Patient-Specific Computational Model and Dosimetry Calculations for PET/CT of a Patient Pregnant with Twins

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

PURPOSE

Data published in the literature concerning the doses received by fetuses exposed to a 18 F-FDG PET are reassuring but were obtained from small and heterogeneous cohorts, and very few data are available concerning the fetal dose received after exposure to both PET and CT. The present study aimed to estimate the fetal dose received following a PET/CT exposure using methods that include anthropomorphic phantoms of pregnant women applied on a large cohort.

PATIENTS AND METHODS

This retrospective multicenter study included 18 pregnant patients in the second and third trimesters. For PET exposure, the fetal volume and mean concentration of radioactivity in the fetus were measured by manually drawing regions of interest. Those data, combined with the time-integrated activities of the fetus and the mother's organs, were entered into the OLINDA/EXM software 2.0 to assess the fetal dose due to PET exposure. To estimate the fetal dose received due to CT exposure, 2 softwares were used: CT-Expo (based on geometric phantom models of nonpregnant patients) and VirtualDose (using pregnant patient phantoms).

RESULTS

The fetal dose exposure for PET/CT examination in the second trimester ranged from 5.7 to 15.8 mGy using CT-Expo (mean, 11.6 mGy) and from 5.1 to 11.6 mGy using VirtualDose (mean, 8.6 mGy). In the third trimester, it ranged from 7.9 to 16.6 mGy using CT-Expo (mean, 10.7 mGy) and from 6.1 to 10.7 mGy using VirtualDose (mean, 7.6 mGy).

CONCLUSIONS

The estimated fetal doses were in the same range of those previously published and are well below the threshold for deterministic effects. Pregnancy does not constitute an absolute contraindication for a clinically justified hybrid 18 F-FDG PET/CT.

Pregnancy and Gastric Cancer: A Narrative Review

Cases of digestive cancers diagnosed during pregnancy are rare. The increasing prevalence of pregnancy in women aged 30-39 years (and not exceptionally 40-49 years) could explain the frequent co-occurrence of cancers and pregnancy. The diagnosis of digestive cancers in pregnancy is difficult due to the overlap between neoplasm symptomatology and the clinical picture of pregnancy. A paraclinical evaluation may also be difficult depending on the trimester of the pregnancy. Diagnosis is also delayed by practitioners' hesitation to use invasive investigations (imaging, endoscopy, etc.) due to fetal safety concerns. Therefore, digestive cancers are often diagnosed during pregnancy in advanced stages, where complications such as occlusions, perforations, and cachexia have already arisen. In this review, we highlight the epidemiology, clinical aspects, paraclinical evaluation, and particularities of the diagnosis and treatment of gastric cancer during pregnancy.

Hybrid computational pregnant female phantom construction for radiation dosimetry applications

The number of patients undergoing diagnostic radiology and radiation therapy procedures has increased drastically owing to improvements in cancer diagnosis and treatment and, consequently, patient survival. However, the risk of the occurrence of secondary malignancies due to radiation exposure remains a matter of concern. There are concerns about the fetus's health when pregnant women are exposed to and/or treated with ionizing radiation at various stages of pregnancy. We previously published three hybrid computational fetus phantoms, which contained 27 fetal organs, as a beginning point for developing the whole hybrid computational pregnant phantom set, which is the second objective of this study. An ICRP reference female voxel model was converted to a non-uniform rational basis spline (NURBS) surface model in order to construct a hybrid computational female phantom as a pregnant mother to each fetus model. Both the fetal and maternal organs were matched with ICRP-89 reference data. In order to create a complete standard pregnant computational phantom set at 20, 30, and 35 weeks of pregnancy, the model mother's reproductive organs were removed, and the fetus phantoms with appropriate placental and uterine models were added female pelvis using a 3D-modeling software. With the aid of radiological image sets that had been initially used to construct the fetus models, each fetus' position and rotation inside the uterus were carefully adjusted to represent the real fetal locations inside the uterus. The resulting fetus phantom was positioned in the appropriate location, matching the original radiological image sets. An obstetrician-gynecologist reviewed the complete internal anatomy of all fetus phantoms and the pregnant female for accuracy, and suggested changes were implemented as needed. This new set of hybrid computational pregnant phantom models has realistic anatomical details that can help evaluate fetal radiation doses where realistic fetal computational human phantoms are needed.

Construction of A Digital Fetus Library for Radiation Dosimetry

PURPOSE

Accurate estimation of fetal absorbed dose and radiation risks are crucial for radiation protection and important for radiological imaging research owing to the high radio-sensitivity of the fetus. Computational anthropomorphic models have been widely used in patient-specific radiation dosimetry calculations. In this work, we aim to build the first digital fetal library for more reliable and accurate radiation dosimetry studies.

ACQUISITION AND VALIDATION METHODS

Computed tomography (CT) images of abdominal and pelvic regions of 46 pregnant females were segmented by experienced medical physicists. The segmented tissues/organs include the body contour, skeleton, uterus, liver, kidney, intestine, stomach, lung, bladder, gall bladder, spleen and pancreas for maternal body, and placenta, amniotic fluid, fetal body, fetal brain and fetal skeleton. Non-Uniform Rational B-Spline (NURBS) surfaces of each identified region was constructed manually using 3D modeling software. The Hounsfield unit (HU) values of each identified organs were gathered from CT images of pregnant patients and converted to tissue density. Organ volumes were further adjusted according to reference measurements for the developing fetus recommended by the World Health Organization (WHO) and International Commission on Radiological Protection (ICRP). A series of anatomical parameters, including femur length (FL), humerus length (HL), biparietal diameter (BPD), abdominal circumference (FAC) and head circumference (HC) were measured and compared with WHO recommendations.

DATA FORMAT AND USAGE NOTES

The first fetal patient-specific model library was developed with the anatomical characteristics of each model derived from the corresponding patient whose gestational age varies between 8-weeks and 35-weeks. Voxelized models are represented in the form of MCNP matrix input files representing the three-dimensional model of the fetus. The size distributions of each model are also provided in text files. All data are stored on Zenodo and are publicly accessible on the following link: https://zenodo.org/record/6471884.

POTENTIAL APPLICATIONS

The constructed fetal models and maternal anatomical characteristics are consistent with the corresponding patients. The resulting computational fetus could be used in radiation dosimetry studies to improve the reliability of fetal dosimetry and radiation risks assessment. The advantages of NURBS surfaces in terms of adapting fetal postures and positions enable us to adequately assess their impact on radiation dosimetry calculations. This article is protected by copyright. All rights reserved.

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.

Feasibility, Safety and Impact of (18F)-FDG PET/CT in patients with pregnancy-associated cancer: experience of the French CALG (Cancer Associé à La Grossesse) network

BACKGROUND

The incidence of pregnancy-associated cancers has been increasing for decades. (18F)-FDG Positron Emission Tomography (PET)/Computed Tomography (CT) imaging has become a golden standard in the staging of many malignant diseases. The aims of the current study were to evaluate the feasibility, safety and impact of (18F)-FDG PET/CT performed during pregnancy.

MATERIAL AND METHODS

A retrospective analysis from the prospective database of the Cancer Associé à La Grossesse (CALG) network (Tenon Hospital, France) including patients who underwent (18F)-FDG PET/CT during their pregnancy between 2015 and 2020.

RESULTS

Of the 536 patients for whom advice from the CALG network was requested during the study period, 359 were diagnosed with cancer during pregnancy. Study population was composed of 63 (17.5%) patients who underwent (18F)-FDG PET/CT. Most cancers were diagnosed during the second trimester. Seventy-five percent were diagnosed with breast cancer, mostly locally advanced invasive ductal carcinomas. Median term of pregnancy at PET/CT was 24.8 weeks of gestation. Twelve (19%), 24 (38.1%) and 22 (34.9%) patients underwent the exam during the 1st, 2nd and 3rd trimester, respectively. (18F)-FDG PET/CT resulted in stage modification for 38 (60.3%) of the patients (28 with more extensive lymph node involvement and 10 with metastatic disease) with subsequently/accordingly modified first-line medical treatment. Fifty patients gave birth to healthy newborns. Two patients had a medical termination of pregnancy, five had a medical abortion, one neonatal death occurred in a patient with severe preeclampsia (unrelated to (18F)-FDG PET/CT). The data of 46 children were available at 6 months, 29 at 12 months, and 15 at 24 months. No cases of mental retardation, childhood cancer, or malformation were reported within 2 years.

CONCLUSION

(18F)-FDG PET/CT has a major impact on the management of pregnancy-associated cancers and does not appear to cause fetal side effects suggesting that the exam is feasible during pregnancy as maternal benefits outweigh fetal risks.

18F-Fludeoxyglucose absorbed dose estimation in fetus during early pregnancy

The purpose of this study is to assess a rare case of fetal radiation absorbed dose here through ¹⁸F-Fludeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) in early pregnancy (5-week-old fetus). The fetal absorbed dose due to the radiation emitted from the mother's body, the fetus self-dose, and the dose received from CT were computed. The 35-year-old patient, weighing 85 kg, was injected with 370 MBq of ¹⁸F-FDG. Imaging started at 1 h with CT acquisition followed by PET imaging. The photon and positron self-dose was calculated by applying the Monte Carlo (MC) GATE (GEANT 4 Application for Tomographic Emission) code. The volume of absorbed dose from the mother's body organs and the absorbed dose from the CT were added to the self-dose to obtain the final dose. The volume of self-dose obtained through MC simulation for the fetus was 3.3 × 10^-2 mGy/MBq, of which 2.97 × 10^-2 mGy/MBq was associated with positrons and 0.33 × 10^-2 mGy/MBq was associated with photons. Biologically, the absorbed dose from CT, 7.3 mGy, had to be added to the total dose. The absorbed dose by the fetus during early pregnancy was higher than the standard value of 2.2 × 10^-2 mGy/MBq (MIRD DER) because, during the examinations, the mother's bladder was full. This issue was a concern during updating standards.

Construction of realistic hybrid computational fetal phantoms from radiological images in three gestational ages for radiation dosimetry applications

Radiation exposure and associated radiation risks are major concerns for fetal development for pregnant patients who undergo radiation therapy or diagnostic imaging procedures. In order to accurately estimate the radiation dose to the fetus and assess the uncertainty of fetal position and rotation, three hybrid computational fetus phantoms were constructed using magnetic resonance imaging (MRI) for each fetus model as a starting point to construct a complete anatomically accurate fetus, gravid uterus, and placenta. A total of 27 fetal organs were outlined from radiological images via the Velocity Treatment Planning System. The DICOM-Structure set was imported to Rhinoceros software for further reconstruction of 3D fetus phantom model sets. All fetal organ masses were compared with ICRP-89 reference data. Our fetal model series corresponds to 20, 31, and 35 weeks of pregnancy, thus covering the second and third trimester. Fetal positions and locations were carefully adapted to represent the real fetus locations inside the uterus for each trimester of pregnancy. The new series of hybrid computational fetus models together with pregnant female models can be used in evaluating fetal radiation doses in diagnostic imaging and radiotherapy procedures.

Estimation of the radiation dose in pregnancy: an automated patient-specific model using convolutional neural networks

OBJECTIVES

The conceptus dose during diagnostic imaging procedures for pregnant patients raises health concerns owing to the high radiosensitivity of the developing embryo/fetus. The aim of this work is to develop a methodology for automated construction of patient-specific computational phantoms based on actual patient CT images to enable accurate estimation of conceptus dose.

METHODS

We developed a 3D deep convolutional network algorithm for automated segmentation of CT images to build realistic computational phantoms. The neural network architecture consists of analysis and synthesis paths with four resolution levels each, trained on manually labeled CT scans of six identified anatomical structures. Thirty-two CT exams were augmented to 128 datasets and randomly split into 80%/20% for training/testing. The absorbed doses for six segmented organs/tissues from abdominal CT scans were estimated using Monte Carlo calculations. The resulting radiation doses were then compared between the computational models generated using automated segmentation and manual segmentation, serving as reference.

RESULTS

The Dice similarity coefficient for identified internal organs between manual segmentation and automated segmentation results varies from 0.92 to 0.98 while the mean Hausdorff distance for the uterus is 16.1 mm. The mean absorbed dose for the uterus is 2.9 mGy whereas the mean organ dose differences between manual and automated segmentation techniques are 0.07%, - 0.45%, - 1.55%, - 0.48%, - 0.12%, and 0.28% for the kidney, liver, lung, skeleton, uterus, and total body, respectively.

CONCLUSION

The proposed methodology allows automated construction of realistic computational models that can be exploited to estimate patient-specific organ radiation doses from radiological imaging procedures.

KEY POINTS

• The conceptus dose during diagnostic radiology and nuclear medicine imaging procedures for pregnant patients raises health concerns owing to the high radiosensitivity of the developing embryo/fetus. • The proposed methodology allows automated construction of realistic computational models that can be exploited to estimate patient-specific organ radiation doses from radiological imaging procedures. • The dosimetric results can be used for the risk-benefit analysis of radiation hazards to conceptus from diagnostic imaging procedures, thus guiding the decision-making process.

Construction of patient-specific computational models for organ dose estimation in radiological imaging

PURPOSE

Diagnostic imaging procedures require optimization depending on the medical task at hand, the apparatus being used, and patient physical and anatomical characteristics. The assessment of the radiation dose and associated risks plays a key role in safety and quality management for radiation protection purposes. In this work, we aim at developing a methodology for personalized organ-level dose assessment in x-ray computed tomography (CT) imaging.

METHODS

Regional voxel models representing reference patient-specific computational phantoms were generated through image segmentation of CT images for four patients. The best-fitting anthropomorphic phantoms were selected from a previously developed comprehensive phantom library according to patient's anthropometric parameters, then registered to the anatomical masks (skeleton, lung, and body contour) of patients to produce a patient-specific whole-body phantom. Well-established image registration metrics including Jaccard's coefficients for each organ, organ mass, body perimeter, organ-surface distance, and effective diameter are compared between the reference patient model, registered model, and anchor phantoms. A previously validated Monte Carlo code is utilized to calculate the absorbed dose in target organs along with the effective dose delivered to patients. The calculated absorbed doses from the reference patient models are then compared with the produced personalized model, anchor phantom, and those reported by commercial dose monitoring systems.

RESULTS

The evaluated organ-surface distance and body effective diameter metrics show a mean absolute difference between patient regional voxel models, serving as reference, and patient-specific models around 4.4% and 4.5%, respectively. Organ-level radiation doses of patient-specific models are in good agreement with those of the corresponding patient regional voxel models with a mean absolute difference of 9.1%. The mean absolute difference of organ doses for the best-fitting model extracted from the phantom library and Radimetrics™ commercial dose tracking software are 15.5% and 41.1%, respectively.

CONCLUSION

The results suggest that the proposed methodology improves the accuracy of organ-level dose estimation in CT, especially for extreme cases [high body mass index (BMI) and large skeleton]. Patient-specific radiation dose calculation and risk assessment can be performed using the proposed methodology for both monitoring of cumulative radiation exposure of patients and epidemiological studies. Further validation using a larger database is warranted.