From the clinician's point of view - What is the status quo of positron emission tomography in patients with brain tumors?

Imaging Glioblastoma Posttreatment: Progression, Pseudoprogression, Pseudoresponse, Radiation Necrosis

Radiographic monitoring of posttreatment glioblastoma is important for clinical trials and determining next steps in management. Evaluation for tumor progression is confounded by the presence of treatment-related radiographic changes, making a definitive determination less straight-forward. The purpose of this article was to describe imaging tools available for assessing treatment response in glioblastoma, as well as to highlight the definitions, pathophysiology, and imaging features typical of true progression, pseudoprogression, pseudoresponse, and radiation necrosis.

Diagnostic and grading accuracy of 18F-FDOPA PET and PET/CT in patients with gliomas: a systematic review and meta-analysis

Background

Positron emission tomography (PET) and PET/computed tomography (PET/CT) imaging with 3,4-dihydroxy-6-[¹⁸F] fluoro-L-phenylalanine (¹⁸F-FDOPA) has been used in the evaluation of gliomas. We performed a meta-analysis to obtain the diagnostic and grading accuracy of ¹⁸F-FDOPA PET and PET/CT in patients with gliomas.

Methods

PubMed, Embase, Cochrane Library and Web of Science were searched through 13 May 2019. We included studies reporting the diagnostic performance of ¹⁸F-FDOPA PET or PET/CT in glioma patients. Pooled sensitivity, specificity, and area under the summary receiver operating characteristic (SROC) curve were calculated from eligible studies on a per-lesion basis.

Results

Eventually, 19 studies were included. Across 13 studies (370 patients) for glioma diagnosis, the pooled sensitivity and specificity of ¹⁸F-FDOPA PET and PET/CT were 0.90 (95%CI: 0.86–0.93) and 0.75 (95%CI: 0.65–0.83). Across 7 studies (219 patients) for glioma grading, ¹⁸F-FDOPA PET and PET/CT showed a pooled sensitivity of 0.88 (95%CI: 0.81–0.93) and a pooled specificity of 0.73 (95%CI: 0.64–0.81).

Conclusions

¹⁸F-FDOPA PET and PET/CT demonstrated good performance for diagnosing gliomas and differentiating high-grade gliomas (HGGs) from low-grade gliomas (LGGs). Further studies implementing standardized PET protocols and investigating the grading parameters are needed.

Diagnostic performance of 18F-fluorodeoxyglucose positron emission tomography in the evaluation of glioma

INTRODUCTION

Identifying glioma grade through imaging allows clinicians to recommend and accurately direct treatment. We sought to quantify the utility of FDG-PET/CT (18F-fluorodeoxyglucose positron emission tomography/computed tomography), alone and in combination with MRI, in identifying high-grade regions of glioma.

METHODS

This is a retrospective review of patients who had an FDG-PET/CT performed as part of the workup of suspected glioma or in follow-up of known glioma. FDG-PET/CT scans were reviewed and uptake in the identifiable lesion coded as none, diffusely or focally increased. Patients also underwent gadolinium-enhanced MRI, noting regions of contrast enhancement. Sensitivity, specificity, positive and negative predictive values (PPV and NPV) were calculated for identification of high-grade histology (WHO III or IV, or metastatic disease) obtained post-FDG-PET/CT.

RESULTS

Thirty-three patients had 36 FDG-PET/CT and MRI scans followed by histological confirmation (biopsy or debulking). Increased FDG uptake demonstrated a sensitivity of 59% and specificity of 79%, PPV of 81% and NPV of 55% for identification of high-grade histology. MRI demonstrated a sensitivity of 77% and specificity of 86%, PPV of 89% and NPV of 71% for identification of high-grade histology. Only 64% of MRI and FDG-PET/CT scan series were concordant. When FDG-PET/CT and MRI were concordant, a specificity of 100% and PPV of 100% was achieved, however, sensitivity was 79% and NPV was 75%.

CONCLUSION

The combination of FDG-PET/CT and gadolinium-enhanced MRI demonstrated marked improvement in identifying potential high-grade disease over each modality alone. Increased FDG uptake without gadolinium enhancement rarely occurred and identified high-grade histology in a small number of patients. Due to limited sensitivity and NPV, a negative FDG-PET/CT alone, or in combination with MRI, should not guide a decision for observation where surgery would otherwise be recommended.

Report of first recurrent glioma patients examined with PET-MRI prior to re-irradiation

Background and purpose

The advantage of combined PET-MRI over sequential PET and MRI is the high spatial conformity and the absence of time delay between the examinations. The benefit of this technique for planning of re-irradiation (re-RT) treatment is unkown yet. Imaging data from a phase 1 trial of re-RT for recurrent glioma was analysed to assess whether planning target volumes and treatment margins in glioma re-RT can be adjusted by PET-MRI with rater independent PET based biological tumour volumes (BTVs).

Patients and methods

Combined PET-MRI with the tracer O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) prior to re-RT was performed in recurrent glioma patients in a phase I trial. GTVs including all regions suspicious of tumour on contrast enhanced MRI were delineated by three experienced radiation oncologists and included into MRI based consensus GTVs (_MRGTVs). BTVs were semi-automatically delineated with a fixed threshold of 1.6 x background activity. Corresponding BTVs and _MRGTVs were fused into union volume _PET-MRGTVs. The Sørensen–Dice coefficient and the conformity index were used to assess the geometric overlap of the BTVs with the _MRGTVs. A recurrence pattern analysis was performed based on the original planning target volumes (PTVs = GTV + 10 mm margin or 5 mm in one case) and the _PET-MRGTVs with margins of 10, 8, 5 and 3 mm.

Results

Seven recurrent glioma patients, who received PET-MRI prior to re-RT, were included into the present planning study. At the time of re-RT, patients were in median 54 years old and had a median Karnofsky Performance Status (KPS) score of 80. Median post-recurrence survival after the beginning of re-RT was 13 months. Concomitant bevacizumab therapy was applied in six patients and one patient received chemoradiation with temozolomide. Median GTV volumes of the three radiation oncologists were 35.0, 37.5 and 40.5 cubic centimeters (cc) and median _MRGTV volume 41.8 cc. Median BTV volume was 36.6 cc and median _PET-MRGTV volume 59.3 cc. The median Sørensen–Dice coefficient for the comparison between _MRGTV and BTV was 0.61 and the median conformity index 0.44. Recurrence pattern analysis revealed two central, two in-field and one distant recurrence within both, the original PTV, as well as the _PET-MRGTV with a reduced margin of 3 mm.

Conclusion

PET-MRI provides radiation treatment planning imaging with high spatial and timely conformity for high-grade glioma patients treated with re-RT with potential advancements for target volume delineation. Prospective randomised trials are warranted to further investigate the treatment benefits of PET-MRI based re-RT planning.

Differentiation of treatment-related changes from tumour progression: a direct comparison between dynamic FET PET and ADC values obtained from DWI MRI

BACKGROUND

Following brain cancer treatment, the capacity of anatomical MRI to differentiate neoplastic tissue from treatment-related changes (e.g., pseudoprogression) is limited. This study compared apparent diffusion coefficients (ADC) obtained by diffusion-weighted MRI (DWI) with static and dynamic parameters of O-(2-[¹⁸F]fluoroethyl)-L-tyrosine (FET) PET for the differentiation of treatment-related changes from tumour progression.

PATIENTS AND METHODS

Forty-eight pretreated high-grade glioma patients with anatomical MRI findings suspicious for progression (median time elapsed since last treatment was 16 weeks) were investigated using DWI and dynamic FET PET. Maximum and mean tumour-to-brain ratios (TBR_max, TBR_mean) as well as dynamic parameters (time-to-peak and slope values) of FET uptake were calculated. For mean ADC calculation, regions-of-interest analyses were performed on ADC maps calculated from DWI coregistered with the contrast-enhanced MR image. Diagnoses were confirmed neuropathologically (21%) or clinicoradiologically. Diagnostic performance was evaluated using receiver-operating-characteristic analyses or Fisher's exact test for a combinational approach.

RESULTS

Ten of 48 patients had treatment-related changes (21%). The diagnostic performance of FET PET was significantly higher (threshold for both TBR_max and TBR_mean, 1.95; accuracy, 83%; AUC, 0.89 ± 0.05; P < 0.001) than that of ADC values (threshold ADC, 1.09 × 10^-3 mm²/s; accuracy, 69%; AUC, 0.73 ± 0.09; P = 0.13). The addition of static FET PET parameters to ADC values increased the latter's accuracy to 89%. The highest accuracy was achieved by combining static and dynamic FET PET parameters (93%). Moreover, in contrast to ADC values, TBRs <1.95 at suspected progression predicted a significantly longer survival (P = 0.01).

CONCLUSIONS

Data suggest that static and dynamic FET PET provide valuable information concerning the differentiation of early treatment-related changes from tumour progression and outperform ADC measurement for this highly relevant clinical question.

Current status of PET imaging in neuro-oncology

Over the past decades, a variety of PET tracers have been used for the evaluation of patients with brain tumors. For clinical routine, the most important clinical indications for PET imaging in patients with brain tumors are the identification of neoplastic tissue including the delineation of tumor extent for the further diagnostic and therapeutic management (ie, biopsy, resection, or radiotherapy planning), the assessment of response to a certain anticancer therapy including its (predictive) effect on the patients’ outcome and the differentiation of treatment-related changes (eg, pseudoprogression and radiation necrosis) from tumor progression at follow-up. To serve medical professionals of all disciplines involved in the diagnosis and care of patients with brain tumors, this review summarizes the value of PET imaging for the latter-mentioned 3 clinically relevant indications in patients with glioma, meningioma, and brain metastases.

Investigation of cis-4-[18F]Fluoro-D-Proline Uptake in Human Brain Tumors After Multimodal Treatment

PURPOSE

Cis-4-[¹⁸F]fluoro-D-proline (D-cis-[¹⁸F]FPro) has been shown to pass the intact blood-brain barrier and to accumulate in areas of secondary neurodegeneration and necrosis in the rat brain while uptake in experimental brain tumors is low. This pilot study explores the uptake behavior of D-cis-[¹⁸F]FPro in human brain tumors after multimodal treatment.

PROCEDURES

In a prospective study, 27 patients with suspected recurrent brain tumor after treatment with surgery, radiotherapy, and/or chemotherapy (SRC) were investigated by dynamic positron emission tomography (PET) using D-cis-[¹⁸F]FPro (22 high-grade gliomas, one unspecified glioma, and 4 metastases). Furthermore, two patients with untreated lesions were included (one glioblastoma, one reactive astrogliosis). Data were compared with the results of PET using O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) which detects viable tumor tissue. Tracer distribution, mean and maximum lesion-to-brain ratios (LBR_mean, LBR_max), and time-to-peak (TTP) of the time activity curve (TAC) of tracer uptake were evaluated. Final diagnosis was determined by histology (n = 9), clinical follow-up (n = 10), or by [¹⁸F]FET PET (n = 10).

RESULTS

D-cis-[¹⁸F]FPro showed high uptake in both recurrent brain tumors (n = 11) and lesions classified as treatment-related changes (TRC) only (n = 16) (LBR_mean 2.2 ± 0.7 and 2.1 ± 0.6, n.s.; LBR_max 3.4 ± 1.2 and 3.2 ± 1.3, n.s.). The untreated glioblastoma and the lesion showing reactive astrogliosis exhibited low D-cis-[¹⁸F]FPro uptake. Distribution of [¹⁸F]FET and D-cis-[¹⁸F]FPro uptake was discordant in 21/29 cases indicating that the uptake mechanisms are different.

CONCLUSION

The high accumulation of D-cis-[¹⁸F]FPro in pretreated brain tumors and TRC supports the hypothesis that tracer uptake is related to cell death. Further studies before and after therapy are needed to assess the potential of D-cis-[¹⁸F]FPro for treatment monitoring.

Comparisons Between PET With 11C-Methyl-L-Methionine and Arterial Spin Labeling Perfusion Imaging in Recurrent Glioblastomas Treated With Bevacizumab

PURPOSE

The aim of this study was to clarify whether arterial spin labeling (ASL) perfusion imaging can assess biological effects from bevacizumab (BEV) therapy as reliably as PET with C-methyl-L-methionine (C-met-PET).

MATERIALS AND METHODS

Twenty-four patients with recurrent glioblastoma were examined using both ASL and C-met-PET before and 4 and 8 weeks after starting BEV treatment. Tumor-to-normal brain (T/N) ratios, fluctuations in T/N ratio, and tumor volumes were compared between ASL and C-met-PET. Accuracy of predicting patient with long progression-free survival (PFS) was assessed for T/N ratios and fluctuations for ASL and C-met-PET in each phase and in each period using receiver operating characteristic curves. Between 2 groups of patients assigned by cutoff values from receiver operating characteristic curves, PFS was compared in each phase or in each period.

RESULTS

T/N ratios, fluctuations in ratio, and tumor volumes correlated significantly between ASL and C-met-PET at all time points and all periods. Arterial spin labeling was eligible as a predictor for long PFS only in assessment of fluctuations in T/N ratio. However, the most accurate predictors for long PFS were T/N ratio from C-met-PET at 8 weeks and the fluctuation from baseline to 4 weeks in T/N ratio from C-met-PET.

CONCLUSIONS

Blood flows on ASL correlated with accumulations of C-met on PET in recurrent glioblastoma under BEV treatment. Although C-met-PET offered superior accuracy for predicting patients with long PFS from time points, ASL offered reliable prediction of long PFS, provided that fluctuations in T/N ratio between consecutive scans are assessed.

Combined Amino Acid Positron Emission Tomography and Advanced Magnetic Resonance Imaging in Glioma Patients

Imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI) provide valuable information about brain tumor patients. Particularly amino acid PET, advanced MRI techniques, and combinations thereof are of great interest for the non-invasive assessment of biological characteristics in patients with primary or secondary brain cancer. A methodological innovation that potentially advances research in patients with brain tumors is the increasing availability of hybrid PET/MRI systems, which enables the simultaneous acquisition of both imaging modalities. Furthermore, the advent of ultra-high field MRI scanners operating at magnetic field strengths of 7 T or more will allow further development of metabolic MR imaging at higher resolution. This review focuses on the combination of amino acid PET with MR spectroscopic imaging, perfusion- and diffusion-weighted imaging, as well as chemical exchange saturation transfer in patients with high-grade gliomas, especially glioblastomas.

Dynamic [18F]FET-PET/MRI using standard MRI-based attenuation correction methods

AIM

To assess if tumour grading based on dynamic [18F]FET positron emission tomography/magnetic resonance imaging (PET/MRI) studies is affected by different MRI-based attenuation correction (AC) methods.

METHODS

Twenty-four patients with suspected brain tumours underwent dynamic [18F]FET-PET/MRI examinations and subsequent low-dose computed tomography (CT) scans of the head. The dynamic PET data was reconstructed using the following AC methods: standard Dixon-based AC and ultra-short echo time MRI-based AC (MR-AC) and a model-based AC approach. All data were reconstructed also using CT-based AC (reference). For all lesions and reconstructions, time-activity curves (TACs) and time to peak (TTP) were extracted using different region-of-interest (ROI) and volume-of-interest (VOI) definitions. According to the most common evaluation approaches, TACs were categorised into two or three distinct curve patterns. Changes in TTP and TAC patterns compared to PET using CT-based AC were reported.

RESULTS

In the majority of cases, TAC patterns did not change. However, TAC pattern changes as well as changes in TTP were observed in up to 8% and 17% of the cases when using different MR-AC methods and ROI/VOI definitions, respectively. However, these changes in TTP and TAC pattern were attributed to different delineations of the ROIs/VOIs in PET corrected with different AC methods.

CONCLUSION

PET/MRI using different MR-AC methods can be used for the assessment of TAC patterns in dynamic [18F]FET studies, as long as a meaningful delineation of the area of interest within the tumour is ensured.

KEY POINTS

• PET/MRI using different MR-AC methods can be used for dynamic [18F]FET studies. • A meaningful segmentation of the area of interest needs to be ensured, mandating a visual validation of the delineation by an experienced reader.

The psychopharmacology of primary and metastatic brain tumors and paraneoplastic syndromes

Brain tumors and paraneoplastic syndromes can cause various neuropsychiatric symptoms. Rarely, psychiatric symptoms may be the initial presentation of the underlying neurologic lesion. Brain imaging studies are crucial in the diagnosis of brain tumors. Paraneoplastic syndromes are mostly immune-mediated, and antineuronal antibodies may be detected in the blood or cerebrospinal fluid. Clinical suspicion is very important in assisting the diagnostic workup. Treatment of the psychiatric symptoms depends on the nature of the symptoms. Selection of the psychotropic agent has to be done carefully to minimize complications such as seizures and delirium secondary to anticholinergic toxicity. With advances in targeted therapies, immunology, and genetics, the future appears more promising.

Joint EANM/EANO/RANO practice guidelines/SNMMI procedure standards for imaging of gliomas using PET with radiolabelled amino acids and [18F]FDG: version 1.0

These joint practice guidelines, or procedure standards, were developed collaboratively by the European Association of Nuclear Medicine (EANM), the Society of Nuclear Medicine and Molecular Imaging (SNMMI), the European Association of Neurooncology (EANO), and the working group for Response Assessment in Neurooncology with PET (PET-RANO). Brain PET imaging is being increasingly used to supplement MRI in the clinical management of glioma. The aim of these standards/guidelines is to assist nuclear medicine practitioners in recommending, performing, interpreting and reporting the results of brain PET imaging in patients with glioma to achieve a high-quality imaging standard for PET using FDG and the radiolabelled amino acids MET, FET and FDOPA. This will help promote the appropriate use of PET imaging and contribute to evidence-based medicine that may improve the diagnostic impact of this technique in neurooncological practice. The present document replaces a former version of the guidelines published in 2006 (Vander Borght et al. Eur J Nucl Med Mol Imaging. 33:1374–80, 2006), and supplements a recent evidence-based recommendation by the PET-RANO working group and EANO on the clinical use of PET imaging in patients with glioma (Albert et al. Neuro Oncol. 18:1199–208, 2016). The information provided should be taken in the context of local conditions and regulations.

PET imaging in glioma: techniques and current evidence

PET holds potential to provide additional information about tumour metabolic processes, which could aid brain tumour differential diagnosis, grading, molecular subtyping and/or the distinction of therapy effects from disease recurrence. This review discusses PET techniques currently in use for untreated and treated glioma characterization and aims to critically assess the evidence for different tracers ([F]Fluorodeoxyglucose, choline and amino acid tracers) in this context.

Distinguishing True Progression From Radionecrosis After Stereotactic Radiation Therapy for Brain Metastases With Machine Learning and Radiomics

PURPOSE

Treatment effect or radiation necrosis after stereotactic radiosurgery (SRS) for brain metastases is a common phenomenon often indistinguishable from true progression. Radiomics is an emerging field that promises to improve on conventional imaging. In this study, we sought to apply a radiomics-based prediction model to the problem of diagnosing treatment effect after SRS.

METHODS AND MATERIALS

We included patients in the Johns Hopkins Health System who were treated with SRS for brain metastases who subsequently underwent resection for symptomatic growth. We also included cases of likely treatment effect in which lesions grew but subsequently regressed spontaneously. Lesions were segmented semiautomatically on preoperative T1 postcontrast and T2 fluid-attenuated inversion recovery magnetic resonance imaging, and radiomic features were extracted with software developed in-house. Top-performing features on univariate logistic regression were entered into a hybrid feature selection/classification model, IsoSVM, with parameter optimization and further feature selection performed using leave-one-out cross-validation. Final model performance was assessed by 10-fold cross-validation with 100 repeats. All cases were independently reviewed by a board-certified neuroradiologist for comparison.

RESULTS

We identified 82 treated lesions across 66 patients, with 77 lesions having pathologic confirmation. There were 51 radiomic features extracted per segmented lesion on each magnetic resonance imaging sequence. An optimized IsoSVM classifier based on top-ranked radiomic features had sensitivity and specificity of 65.38% and 86.67%, respectively, with an area under the curve of 0.81 on leave-one-out cross-validation. Only 73% of cases were classifiable by the neuroradiologist, with a sensitivity of 97% and specificity of 19%.

CONCLUSIONS

Radiomics holds promise for differentiating between treatment effect and true progression in brain metastases treated with SRS. A predictive model built on radiomic features from an institutional cohort performed well on cross-validation testing. These results warrant further validation in independent datasets. Such work could prove invaluable for guiding management of individual patients and assessing outcomes of novel interventions.

The role of 13N-ammonia in the differential diagnosis of gliomas and brain inflammatory lesions

OBJECTIVE

To investigate the utility of ¹³N-ammonia PET/CT imaging in the differential diagnosis of gliomas and brain inflammations.

METHODS

¹³N-ammonia PET/CT imaging data of 77 patients with gliomas and 34 patients with brain inflammations were retrospectively analyzed. No patients received any treatment before ¹³N-ammonia imaging. All the patients were diagnosed by stereotactic biopsy or clinical follow-up. Visual and semi-quantitative analysis was performed to analyze the results of ¹³N-ammonia imaging. Finally, the uptake ratios of each lesion were calculated and its differences among different groups were tested with one-way ANOVA.

RESULTS

29.4% inflammations, 51.6% low-grade gliomas and 91.3% high-grade gliomas were positive by visual analysis in ¹³N-ammonia imaging. The sensitivity, specificity and accuracy for the diagnosis of gliomas were 75.3%, 55.8% and 67.8%, respectively. As for semi-quantitative analysis, the T/G ratios of inflammatory lesions, low-grade gliomas and high-grade gliomas were 0.88 ± 0.24, 1.04 ± 0.43 and 1.43 ± 0.49, respectively. One-way ANOVA revealed that the T/G ratios of high-grade gliomas were significantly higher than those of low-grade gliomas and inflammations (P < 0.05), but there was no statistical difference between low-grade gliomas and inflammations (P = 0.118). Among the inflammatory lesions, T/G ratios were not statistically different between infectious and demyelinating lesions (P > 0.05). ROC curve analysis showed that the optimal cut-off value of T/G ratio in distinguishing gliomas from inflammations was 1.21 with the AUC 0.78. The sensitivity, specificity, accuracy, PPV and NPV were 52.9%, 94.4%, 65.3%, 95.7% and 45.9%, respectively. ROC curve analysis showed that the optimal cut-off value of T/G ratio in distinguishing high-grade gliomas from low-grade gliomas was 1.06 with the AUC 0.78. The sensitivity, specificity, accuracy, PPV and NPV were 81.5%, 67.7%, 76.5%, 81.5% and 67.7%, respectively. ROC curve analysis showed that the optimal cut-off value of T/G ratio in distinguishing high-grade gliomas from low-grade gliomas and inflammations was 1.19 with the AUC 0.84. The sensitivity, specificity, accuracy, PPV and NPV were 70.4%, 85.1%, 78.5%, 79.2% and 78.1%, respectively.

CONCLUSIONS

¹³N-ammonia imaging is effective in distinguishing high-grade gliomas from low-grade gliomas and inflammations, but its role in the differential diagnosis of low-grade gliomas and brain inflammatory lesions is limited, and the accuracy needs to be improved.

Voxel-wise analysis of dynamic 18F-FET PET: a novel approach for non-invasive glioma characterisation

BACKGROUND

Glioma grading with dynamic ¹⁸F-FET PET (0-40 min p.i.) is typically performed by analysing the mean time-activity curve of the entire tumour or a suspicious area within a heterogeneous tumour. This work aimed to ensure a reader-independent glioma characterisation and identification of aggressive sub-volumes by performing a voxel-based analysis with diagnostically relevant kinetic and static ¹⁸F-FET PET parameters. One hundred sixty-two patients with a newly diagnosed glioma classified according to histologic and molecular genetic properties were evaluated. The biological tumour volume (BTV) was segmented in static 20-40 min p.i. ¹⁸F-FET PET images using the established threshold of 1.6 × background activity. For each enclosed voxel, the time-to-peak (TTP), the late slope (Slope_15-40), and the tumour-to-background ratios (TBR_5-15, TBR_20-40) obtained from 5 to 15 min p.i. and 20 to 40 min p.i. images were determined. The percentage portion of these values within the BTV was evaluated with percentage volume fractions (PVFs) and cumulated percentage volume histograms (PVHs). The ability to differentiate histologic and molecular genetic classes was assessed and compared to volume-of-interest (VOI)-based parameters.

RESULTS

Aggressive WHO grades III and IV and IDH-wildtype gliomas were dominated by a high proportion of voxels with an early peak, negative slope, and high TBR, whereby the PVHs with TTP < 20 min p.i., Slope_15-40 < 0 SUV/h, and TBR_5-15 and TBR_20-40 > 2 yielded the most significant differences between glioma grades. We found significant differences of the parameters between WHO grades and IDH mutation status, where the effect size was predominantly higher for voxel-based PVHs compared to the corresponding VOI-based parameters. A low overlap of BTV sub-volumes defined by TTP < 20 min p.i. and negative Slope_15-40 with TBR_5-15 > 2- and TBR_{20-40 > 2}-defined hotspots was observed.

CONCLUSIONS

The presented approach applying voxel-wise analysis of dynamic ¹⁸F-FET PET enables an enhanced characterisation of gliomas and might potentially provide a fast identification of aggressive sub-volumes within the BTV. Parametric 3D ¹⁸F-FET PET information as investigated in this study has the potential to guide individual therapy instrumentation and may be included in future biopsy studies.

Investigational PET tracers for high-grade gliomas

High-grade gliomas (HGGs) are the most common primary malignant tumors of the brain, with glioblastoma (GBM) constituting over 50% of all the gliomas in adults. The disease carries very high mortality, and even with optimal treatment, the median survival is 2-5 years for anaplastic tumors and 1-2 years for GBMs. Neuroimaging is critical to managing patients with HGG for diagnosis, treatment planning, response assessment, and detecting recurrent disease. Magnetic resonance imaging (MRI) is the cornerstone of imaging in neuro-oncology, but molecular imaging with positron emission tomography (PET) can overcome some of the inherent limitations of MRI. Additionally, PET has the potential to target metabolic and molecular alterations in HGGs relevant to prognosis and therapy that cannot be assessed with anatomic imaging. Many classes of PET tracers have been evaluated in HGG including agents that target cell membrane biosynthesis, protein synthesis, amino acid transport, DNA synthesis, the tricarboxylic acid (TCA) cycle, hypoxic environments, cell surface receptors, blood flow, vascular endothelial growth factor (VEGF), epidermal growth factor (EGFR), and the 18-kDa translocator protein (TSPO), among others. This chapter will provide an overview of PET tracers for HGG that have been evaluated in human subjects with a focus on tracers that are not yet in widespread use for neuro-oncology.

Correlation of Dynamic O-(2-[18F]Fluoroethyl)-L-Tyrosine Positron Emission Tomography, Conventional Magnetic Resonance Imaging, and Whole-Brain Histopathology in a Pretreated Glioblastoma: A Postmortem Study

OBJECTIVE

Amino acid positron emission tomography (PET) using O-(2-[¹⁸F]fluoroethyl)-L-tyrosine (FET) provides important additional information on the extent of viable tumor tissue of glioblastoma compared with magnetic resonance imaging (MRI). Especially after radiochemotherapy, progression of contrast enhancement in MRI is equivocal and may represent either tumor progression or treatment-related changes. Here, the first case comparing postmortem whole-brain histology of a patient with pretreated glioblastoma with dynamic in vivo FET PET and MRI is presented.

METHODS

A 61-year-old patient with glioblastoma initially underwent partial tumor resection and died 11 weeks after completion of chemoradiation with concurrent temozolomide. Three days before the patient died, a follow-up FET PET and MRI scan indicated tumor progression. Autopsy was performed 48 hours after death. After formalin fixation, a 7-cm bihemispherical segment of the brain containing the entire tumor mass was cut into 3500 consecutive 20μm coronal sections. Representative sections were stained with hematoxylin and eosin stain, cresyl violet, and glial fibrillary acidic protein immunohistochemistry. An experienced neuropathologist identified areas of dense and diffuse neoplastic infiltration, astrogliosis, and necrosis. In vivo FET PET, MRI datasets, and postmortem histology were co-registered and compared by 3 experienced physicians.

RESULTS

Increased uptake of FET in the area of equivocal contrast enhancement on MRI correlated very well with dense infiltration by vital tumor cells and showed tracer kinetics typical for malignant gliomas. An area of predominantly reactive astrogliosis showed only moderate uptake of FET and tracer kinetics usually observed in benign lesions.

CONCLUSIONS

This case report impressively documents the correct imaging of a progressive glioblastoma by FET PET.

Early treatment response evaluation using FET PET compared to MRI in glioblastoma patients at first progression treated with bevacizumab plus lomustine

BACKGROUND

The goal of this prospective study was to compare the value of both conventional MRI and O-(2-¹⁸F-fluoroethyl)-L-tyrosine (FET) PET for response evaluation in glioblastoma patients treated with bevacizumab plus lomustine (BEV/LOM) at first progression.

METHODS

After chemoradiation with concomitant and adjuvant temozolomide, 21 IDH wild-type glioblastoma patients at first progression (age range, 33-75 years; MGMT promoter unmethylated, 81%) were treated with BEV/LOM. Contrast-enhanced MRI and FET-PET scans were performed at baseline and after 8-10 weeks. We obtained FET metabolic tumor volumes (MTV) and tumor/brain ratios. Threshold values of FET-PET parameters for treatment response were established by ROC analyses using the post-progression overall survival (OS) ≤/>9 months as the reference. MRI response assessment was based on RANO criteria. The predictive ability of FET-PET thresholds and MRI changes on early response assessment was evaluated subsequently concerning OS using uni- and multivariate survival estimates.

RESULTS

Early treatment response as assessed by RANO criteria was not predictive for an OS>9 months (P = 0.203), whereas relative reductions of all FET-PET parameters significantly predicted an OS>9 months (P < 0.05). The absolute MTV at follow-up enabled the most significant OS prediction (sensitivity, 85%; specificity, 88%; P = 0.001). Patients with an absolute MTV below 5 ml at follow-up survived significantly longer (12 vs. 6 months, P < 0.001), whereas early responders defined by RANO criteria lived only insignificantly longer (9 vs. 6 months; P = 0.072). The absolute MTV at follow-up remained significant in the multivariate survival analysis (P = 0.006).

CONCLUSIONS

FET-PET appears to be useful for identifying responders to BEV/LOM early after treatment initiation.

O-(2-18F-fluoroethyl)-L-tyrosine PET for evaluation of brain metastasis recurrence after radiotherapy: an effectiveness and cost-effectiveness analysis

Background

Conventional MRI is the standard method to diagnose recurrence of brain metastases after radiation. However, following radiation therapy, reactive transient blood-brain barrier alterations with consecutive contrast enhancement can mimic brain metastasis recurrence. Recent studies have suggested that O-(2-18F-fluoroethyl)-L-tyrosine (FET) PET improves the correct differentiation of brain metastasis recurrence from radiation injury. Based on published evidence and clinical expert opinion, we analyzed effectiveness and cost-effectiveness of the use of FET PET in addition to MRI compared with MRI alone for the diagnosis of recurrent brain metastases.

Methods

A decision-tree model was designed to compare the 2 diagnostic strategies from the perspective of the German Statutory Health Insurance (SHI) system. Effectiveness was defined as correct diagnosis of recurrent brain metastasis and was compared between FET PET with MRI and MRI alone. Costs were calculated for a baseline scenario and for a more expensive scenario. Robustness of the results was tested using sensitivity analyses.

Results

Compared with MRI alone, FET PET in combination with MRI increases the rate of correct diagnoses by 42% (number needed to diagnose of 3) with an incremental cost-effectiveness ratio of €2821 (baseline scenario) and €4014 (more expensive scenario) per correct diagnosis. The sensitivity analyses confirmed the robustness of the results.

Conclusions

The model suggests that the additional use of FET PET with conventional MRI for the diagnosis of recurrent brain metastases may be cost-effective. Integration of FET PET has the potential to avoid overtreatment with corresponding costs as well as unnecessary side effects.

Discriminating radiation injury from recurrent tumor with [18F]PARPi and amino acid PET in mouse models

Background

Radiation injury can be indistinguishable from recurrent tumor on standard imaging. Current protocols for this differential diagnosis require one or more follow-up imaging studies, long dynamic acquisitions, or complex image post-processing; despite much research, the inability to confidently distinguish between these two entities continues to pose a significant dilemma for the treating clinician. Using mouse models of both glioblastoma and radiation necrosis, we tested the potential of poly(ADP-ribose) polymerase (PARP)-targeted PET imaging with [¹⁸F]PARPi to better discriminate radiation injury from tumor.

Results

In mice with experimental radiation necrosis, lesion uptake on [¹⁸F]PARPi-PET was similar to contralateral uptake (1.02 ± 0.26 lesion/contralateral %IA/cc_max ratio), while [¹⁸F]FET-PET clearly delineated the contrast-enhancing region on MR (2.12 ± 0.16 lesion/contralateral %IA/cc_max ratio). In mice with focal intracranial U251 xenografts, tumor visualization on PARPi-PET was superior to FET-PET, and lesion-to-contralateral activity ratios (max/max, p = 0.034) were higher on PARPi-PET than on FET-PET.

Conclusions

A murine model of radiation necrosis does not demonstrate [¹⁸F]PARPi avidity, and [¹⁸F]PARPi-PET is better than [¹⁸F]FET-PET in distinguishing radiation injury from brain tumor. [¹⁸F]PARPi-PET can be used for discrimination between recurrent tumor and radiation injury within a single, static imaging session, which may be of value to resolve a common dilemma in neuro-oncology.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0399-z) contains supplementary material, which is available to authorized users.

The Role of Advanced Brain Tumor Imaging in the Care of Patients with Central Nervous System Malignancies

OPINION STATEMENT

T1-weighted post-contrast and T2-weighted fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) constitute the gold standard for diagnosis and response assessment in neuro-oncologic patients but are limited in their ability to accurately reflect tumor biology and metabolism, particularly over the course of a patient's treatment. Advanced MR imaging methods are sensitized to different biophysical processes in tissue, including blood perfusion, tumor metabolism, and chemical composition of tissue, and provide more specific information on tissue physiology than standard MRI. This review provides an overview of the most common and emerging advanced imaging modalities in the field of brain tumor imaging and their applications in the care of neuro-oncologic patients.

18F-FET-PET as a biomarker for therapy response in non-contrast enhancing glioma following chemotherapy

BACKGROUND

Monitoring treatment response after chemotherapy of gadolinium-(Gd)-negative gliomas is challenging as conventional MRI often indicates no radiological changes. We hypothesize that ¹⁸F-FET-PET can be used as a biomarker for response assessment in Gd-negative gliomas undergoing chemotherapy.

METHODS

Sixty-one patients harboring Gd-negative WHO grade II or III glioma receiving alkylating agents (temozolomide or CCNU/procarbacine) were included. All patients underwent MRI and ¹⁸F-FET-PET before chemotherapy and 6 months later. We calculated T₂-volume, ¹⁸F-FET-PET based biological tumour volume (BTV) and maximal tumour-to-brain ratio (TBR_max). Moreover, dynamic PET acquisition was performed using time-activity-curves (TACs) analysis. For MRI-based response assessment, RANO criteria for low-grade glioma were used. For ¹⁸F-FET-PET, following classification scheme was tested: responsive disease (RD) when a decrease in either BTV ≥ 25% and/or TBR_max ≥ 10% occurred, an increase in BTV ≥ 25% and/or TBR_max increase > 10% characterized progressive disease (PD), minor changes ± 25% for BTV and ± 10% for TBR_max were regarded as stable disease (SD). Post-chemotherapy survival (PCS) and time-to-treatment failure (TTF) were calculated using the Kaplan-Meier method.

RESULTS

¹⁸F-FET-PET based response has shown patients with RD to have the longest TTF time (78.5 vs 24.6 vs 24.1 months, p = 0.001), while there was no significant difference between patients with a SD and PD. A comparable pattern was observed for PCS (p < 0.001). T₂-volume based assessment was not associated with outcome.

CONCLUSION

¹⁸F-FET-PET is a promising biomarker for early response assessment in Gd-negative gliomas undergoing chemotherapy. It might be helpful for a timely adjustment of potentially ineffective treatment concepts and overcomes limitations of conventional structural imaging.

Multimodal Imaging of Patients With Gliomas Confirms 11C-MET PET as a Complementary Marker to MRI for Noninvasive Tumor Grading and Intraindividual Follow-Up After Therapy

The value of combined L-( methyl-[¹¹C]) methionine positron-emitting tomography (MET-PET) and magnetic resonance imaging (MRI) with regard to tumor extent, entity prediction, and therapy effects in clinical routine in patients with suspicion of a brain tumor was investigated. In n = 65 patients with histologically verified brain lesions n = 70 MET-PET and MRI (T1-weighted gadolinium-enhanced [T1w-Gd] and fluid-attenuated inversion recovery or T2-weighted [FLAIR/T2w]) examinations were performed. The computer software "visualization and analysis framework volume rendering engine (Voreen)" was used for analysis of extent and intersection of tumor compartments. Binary logistic regression models were developed to differentiate between World Health Organization (WHO) tumor types/grades. Tumor sizes as defined by thresholding based on tumor-to-background ratios were significantly different as determined by MET-PET (21.6 ± 36.8 cm³), T1w-Gd-MRI (3.9 ± 7.8 cm³), and FLAIR/T2-MRI (64.8 ± 60.4 cm³; P < .001). The MET-PET visualized tumor activity where MRI parameters were negative: PET positive tumor volume without Gd enhancement was 19.8 ± 35.0 cm³ and without changes in FLAIR/T2 10.3 ± 25.7 cm³. FLAIR/T2-MRI visualized greatest tumor extent with differences to MET-PET being greater in posttherapy (64.6 ± 62.7 cm³) than in newly diagnosed patients (20.5 ± 52.6 cm³). The binary logistic regression model differentiated between WHO tumor types (fibrillary astrocytoma II n = 10 from other gliomas n = 16) with an accuracy of 80.8% in patients at primary diagnosis. Combined PET and MRI improve the evaluation of tumor activity, extent, type/grade prediction, and therapy-induced changes in patients with glioma and serve information highly relevant for diagnosis and management.

Diagnostic Accuracy of Amino Acid and FDG-PET in Differentiating Brain Metastasis Recurrence from Radionecrosis after Radiotherapy: A Systematic Review and Meta-Analysis

BACKGROUND

Current studies that analyze the usefulness of amino acid and FDG-PET in distinguishing brain metastasis recurrence and radionecrosis after radiation therapy are limited by small cohort size.

PURPOSE

Our aim was to assess the diagnostic accuracy of amino acid and FDG-PET in differentiating brain metastasis recurrence from radionecrosis after radiation therapy.

DATA SOURCES

Studies were retrieved from PubMed, Embase, and the Cochrane Library.

STUDY SELECTION

Fifteen studies were included from the literature. Each study used PET to differentiate radiation necrosis from tumor recurrence in contrast-enhancing lesions on follow-up brain MR imaging after treating brain metastasis with radiation therapy.

DATA ANALYSIS

Data were analyzed with a bivariate random-effects model. Sensitivity, specificity, positive likelihood ratio, negative likelihood ratio, and diagnostic odds ratio were pooled, and a summary receiver operating characteristic curve was fit to the data.

DATA SYNTHESIS

The overall pooled sensitivity, specificity, positive likelihood ratio, negative likelihood ratio, and diagnostic odds ratio of PET were 0.85, 0.88, 7.0, 0.17, and 40, respectively. The area under the receiver operating characteristic curve was 0.93. On subgroup analysis of different tracers, amino acid and FDG-PET had similar diagnostic accuracy. Meta-regression analysis demonstrated that the method of quantification based on patient, lesion, or PET scan (based on lesion versus not, P = .07) contributed to the heterogeneity.

LIMITATIONS

Our study was limited by small sample size, and 60% of the included studies were of retrospective design.

CONCLUSIONS

Amino acid and FDG-PET had good diagnostic accuracy in differentiating brain metastasis recurrence from radionecrosis after radiation therapy.

Relationship between Glioblastoma Dose Volume Parameters Measured by Dual Time Point Fluoroethylthyrosine-PET and Clinical Outcomes

Glioblastoma multiforme (GBM) is highly invasive. Despite irradiation with wide margins, GBM usually recurs in-field. Recent in vitro data have suggested that progression might be promoted by sublethal irradiation. Fluoroethylthyrosine-PET (FET-PET) can be used to detect glioblastoma invasion not apparent on MRI. We therefore performed a retrospective analysis of a prospective clinical study to examine whether glioblastoma outcomes depend on dose volume parameters measured by MRI and FET-PET. Twenty-three patients were prospectively recruited to a study examining the role of dual time point FET-PET in the treatment planning of GBM radiotherapy. The dose delivered to the site of recurrence was subdivided into suboptimal-dose (SOD) and high-dose (HD) areas. Types of progression were defined for correlation with dosimetric parameters including V100% of gross tumor volume (GTV)_PET, GTV_PETMRI, and GTV_MRI. The HD area did not cover the entire GTV_PETMRI in any case. Recurrences were significantly more frequent in the SubD area (chi-squared test, p = 0.004). There was no relationship between increasing dose volume and progression. The V100% for GTV_PET and progression-free survival (PFS) was positively correlated (Spearman’s rho 0.417; p = 0.038). Progression is more common in areas with suboptimal dosing. Dose heterogeneity within GTV_PET may be responsible for shorter PFS.

One-step synthesis of an 18F-labeled boron-derived methionine analog: a substitute for 11C-methionine?

Amino acid-based tracers have been extensively investigated for positron emission tomography (PET) imaging of brain tumors, and ¹¹C-methionine (¹¹C-MET) is one of the most extensively investigated. However, widespread clinical use of ¹¹C-MET is challenging due to the short half-life of ¹¹C and low radiolabeling yield. In this issue of the European Journal of Nuclear Medicine and Molecular Imaging, Yang and colleagues report an ¹⁸F-labeled boron-derived methionine analog, ¹⁸F-B-MET, as a potential substitute for ¹¹C-MET in PET imaging of glioma. The push-button synthesis, highly efficient radiolabeling, and good imaging performance in glioma models make this tracer a promising candidate for future clinical translation.

Brain metastases: neuroimaging

Magnetic resonance imaging (MRI) is the cornerstone for evaluating patients with brain masses such as primary and metastatic tumors. Important challenges in effectively detecting and diagnosing brain metastases and in accurately characterizing their subsequent response to treatment remain. These difficulties include discriminating metastases from potential mimics such as primary brain tumors and infection, detecting small metastases, and differentiating treatment response from tumor recurrence and progression. Optimal patient management could be benefited by improved and well-validated prognostic and predictive imaging markers, as well as early response markers to identify successful treatment prior to changes in tumor size. To address these fundamental needs, newer MRI techniques including diffusion and perfusion imaging, MR spectroscopy, and positron emission tomography (PET) tracers beyond traditionally used 18-fluorodeoxyglucose are the subject of extensive ongoing investigations, with several promising avenues of added value already identified. These newer techniques provide a wealth of physiologic and metabolic information that may supplement standard MR evaluation, by providing the ability to monitor and characterize cellularity, angiogenesis, perfusion, pH, hypoxia, metabolite concentrations, and other critical features of malignancy. This chapter reviews standard and advanced imaging of brain metastases provided by computed tomography, MRI, and amino acid PET, focusing on potential biomarkers that can serve as problem-solving tools in the clinical management of patients with brain metastases.

PET imaging in patients with meningioma-report of the RANO/PET Group

Meningiomas are the most frequent nonglial primary brain tumors and represent about 30% of brain tumors. Usually, diagnosis and treatment planning are based on neuroimaging using mainly MRI or, rarely, CT. Most common treatment options are neurosurgical resection and radiotherapy (eg, radiosurgery, external fractionated radiotherapy). For follow-up after treatment, a structural imaging technique such as MRI or CT is used. However, these structural imaging modalities have limitations, particularly in terms of tumor delineation as well as diagnosis of posttherapeutic reactive changes. Molecular imaging techniques such as PET can characterize specific metabolic and cellular features which may provide clinically relevant information beyond that obtained from structural MR or CT imaging alone. Currently, the use of PET in meningioma patients is steadily increasing. In the present article, we provide recommendations for the use of PET imaging in the clinical management of meningiomas based on evidence generated from studies being validated by histology or clinical course.

Dynamic O-(2-18F-fluoroethyl)-L-tyrosine positron emission tomography differentiates brain metastasis recurrence from radiation injury after radiotherapy

Background

The aim of this study was to investigate the potential of dynamic O-(2-[18F]fluoroethyl)-L-tyrosine (18F-FET) PET for differentiating local recurrent brain metastasis from radiation injury after radiotherapy since contrast-enhanced MRI often remains inconclusive.

Methods

Sixty-two patients (mean age, 55 ± 11 y) with single or multiple contrast-enhancing brain lesions (n = 76) on MRI after radiotherapy of brain metastases (predominantly stereotactic radiosurgery) were investigated with dynamic 18F-FET PET. Maximum and mean tumor-to-brain ratios (TBRmax, TBRmean) of 18F-FET uptake were determined (20-40 min postinjection) as well as tracer uptake kinetics (ie, time-to-peak and slope of time-activity curves). Diagnoses were confirmed histologically (34%; 26 lesions in 25 patients) or by clinical follow-up (66%; 50 lesions in 37 patients). Diagnostic accuracies of PET parameters for the correct identification of recurrent brain metastasis were evaluated by receiver-operating-characteristic analyses or the chi-square test.

Results

TBRs were significantly higher in recurrent metastases (n = 36) than in radiation injuries (n = 40) (TBRmax 3.3 ± 1.0 vs 2.2 ± 0.4, P < .001; TBRmean 2.2 ± 0.4 vs 1.7 ± 0.3, P < .001). The highest accuracy (88%) for diagnosing local recurrent metastasis could be obtained with TBRs in combination with the slope of time-activity curves (P < .001).

Conclusions

The results of this study confirm previous preliminary observations that the combined evaluation of the TBRs of 18F-FET uptake and the slope of time-activity curves can differentiate local brain metastasis recurrence from radiation-induced changes with high accuracy. 18F-FET PET may thus contribute significantly to the management of patients with brain metastases.

Pseudoprogression after glioma therapy: an update

INTRODUCTION

Initial diagnostics and follow-up of gliomas is usually based on contrast-enhanced MRI. However, the capacity of standard MRI to differentiate neoplastic tissue from posttherapeutic effects such as pseudoprogression is limited. Advanced neuroimaging methods may provide relevant additional information, which allow for a more accurate diagnosis especially in clinically equivocal situations. This review article focuses predominantly on PET using radiolabeled amino acids and advanced MRI techniques such as perfusion-weighted imaging (PWI) and summarizes the efforts of these methods regarding the identification of pseudoprogression after glioma therapy. Areas covered: The current literature on pseudoprogression in the field of brain tumors, with a focus on gliomas is summarized. A literature search was performed using the terms 'pseudoprogression', 'temozolomide', 'glioblastoma', 'PET', 'PWI', 'radiochemotherapy', and derivations thereof. Expert commentary: The present literature provides strong evidence that PWI MRI and amino acid PET can be of great value by providing valuable additional diagnostic information in order to overcome the diagnostic challenge of pseudoprogression. Despite various obstacles such as the still limited availability of amino acid PET and the lack of standardization of PWI, the diagnostic improvement probably results in relevant benefits for brain tumor patients and justifies a more widespread use of these diagnostic tools.

Comparison of 18F-FET PET and perfusion-weighted MRI for glioma grading: a hybrid PET/MR study

PURPOSE

Both perfusion-weighted MR imaging (PWI) and O-(2-¹⁸F-fluoroethyl)-L-tyrosine PET (¹⁸F-FET) provide grading information in cerebral gliomas. The aim of this study was to compare the diagnostic value of ¹⁸F-FET PET and PWI for tumor grading in a series of patients with newly diagnosed, untreated gliomas using an integrated PET/MR scanner.

METHODS

Seventy-two patients with untreated gliomas [22 low-grade gliomas (LGG), and 50 high-grade gliomas (HGG)] were investigated with ¹⁸F-FET PET and PWI using a hybrid PET/MR scanner. After visual inspection of PET and PWI maps (rCBV, rCBF, MTT), volumes of interest (VOIs) with a diameter of 16 mm were centered upon the maximum of abnormality in the tumor area in each modality and the contralateral unaffected hemisphere. Mean and maximum tumor-to-brain ratios (TBR_mean, TBR_max) were calculated. In addition, Time-to-Peak (TTP) and slopes of time-activity curves were calculated for ¹⁸F-FET PET. Diagnostic accuracies of ¹⁸F-FET PET and PWI for differentiating low-grade glioma (LGG) from high-grade glioma (HGG) were evaluated by receiver operating characteristic analyses (area under the curve; AUC).

RESULTS

The diagnostic accuracy of ¹⁸F-FET PET and PWI to discriminate LGG from HGG was similar with highest AUC values for TBR_mean and TBR_max of ¹⁸F-FET PET uptake (0.80, 0.83) and for TBR_mean and TBR_max of rCBV (0.80, 0.81). In case of increased signal in the tumor area with both methods (n = 32), local hot-spots were incongruent in 25 patients (78%) with a mean distance of 10.6 ± 9.5 mm. Dynamic FET PET and combination of different parameters did not further improve diagnostic accuracy.

CONCLUSIONS

Both ¹⁸F-FET PET and PWI discriminate LGG from HGG with similar diagnostic performance. Regional abnormalities in the tumor area are usually not congruent indicating that tumor grading by ¹⁸F-FET PET and PWI is based on different pathophysiological phenomena.

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.

Brain Tumor Imaging

Modern imaging techniques, particularly functional imaging techniques that interrogate some specific aspect of underlying tumor biology, have enormous potential in neuro-oncology for disease detection, grading, and tumor delineation to guide biopsy and resection; monitoring treatment response; and targeting radiotherapy. This brief review considers the role of magnetic resonance imaging and spectroscopy, and positron emission tomography in these areas and discusses the factors that limit translation of new techniques to the clinic, in particular, the cost and difficulties associated with validation in multicenter clinical trials.

Melanoma patient imaging in the era of effective systemic therapies

Imaging plays a critical role in the current multi-disciplinary management of patients with melanoma. It is used for primary disease staging, surgical planning, and surveillance in high-risk patients, and for monitoring the effects of systemic or loco-regional therapies. Several different imaging modalities have been utilised in the past. Contemporary imaging practises vary geographically depending on clinical guidelines, physician preferences, availability and cost. Targeted therapies and immunotherapies have revolutionised the treatment of patients with metastatic melanoma over the last few years. With this have come new patterns of disease that were not observed after conventional therapies, and new criteria to assess therapeutic responses. In this article we review the role of imaging for patients with melanoma in the era of effective systemic therapies and discuss likely future developments.

The use of O-(2-18F-fluoroethyl)-L-tyrosine PET in the diagnosis of gliomas located in the brainstem and spinal cord

Background

Despite an increasing number of O-(2-18F-fluoroethyl)-L-tyrosine (18F-FET) PET studies in supratentorial gliomas, studies regarding the usefulness of 18F-FET PET in brainstem and spinal cord gliomas to date remain scarce.

Methods

Thirty-six 18F-FET PET scans were performed in 29 patients with brainstem (n = 29 scans) or spinal cord glioma (n = 7 scans). In 32 of 36 PET scans, a dynamic acquisition was performed. Fifteen scans in 15 patients were performed to assess newly diagnosed lesions, and 21 scans were obtained during follow-up: for diagnosing tumor progression (n = 15 scans in 14 patients) as well as for treatment monitoring (n = 6 scans in 3 patients). Four patients underwent additional serial scans (range, 1-2), and 3 of these 4 patients were examined for more than one indication. Maximum and mean tumor/brain ratios (TBRmax/mean) of 18F-FET uptake (20-40 min post injection) as well as kinetic 18F-FET uptake parameters were determined. Final diagnoses were confirmed histologically (54%) or by clinical follow-up (46%).

Results

In all newly diagnosed high-grade (n = 3 patients) and in 5 of 11 patients with low-grade gliomas, 18F-FET uptake was increased (TBRmax ≥2.5 and/or TBRmean ≥1.9). In 2 patients with newly diagnosed gliomas without MR contrast enhancement, 18F-FET PET nevertheless showed increased metabolism. At suspected progression, the combination of TBRs with kinetic 18F-FET parameters correctly identified presence or absence of progressive disease in 9 of 11 patients (82%).

Conclusions

This preliminary study suggests that 18F-FET PET adds valuable diagnostic information in brainstem and spinal cord glioma, particularly when the diagnostic information derived from MRI is equivocal.

Advances in neuro-oncology imaging

Despite the fact that MRI has evolved to become the standard method for diagnosis and monitoring of patients with brain tumours, conventional MRI sequences have two key limitations: the inability to show the full extent of the tumour and the inability to differentiate neoplastic tissue from nonspecific, treatment-related changes after surgery, radiotherapy, chemotherapy or immunotherapy. In the past decade, PET involving the use of radiolabelled amino acids has developed into an important diagnostic tool to overcome some of the shortcomings of conventional MRI. The Response Assessment in Neuro-Oncology working group - an international effort to develop new standardized response criteria for clinical trials in brain tumours - has recommended the additional use of amino acid PET imaging for brain tumour management. Concurrently, a number of advanced MRI techniques such as magnetic resonance spectroscopic imaging and perfusion weighted imaging are under clinical evaluation to target the same diagnostic problems. This Review summarizes the clinical role of amino acid PET in relation to advanced MRI techniques for differential diagnosis of brain tumours; delineation of tumour extent for treatment planning and biopsy guidance; post-treatment differentiation between tumour progression or recurrence versus treatment-related changes; and monitoring response to therapy. An outlook for future developments in PET and MRI techniques is also presented.

Determination of an Optimal Pharmacokinetic Model of 18F-FET for Quantitative Applications in Rat Brain Tumors

O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) is a radiolabeled artificial amino acid used in PET for tumor delineation and grading. The present study compares different kinetic models to determine which are more appropriate for ¹⁸F-FET in rats. Methods: Rats were implanted with F98 glioblastoma cells in the right hemisphere and scanned 9-15 d later. PET data were acquired during 50 min after a 1-min bolus of ¹⁸F-FET. Arterial blood samples were drawn for arterial input function determination. Two compartmental pharmacokinetic models were tested: the 2-tissue model and the 1-tissue model. Their performance at fitting concentration curves from regions of interest was evaluated using the Akaike information criterion, F test, and residual plots. Graphical models were assessed qualitatively. Results: Metrics indicated that the 2-tissue model was superior to the 1-tissue model for the current dataset. The 2-tissue model allowed adequate decoupling of ¹⁸F-FET perfusion and internalization by cells in the different regions of interest. Of the 2 graphical models tested, the Patlak plot provided adequate results for the tumor and brain, whereas the Logan plot was appropriate for muscles. Conclusion: The 2-tissue-compartment model is appropriate to quantify the perfusion and internalization of ¹⁸F-FET by cells in various tissues of the rat, whereas graphical models provide a global measure of uptake.

Amino acid PET and MR perfusion imaging in brain tumours

Purpose

Despite the excellent capacity of the conventional MRI to image brain tumours, problems remain in answering a number of critical diagnostic questions. To overcome these diagnostic shortcomings, PET using radiolabeled amino acids and perfusion-weighted imaging (PWI) are currently under clinical evaluation. The role of amino acid PET and PWI in different diagnostic challenges in brain tumours is controversial.

Methods

Based on the literature and experience of our centres in correlative imaging with PWI and PET using O-(2-[¹⁸F]fluoroethyl)-l-tyrosine or 3,4-dihydroxy-6-[¹⁸F]-fluoro-l-phenylalanine, the current role and shortcomings of amino acid PET and PWI in different diagnostic challenges in brain tumours are reviewed. Literature searches were performed on PubMed, and additional literature was retrieved from the reference lists of identified articles. In particular, all studies in which amino acid PET was directly compared with PWI were included.

Results

PWI is more readily available, but requires substantial expertise and is more sensitive to artifacts than amino acid PET. At initial diagnosis, PWI and amino acid PET can help to define a site for biopsy but amino acid PET appears to be more powerful to define the tumor extent. Both methods are helpful to differentiate progression or recurrence from unspecific posttherapeutic changes. Assessment of therapeutic efficacy can be achieved especially with amino acid PET, while the data with PWI are sparse.

Conclusion

Both PWI and amino acid PET add valuable diagnostic information to the conventional MRI in the assessment of patients with brain tumours, but further studies are necessary to explore the complementary nature of these two methods.