Usefulness of PET Imaging to Guide Treatment Options in Gliomas

DeepDixon synthetic CT for [18F]FET PET/MRI attenuation correction of post-surgery glioma patients with metal implants

Purpose

Conventional magnetic resonance imaging (MRI) can for glioma assessment be supplemented by positron emission tomography (PET) imaging with radiolabeled amino acids such as O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET), which provides additional information on metabolic properties. In neuro-oncology, patients often undergo brain and skull altering treatment, which is known to challenge MRI-based attenuation correction (MR-AC) methods and thereby impact the simplified semi-quantitative measures such as tumor-to-brain ratio (TBR) used in clinical routine. The aim of the present study was to examine the applicability of our deep learning method, DeepDixon, for MR-AC in [¹⁸F]FET PET/MRI scans of a post-surgery glioma cohort with metal implants.

Methods

The MR-AC maps were assessed for all 194 included post-surgery glioma patients (318 studies). The subgroup of 147 patients (222 studies, 200 MBq [¹⁸F]FET PET/MRI) with tracer uptake above 1 ml were subsequently reconstructed with DeepDixon, vendor-default atlas-based method, and a low-dose computed tomography (CT) used as reference. The biological tumor volume (BTV) was delineated on each patient by isocontouring tracer uptake above a TBR threshold of 1.6. We evaluated the MR-AC methods using the recommended clinical metrics BTV and mean and maximum TBR on a patient-by-patient basis against the reference with CT-AC.

Results

Ninety-seven percent of the studies (310/318) did not have any major artifacts using DeepDixon, which resulted in a Dice coefficient of 0.89/0.83 for tissue/bone, respectively, compared to 0.84/0.57 when using atlas. The average difference between DeepDixon and CT-AC was within 0.2% across all clinical metrics, and no statistically significant difference was found. When using DeepDixon, only 3 out of 222 studies (1%) exceeded our acceptance criteria compared to 72 of the 222 studies (32%) with the atlas method.

Conclusion

We evaluated the performance of a state-of-the-art MR-AC method on the largest post-surgical glioma patient cohort to date. We found that DeepDixon could overcome most of the issues arising from irregular anatomy and metal artifacts present in the cohort resulting in clinical metrics within acceptable limits of the reference CT-AC in almost all cases. This is a significant improvement over the vendor-provided atlas method and of particular importance in response assessment.

Applications of Artificial Intelligence Based on Medical Imaging in Glioma: Current State and Future Challenges

Glioma is one of the most fatal primary brain tumors, and it is well-known for its difficulty in diagnosis and management. Medical imaging techniques such as magnetic resonance imaging (MRI), positron emission tomography (PET), and spectral imaging can efficiently aid physicians in diagnosing, treating, and evaluating patients with gliomas. With the increasing clinical records and digital images, the application of artificial intelligence (AI) based on medical imaging has reduced the burden on physicians treating gliomas even further. This review will classify AI technologies and procedures used in medical imaging analysis. Additionally, we will discuss the applications of AI in glioma, including tumor segmentation and classification, prediction of genetic markers, and prediction of treatment response and prognosis, using MRI, PET, and spectral imaging. Despite the benefits of AI in clinical applications, several issues such as data management, incomprehension, safety, clinical efficacy evaluation, and ethical or legal considerations, remain to be solved. In the future, doctors and researchers should collaborate to solve these issues, with a particular emphasis on interdisciplinary teamwork.

Use of PET Imaging in Neuro-Oncological Surgery

Simple Summary

The use of positron emission tomography (PET) imaging in neuro-oncological surgery is an exciting field with thriving perspectives. Increasing evidence exists for amino acid-based PET to facilitate interpretation of imaging findings following therapeutic interventions in patients with glioma and brain metastases. In meningioma patients, radiolabeled somatostatin receptor ligands provide an improved tumor tissue visualization in lesions located in the vicinity of the skull base and differentiate between scar tissue and tumor recurrence. Moreover, they can be applied as an individual treatment option in recurrent atypical and anaplastic meningioma not eligible for further surgery and radiotherapy. With a focus on its clinical application, this review provides an overview of the emerging field of PET imaging in neuro-oncological surgery.

Abstract

This review provides an overview of current applications and perspectives of PET imaging in neuro-oncological surgery. The past and future of PET imaging in the management of patients with glioma and brain metastases are elucidated with an emphasis on amino acid tracers, such as O-(2-[¹⁸F]fluoroethyl)-L-tyrosine (¹⁸F-FET). The thematic scope includes surgical resection planning, prognostication, non-invasive prediction of molecular tumor characteristics, depiction of intratumoral heterogeneity, response assessment, differentiation between tumor progression and treatment-related changes, and emerging new tracers. Furthermore, the role of PET using specific somatostatin receptor ligands for the management of patients with meningioma is discussed. Further advances in neuro-oncological imaging can be expected from promising new techniques, such as hybrid PET/MR scanners and the implementation of artificial intelligence methods, such as radiomics.

Glioblastoma Exhibits Inter-Individual Heterogeneity of TSPO and LAT1 Expression in Neoplastic and Parenchymal Cells

Molecular imaging is essential for diagnosis and treatment planning for glioblastoma patients. Positron emission tomography (PET) with tracers for the detection of the solute carrier family 7 member 5 (SLC7A5; also known as the amino acid transporter light chain L system, LAT1) and for the mitochondrial translocator protein (TSPO) is successfully used to provide additional information on tumor volume and prognosis. The current approaches for TSPO-PET and the visualization of tracer ([18F] Fluoroethyltyrosine, FET) uptake by LAT1 (FET-PET) do not yet exploit the full diagnostic potential of these molecular imaging techniques. Therefore, we investigated the expression of TSPO and LAT1 in patient glioblastoma (GBM) samples, as well as in various GBM mouse models representing patient GBMs of different genetic subtypes. By immunohistochemistry, we found that TSPO and LAT1 are upregulated in human GBM samples compared to normal brain tissue. Next, we orthotopically implanted patient-derived GBM cells, as well as genetically engineered murine GBM cells, representing different genetic subtypes of the disease. To determine TSPO and LAT1 expression, we performed immunofluorescence staining. We found that both TSPO and LAT1 expression was increased in tumor regions of the implanted human or murine GBM cells when compared to the neighboring mouse brain tissue. While LAT1 was largely restricted to tumor cells, we found that TSPO was also expressed by microglia, tumor-associated macrophages, endothelial cells, and pericytes. The Cancer Genome Atlas (TCGA)-data analysis corroborates the upregulation of TSPO in a bigger cohort of GBM patient samples compared to tumor-free brain tissue. In addition, AIF1 (the gene encoding for the myeloid cell marker Iba1) was also upregulated in GBM compared to the control. Interestingly, TSPO, as well as AIF1, showed significantly different expression levels depending on the GBM genetic subtype, with the highest expression being exhibited in the mesenchymal subtype. High TSPO and AIF1 expression also correlated with a significant decrease in patient survival compared to low expression. In line with this finding, the expression levels for TSPO and AIF1 were also significantly higher in (isocitrate-dehydrogenase wild-type) IDH^WT compared to IDH mutant (IDH^MUT) GBM. LAT1 expression, on the other hand, was not different among the individual GBM subtypes. Therefore, we could conclude that FET- and TSPO-PET confer different information on pathological features based on different genetic GBM subtypes and may thus help in planning individualized strategies for brain tumor therapy in the future. A combination of TSPO-PET and FET-PET could be a promising way to visualize tumor-associated myeloid cells and select patients for treatment strategies targeting the myeloid compartment.

MEG-3-mediated Wnt/β-catenin signaling pathway controls the inhibition of tunicamycin-mediated viability in glioblastoma

Glioblastoma is the most common primary brain carcinoma and leads to a poor survival rate of patients worldwide. Results of previous studies have suggested that tunicamycin may inhibit aggressiveness by promoting apoptosis of glioblastoma cells. In the present study, the effects of tunicamycin and its potential molecular mechanisms underlying the viability and aggressiveness of glioblastoma cells were investigated. Western blot analysis, the reverse transcription-quantitative polymerase chain reaction, immunohistochemistry, apoptosis assays and immunofluorescence were employed to examine the effects of tunicamycin on apoptosis, viability, aggressiveness and cell cycle arrest of glioblastoma cells by downregulation of the expression levels of fibronectin and epithelial cadherin. In vitro experiments demonstrated that tunicamycin significantly inhibited the viability, migration and invasion of glioblastoma cells. Results demonstrated that tunicamycin administration promoted apoptosis of glioblastoma cells through the upregulation of poly(ADP-ribose) polymerase and caspase-9. Cell cycle assays revealed that tunicamycin suppressed the proliferation of, and induced cell cycle arrest at S phase in, glioblastoma cells. Additionally, tunicamycin increased the expression of maternally expressed gene-3 (MEG-3) and wingless/integrated (Wnt)/β-catenin in glioblastoma cells. Results also indicated that tunicamycin administration promoted the Wnt/β-catenin signaling pathway in glioblastoma cells. Knockdown of MEG-3 inhibited tunicamycin-mediated downregulation of the Wnt/β-catenin signaling pathway, which was inhibited further by tunicamycin-mediated inhibition of viability and aggressiveness in glioblastoma. In vivo assays demonstrated that tunicamycin treatment significantly inhibited tumor viability and promoted apoptosis, which further led to an increased survival rate of tumor-bearing mice compared with that of the control group. In conclusion, these results indicate that tunicamycin may inhibit the viability and aggressiveness by regulating MEG-3-mediated Wnt/β-catenin signaling, suggesting that tunicamycin may be a potential anticancer agent for glioblastoma therapy.

Natural Diterpenoid Isoferritin A (IsoA) Inhibits Glioma Cell Growth and Metastasis via Regulating of TGFβ-Induced EMT Signal Pathway

Background

Malignant glioma is intractable primary brain carcinoma that has a poor survival rate. Natural diterpenoid isoferritin A (IsoA) presents antitumor effects by regulating signal pathways in tumor cells. In the present study we investigated the inhibitory effects of IsoA on glioma cells.

Material/Methods

The potential molecular mechanism of IsoA-mediated glioma cell growth and metastasis were investigated using Western blot, gene knockdown, immunofluorescence, and immunohistochemistry.

Results

Results showed that IsoA significantly inhibits growth and metastasis of glioma cells in multiple preclinical settings. In vitro assay showed that IsoA (4 mg/ml) treatment significantly induced apoptosis of glioma cells. Mechanism analysis demonstrated that IsoA (4 mg/ml) treatment decreased TGFβ and regulated EMT markers expression in glioma cells. Reduced expression of TGFβ in glioma cells was closely correlated with inhibitory effects of IsoA on growth and metastasis of glioma cells. TGFβ overexpression promoted glioma cell growth and invasion. Results also showed that IsoA treatment significantly decreased Fibronectin and Vimentin and increased E-cadherin, while TGFβ overexpression abolished the regulation mediated by IsoA in glioma cells. In vivo assay showed that IsoA treatment inhibited tumor growth in a glioma-bearing mouse model.

Conclusions

Results indicate that IsoA could be regarded as a potential anti-cancer agent by regulating TGFβ-induced EMT signal pathway.

The role of amino-acid PET in the light of the new WHO classification 2016 for brain tumors

Since its introduction in 2016, the revision of the World Health Organization (WHO) classification of central nervous system tumors has already changed the diagnostic and therapeutic approach in glial tumors. Blurring the lines between entities formerly labelled as "high-grade" or "low-grade", molecular markers define distinct biological subtypes with different clinical course. This new classification raises the demand for non-invasive imaging methods focusing on depicting metabolic processes. We performed a review of current literature on the use of amino-acid PET (AA-PET) for obtaining diagnostic or prognostic information on glioma in the setting of the current WHO 2016 classification. So far, only a few studies have focused on combining molecular genetic information and metabolic imaging using AA-PET. The current review summarizes the information available on "molecular grading" as well as prognostic information obtained from AA-PET and delivers an insight into a possible interrelation between metabolic imaging and glioma genetics. Within the framework of molecular characterization of gliomas, metabolic imaging using AA-PET is a promising tool for non-invasive characterization of molecular features and to provide additional prognostic information. Further studies incorporating molecular and metabolic features are necessary to improve the explanatory power of AA-PET in glial tumors.

O-(2-[18F]fluoroethyl)-L-tyrosine PET in gliomas: influence of data processing in different centres

BACKGROUND

PET using O-(2-[18F]fluoroethyl)-L-tyrosine (18F-FET) is an established method for brain tumour diagnostics, but data processing varies in different centres. This study analyses the influence of methodological differences between two centres for tumour characterization with 18F-FET PET using the same PET scanner. Methodological differences between centres A and B in the evaluation of 18F-FET PET data were identified for (1) framing of PET dynamic data, (2) data reconstruction, (3) cut-off values for tumour delineation to determine tumour-to-brain ratios (TBR) and tumour volume (Tvol) and (4) ROI definition to determine time activity curves (TACs) in the tumour. Based on the 18F-FET PET data of 40 patients with untreated cerebral gliomas (20 WHO grade II, 10 WHO grade III, 10 WHO grade IV), the effect of different data processing in the two centres on TBRmean, TBRmax, Tvol, time-to-peak (TTP) and slope of the TAC was compared. Further, the effect on tumour grading was evaluated by ROC analysis.

RESULTS

Significant differences between centres A and B were found especially for TBRmax (2.84 ± 0.99 versus 3.34 ± 1.13; p < 0.001), Tvol (1.14 ± 1.28 versus 1.51 ± 1.44; p < 0.001) and TTP (22.4 ± 8.3 min versus 30.8 ± 6.3 min; p < 0.001) and minor differences for TBRmean and slope. Tumour grading was not influenced by different data processing.

CONCLUSIONS

Variable data processing of 18F-FET PET in different centres leads to significant differences especially for TBRmax and Tvol. A standardization of data processing and evaluation is needed to make 18F-FET PET comparable between different centres.

Suspected recurrence of brain metastases after focused high dose radiotherapy: can [18F]FET- PET overcome diagnostic uncertainties?

Background

After focused high dose radiotherapy of brain metastases, differentiation between tumor recurrence and radiation-induced lesions by conventional MRI is challenging. This study investigates the usefulness of dynamic O-(2-¹⁸F-Fluoroethyl)-L-Tyrosine positron emission tomography (¹⁸F-FET PET) in patients with MRI-based suspicion of tumor recurrence after focused high dose radiotherapy of brain metastases.

Methods

Twenty-two patients with 34 brain metastases (median age 61.9 years) were included. Due to follow-up scan evaluations after repeated treatment in a subset of patients, a total of 50 lesions with MRI-based suspicion of tumor recurrence after focused high dose radiotherapy could be evaluated. ¹⁸F-FET PET analysis included the assessment of maximum and mean tumor-to-background ratio (TBR_max and TBR_mean) and analysis of time-activity-curves (TAC; increasing vs. decreasing) including minimal time-to-peak (TTP_min). PET parameters were correlated with histological findings and radiological-clinical follow-up evaluation.

Results

Tumor recurrence was found in 21/50 cases (15/21 verified by histology, 6/21 by radiological-clinical follow-up) and radiation-induced changes in 29/50 cases (5/29 verified by histology, 24/29 by radiological-clinical follow-up). Median clinical-radiological follow-up was 28.3 months (range 4.2–99.1 months). ¹⁸F-FET uptake was higher in tumor recurrence compared to radiation-induced changes (TBR_max 2.9 vs. 2.0, p < 0.001; TBR_mean 2.2 vs. 1.7, p < 0.001). Receiver-operating-characteristic (ROC) curve analysis revealed optimal cut-off values of 2.15 for TBR_max and 1.95 for TBR_mean (sensitivity 86 %, specificity 79 %). Increasing TACs and long TTP_min were associated with radiation-induced changes, decreasing TACs with tumor recurrence (p = 0.01). By combination of TBR and TACs, sensitivity and specificity could be increased to 93 and 84 %.

Conclusions

In patients with MRI-suspected tumor recurrence after focused high dose radiotherapy, ¹⁸F-FET PET has a high sensitivity and specificity for the differentiation of vital tumor tissue and radiation-induced lesions.