18F-FET-PET imaging in high-grade gliomas and brain metastases: a systematic review and meta-analysis

Autoradiography of Intracerebral Tumours in the Chick Embryo Model: A Feasibility Study Using Different PET Tracers

PURPOSE

In addition to rodent models, the chick embryo model has gained attention for radiotracer evaluation. Previous studies have investigated tumours on the chorioallantoic membrane (CAM), but its value for radiotracer imaging of intracerebral tumours has yet to be demonstrated.

PROCEDURES

Human U87 glioblastoma cells and U87-IDH1 mutant glioma cells were implanted into the brains of chick embryos at developmental day 5. After 12-14 days of tumour growth, blood-brain-barrier integrity was evaluated in vivo using MRI contrast enhancement or ex vivo with Evans blue dye. The tracers O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) (n = 5), 3,4-dihydroxy-6-[18F]-fluoro-L-phenylalanine ([18F]FDOPA) (n = 3), or [68Ga] labelled quinoline-based small molecule fibroblast activation protein inhibitor ([68Ga]FAPI-46) (n = 4) were injected intravenously if solid tumours were detected with MRI. For time-activity curves for [18F]FET, additional micro PET (µPET) was performed. The chick embryos were sacrificed 60 min post-injection, and cryosections of the tumour-bearing brains were produced and evaluated with autoradiography and immunohistochemistry.

RESULTS

Intracerebral tumours were produced with a 100% success rate in viable chick embryos at the experimental endpoint. However, 52% of chick embryos (n = 85) did not survive the procedure to embryonic development day 20. For the evaluated radiotracers, the tumour-to-brain ratios (TBR) derived from ex vivo autoradiography, as well as the tracer kinetics derived from µPET for intracerebral chick embryo tumours, were comparable to those previously reported in rodents and patients: the TBRmean for [18F]FET was 1.69 ± 0.54 (n = 5), and 3.8 for one hypermetabolic tumour and < 2.0 for two isometabolic tumors using [18F]FDOPA, with a TBRmean of 1.92 ± 1,11 (n = 3). The TBRmean of [68Ga]FAPI-46 for intracerebral chick embryo tumours was 19.13 ± 0.64 (n = 4). An intact blood-tumour barrier was observed in one U87-MG tumour (n = 5).

CONCLUSIONS

Radiotracer imaging of intracerebral tumours in the chick embryo offers a fast model for the evaluation of radiotracer uptake, accumulation, and kinetics. Our results indicate a high comparability between intracerebral tumour imaging in chick embryos and xenograft rodent models or brain tumour patients.

Precision radiotherapy with molecular-profiling of CNS tumours

Diagnoses of CNS malignancies in the primary and metastatic setting have significantly advanced in the last decade with the advent of molecular pathology. Using a combination of immunohistochemistry, next-generation sequencing, and methylation profiling integrated with traditional histopathology, patient prognosis and disease characteristics can be understood to a much greater extent. This has recently manifested in predicting response to targeted drug therapies that are redefining management practices of CNS tumours. Radiotherapy, along with surgery, still remains an integral part of treating the majority of CNS tumours. However, the rapid advances in CNS molecular diagnostics have not yet been effectively translated into improving CNS radiotherapy. We explore several promising strategies under development to integrate molecular oncology into radiotherapy, and explore future directions that can serve to use molecular diagnostics to personalize radiotherapy. Evolving the management of CNS tumours with molecular profiling will be integral to supporting the future of precision radiotherapy.

FET PET to differentiate between post-treatment changes and recurrence in high-grade gliomas: a single center multidisciplinary clinic controlled study

PURPOSE

The clinico-radiological dilemma in post-treatment high-grade gliomas, between disease recurrence (TR) and treatment-related changes (TRC), still persists. FET (Fluoro-ethyl-tyrosine) PET has been extensively used as problem-solving modality for cases where MR imaging is inconclusive. We incorporated a systematic imaging and clinical follow-up algorithm in a multi-disciplinary clinic (MDC) setting to analyse our cohort of FET PET in post-treatment gliomas.

METHODS

We retrospectively analyzed 171 patients of post-treatment grade III and IV glioma with equivocal findings on MRI. 185-222 MBq of 18 F-FET was injected and dedicated static imaging of brain was performed at 20 min. TBR (Tumor to background ratio) was used as semi-quantitative parameter. Cutoff of 2.5 was used for image interpretation. Imaging findings were confirmed with histopathological diagnosis, wherever available or in a multidisciplinary joint clinic based on serial imaging.

RESULTS

121 of 171 patients showed recurrent disease on FET PET, on follow up, 109 were confirmed with recurrence; 7 patients showed TRC, whereas 5 were treated with bevacizumab, with no further clinico-radiological deterioration, thus confirming TRC. 50 patients showed TRC on FET PET, on follow up on follow up, 40 were confirmed as true-negative. 10 patients who showed TBR less than 2.5 had confirmed TR on subsequent MR imaging. The overall sensitivity and specificity was 91.6 and 76.9% respectively, with a diagnostic accuracy of 87.13%.

CONCLUSION

There is potential for FET PET to be used along with MRI in the post treatment algorithm of high-grade glial tumors.

Brain Tumor Assessment: Integrating PET/Computed Tomography and MR Imaging Modalities

While MR imaging is the main imaging modality to assess brain tumors, PET imaging has a specific role. Among the many tracers that have been proposed and are still being developed, 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) and O-(2-[18F]-fluoroethyl)-l-tyrosine ([18F]FET) PET remain the most solidly established in the clinics. In particular, [18F]FET has gained increased acceptance due to its higher sensitivity. In this paper, we present an overview of the current clinical status of brain tumor imaging, with emphasis on PET imaging.

Synthetic O-(2-18F-fluoroethyl)-l-tyrosine-positron emission tomography generation and hotspot prediction via preoperative MRI fusion of gliomas lacking radiographic high-grade characteristics

Background

Limited amino acid availability for positron emission tomography (PET) imaging hinders therapeutic decision-making for gliomas without typical high-grade imaging features. To address this gap, we evaluated a generative artificial intelligence (AI) approach for creating synthetic O-(2-18F-fluoroethyl)-l-tyrosine ([18F]FET)-PET and predicting high [18F]FET uptake from magnetic resonance imaging (MRI).

Methods

We trained a deep learning (DL)-based model to segment tumors in MRI, extracted radiomic features using the Python PyRadiomics package, and utilized  a Random Forest classifier to predict high [18F]FET uptake. To generate [18F]FET-PET images, we employed a generative adversarial network framework and utilized a split-input fusion module for processing different MRI sequences through feature extraction, concatenation, and self-attention.

Results

We included magnetic resonance imaging (MRI) and PET images from 215 studies for the hotspot classification and 211 studies for the synthetic PET generation task. The top-performing radiomic features achieved 80% accuracy for hotspot prediction. From the synthetic [18F]FET-PET, 85% were classified as clinically useful by senior physicians. Peak signal-to-noise ratio analysis indicated high signal fidelity with a peak at 40 dB, while structural similarity index values showed structural congruence. Root mean square error analysis demonstrated lower values below 5.6. Most visual information fidelity scores ranged between 0.6 and 0.7. This indicates that synthetic PET images retain the essential information required for clinical assessment and diagnosis.

Conclusion

For the first time, we demonstrate that predicting high [18F]FET uptake and generating synthetic PET images from preoperative MRI in lower-grade and high-grade glioma are feasible. Advanced MRI modalities and other generative AI models will be used to improve the algorithm further in future studies.

Umbrella review and network meta-analysis of diagnostic imaging test accuracy studies in Differentiating between brain tumor progression versus pseudoprogression and radionecrosis

PURPOSE

In this study we gathered and analyzed the available evidence regarding 17 different imaging modalities and performed network meta-analysis to find the most effective modality for the differentiation between brain tumor recurrence and post-treatment radiation effects.

METHODS

We conducted a comprehensive systematic search on PubMed and Embase. The quality of eligible studies was assessed using the Assessment of Multiple Systematic Reviews-2 (AMSTAR-2) instrument. For each meta-analysis, we recalculated the effect size, sensitivity, specificity, positive and negative likelihood ratios, and diagnostic odds ratio from the individual study data provided in the original meta-analysis using a random-effects model. Imaging technique comparisons were then assessed using NMA. Ranking was assessed using the multidimensional scaling approach and by visually assessing surface under the cumulative ranking curves.

RESULTS

We identified 32 eligible studies. High confidence in the results was found in only one of them, with a substantial heterogeneity and small study effect in 21% and 9% of included meta-analysis respectively. Comparisons between MRS Cho/NAA, Cho/Cr, DWI, and DSC were most studied. Our analysis showed MRS (Cho/NAA) and 18F-DOPA PET displayed the highest sensitivity and negative likelihood ratios. 18-FET PET was ranked highest among the 17 studied techniques with statistical significance. APT MRI was the only non-nuclear imaging modality to rank higher than DSC, with statistical insignificance, however.

CONCLUSION

The evidence regarding which imaging modality is best for the differentiation between radiation necrosis and post-treatment radiation effects is still inconclusive. Using NMA, our analysis ranked FET PET to be the best for such a task based on the available evidence. APT MRI showed promising results as a non-nuclear alternative.

18F-Fluoroethyl-L Tyrosine Positron Emission Tomography Radiomics in the Differentiation of Treatment-Related Changes from Disease Progression in Patients with Glioblastoma

The follow-up of glioma patients after therapeutic intervention remains a challenging topic, as therapy-related changes can emulate true progression in contrast-enhanced magnetic resonance imaging. 18F-fluoroethyl-tyrosine (18F-FET) is a radiopharmaceutical that accumulates in glioma cells due to an increased expression of L-amino acid transporters and, contrary to gadolinium, does not depend on blood-brain barrier disruption to reach tumoral cells. It has demonstrated a high diagnostic value in the differentiation of tumoral viability and pseudoprogression or any other therapy-related changes, especially when combining traditional visual analysis with modern radiomics. In this review, we aim to cover the potential role of 18F-FET positron emission tomography in everyday clinical practice when applied to the follow-up of patients after the first therapeutical intervention, early response evaluation, and the differential diagnosis between therapy-related changes and progression.

Low-Dose Bevacizumab for the Treatment of Focal Radiation Necrosis of the Brain (fRNB): A Single-Center Case Series

Focal radiation necrosis of the brain (fRNB) is a late adverse event that can occur following the treatment of benign or malignant brain lesions with stereotactic radiation therapy (SRT) or stereotactic radiosurgery (SRS). Recent studies have shown that the incidence of fRNB is higher in cancer patients who received immune checkpoint inhibitors. The use of bevacizumab (BEV), a monoclonal antibody that targets the vascular endothelial growth factor (VEGF), is an effective treatment for fRNB when given at a dose of 5-7.5 mg/kg every two weeks. In this single-center retrospective case series, we investigated the effectiveness of a low-dose regimen of BEV (400 mg loading dose followed by 100 mg every 4 weeks) in patients diagnosed with fRNB. A total of 13 patients were included in the study; twelve of them experienced improvement in their existing clinical symptoms, and all patients had a decrease in the volume of edema on MRI scans. No clinically significant treatment-related adverse effects were observed. Our preliminary findings suggest that this fixed low-dose regimen of BEV can be a well-tolerated and cost-effective alternative treatment option for patients diagnosed with fRNB, and it is deserving of further investigation.