose were added to obtain the final dose.

RESULTS

The maximum standardized uptake value in fetal tissues was 4.5. Monte Carlo simulation showed that the fetal self-dose was 3.0 x 10(-2) mGy/MBq (2.7 x 10(-2) mGy/MBq from positrons and 0.3 x 10(-2) mGy/MBq from photons). The estimated photon dose to the fetus from maternal tissues was 1.04 x 10(-2) mGy/MBq. Accordingly, the specific (18)F-FDG dose to the fetus was about 4.0 x 10(-2) mGy/MBq (11.8 mGy in this patient). The CT scan added a further 10 mGy.

CONCLUSION

The dose to the fetus during early pregnancy can be as high as 4.0 x 10(-2) mGy/MBq of (18)F-FDG. Current dosimetric standards in early pregnancy may need to be revised.