Ability of (18)F-DOPA PET/CT and fused (18)F-DOPA PET/MRI to assess striatal involvement in paediatric glioma

Towards an Automated Approach to the Semi-Quantification of [18F]F-DOPA PET in Pediatric-Type Diffuse Gliomas

BACKGROUND

This study aims to evaluate the use of a computer-aided, semi-quantification approach to [¹⁸F]F-DOPA positron emission tomography (PET) in pediatric-type diffuse gliomas (PDGs) to calculate the tumor-to-background ratio.

METHODS

A total of 18 pediatric patients with PDGs underwent magnetic resonance imaging and [¹⁸F]F-DOPA PET, which were analyzed using both manual and automated procedures. The former provided a tumor-to-normal-tissue ratio (T_N) and tumor-to-striatal-tissue ratio (T_S), while the latter provided analogous scores (t_n, t_s). We tested the correlation, consistency, and ability to stratify grading and survival between these methods.

RESULTS

High Pearson correlation coefficients resulted between the ratios calculated with the two approaches: ρ = 0.93 (p < 10^-4) and ρ = 0.814 (p < 10^-4). The analysis of the residuals suggested that t_n and t_s were more consistent than T_N and T_S. Similarly to T_N and T_S, the automatically computed scores showed significant differences between low- and high-grade gliomas (p ≤ 10^-4, t-test) and the overall survival was significantly shorter in patients with higher values when compared to those with lower ones (p < 10^-3, log-rank test).

CONCLUSIONS

This study suggested that the proposed computer-aided approach could yield similar results to the manual procedure in terms of diagnostic and prognostic information.

A Systematic Review of Amino Acid PET Imaging in Adult-Type High-Grade Glioma Surgery: A Neurosurgeon's Perspective

Amino acid PET imaging has been used for a few years in the clinical and surgical management of gliomas with satisfactory results in diagnosis and grading for surgical and radiotherapy planning and to differentiate recurrences. Biological tumor volume (BTV) provides more meaningful information than standard MR imaging alone and often exceeds the boundary of the contrast-enhanced nodule seen in MRI. Since a gross total resection reflects the resection of the contrast-enhanced nodule and the majority of recurrences are at a tumor's margins, an integration of PET imaging during resection could increase PFS and OS. A systematic review of the literature searching for "PET" [All fields] AND "glioma" [All fields] AND "resection" [All fields] was performed in order to investigate the diffusion of integration of PET imaging in surgical practice. Integration in a neuronavigation system and intraoperative use of PET imaging in the primary diagnosis of adult high-grade gliomas were among the criteria for article selection. Only one study has satisfied the inclusion criteria, and a few more (13) have declared to use multimodal imaging techniques with the integration of PET imaging to intentionally perform a biopsy of the PET uptake area. Despite few pieces of evidence, targeting a biologically active area in addition to other tools, which can help intraoperatively the neurosurgeon to increase the amount of resected tumor, has the potential to provide incremental and complementary information in the management of brain gliomas. Since supramaximal resection based on the extent of MRI FLAIR hyperintensity resulted in an advantage in terms of PFS and OS, PET-based biological tumor volume, avoiding new neurological deficits, deserves further investigation.

The role of [18F]fluorodopa positron emission tomography in grading of gliomas

PURPOSE

Gliomas are the most commonly occurring brain tumour in adults and there remains no cure for these tumours with treatment strategies being based on tumour grade. All treatment options aim to prolong survival, maintain quality of life and slow the inevitable progression from low-grade to high-grade. Despite imaging advancements, the only reliable method to grade a glioma is to perform a biopsy, and even this is fraught with errors associated with under grading. Positron emission tomography (PET) imaging with amino acid tracers such as [18F]fluorodopa (18F-FDOPA), [11C]methionine (11C-MET), [18F]fluoroethyltyrosine (18F-FET), and 18F-FDOPA are being increasingly used in the diagnosis and management of gliomas.

METHODS

In this review we discuss the literature available on the ability of 18F-FDOPA-PET to distinguish low- from high-grade in newly diagnosed gliomas.

RESULTS

In 2016 the Response Assessment in Neuro-Oncology (RANO) and European Association for Neuro-Oncology (EANO) published recommendations on the clinical use of PET imaging in gliomas. However, since these recommendations there have been a number of studies performed looking at whether 18F-FDOPA-PET can identify areas of high-grade transformation before the typical radiological features of transformation such as contrast enhancement are visible on standard magnetic resonance imaging (MRI).

CONCLUSION

Larger studies are needed to validate 18F-FDOPA-PET as a non-invasive marker of glioma grade and prediction of tumour molecular characteristics which could guide decisions surrounding surgical resection.

Advanced Neuroimaging Approaches to Pediatric Brain Tumors

Simple Summary

After leukemias, brain tumors are the most common cancers in children, and early, accurate diagnosis is critical to improve patient outcomes. Beyond the conventional imaging methods of computed tomography (CT) and magnetic resonance imaging (MRI), advanced neuroimaging techniques capable of both structural and functional imaging are moving to the forefront to improve the early detection and differential diagnosis of tumors of the central nervous system. Here, we review recent developments in neuroimaging techniques for pediatric brain tumors.

Abstract

Central nervous system tumors are the most common pediatric solid tumors; they are also the most lethal. Unlike adults, childhood brain tumors are mostly primary in origin and differ in type, location and molecular signature. Tumor characteristics (incidence, location, and type) vary with age. Children present with a variety of symptoms, making early accurate diagnosis challenging. Neuroimaging is key in the initial diagnosis and monitoring of pediatric brain tumors. Conventional anatomic imaging approaches (computed tomography (CT) and magnetic resonance imaging (MRI)) are useful for tumor detection but have limited utility differentiating tumor types and grades. Advanced MRI techniques (diffusion-weighed imaging, diffusion tensor imaging, functional MRI, arterial spin labeling perfusion imaging, MR spectroscopy, and MR elastography) provide additional and improved structural and functional information. Combined with positron emission tomography (PET) and single-photon emission CT (SPECT), advanced techniques provide functional information on tumor metabolism and physiology through the use of radiotracer probes. Radiomics and radiogenomics offer promising insight into the prediction of tumor subtype, post-treatment response to treatment, and prognostication. In this paper, a brief review of pediatric brain cancers, by type, is provided with a comprehensive description of advanced imaging techniques including clinical applications that are currently utilized for the assessment and evaluation of pediatric brain tumors.

Role of Dynamic Parameters of 18F-DOPA PET/CT in Pediatric Gliomas

PURPOSE OF THE REPORT

PET with 18F-DOPA can be used to evaluate grading and aggressiveness of pediatric cerebral gliomas. However, standard uptake parameters may underperform in circumscribed lesions and in diffuse pontine gliomas. In this study, we tested whether dynamic 18F-DOPA PET could overcome these limitations.

PATIENTS AND METHODS

Patients with available dynamic 18F-DOPA PET were included retrospectively. Static parameters (tumor/striatum ratio [T/S] and tumor/cortex ratio [T/N]) and dynamic ones, calculated on the tumor time activity curve (TAC), including time-to-peak (TTP), slope steepness, the ratio between tumor and striatum TAC steepness (dynamic slope ratio [DSR]), and TAC shape (accumulation vs plateau), were evaluated as predictors of high/low grading (HG and LG) and of progression-free survival and overall survival.

RESULTS

Fifteen patients were included; T/S, T/N, TTP, TAC slope steepness, and DSR were not significantly different between HG and LG. The accumulation TAC shape was more prevalent in the LG than in the HG group (75% vs 27%). On progression-free survival univariate analysis, TAC accumulation shape predicted longer survival (P < 0.001), whereas T/N and DSR showed borderline significance; on multivariate analyses, only TAC shape was retained (P < 0.01, Harrell C index, 0.93-0.95). On overall survival univariate analysis, T/N (P < 0.05), DSR (P < 0.05), and TAC "accumulating" shape predicted survival (P < 0.001); once more, only this last parameter was retained in the multivariate models (P < 0.05, Harrell C index, 0.86-0.89).

CONCLUSIONS

Dynamic 18F-DOPA PET analysis outperforms the static parameter evaluation in grading assessment and survival prediction. Evaluation of the curve shape is a simple-to-use parameter with strong predictive power.

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

Positron emission tomography (PET) has been widely used in paediatric oncology. 2-Deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) is the most commonly used radiopharmaceutical for PET imaging. For oncological brain imaging, different amino acid PET radiopharmaceuticals have been introduced in the last years. The purpose of this document is to provide imaging specialists and clinicians guidelines for indication, acquisition, and interpretation of [¹⁸F]FDG and radiolabelled amino acid PET in paediatric patients affected by brain gliomas. There is no high level of evidence for all recommendations suggested in this paper. These recommendations represent instead the consensus opinion of experienced leaders in the field. Further studies are needed to reach evidence-based recommendations for the applications of [¹⁸F]FDG and radiolabelled amino acid PET in paediatric neuro-oncology. These recommendations are not intended to be a substitute for national and international legal or regulatory provisions and should be considered in the context of good practice in nuclear medicine. The present guidelines/standards were developed collaboratively by the EANM and SNMMI with the European Society for Paediatric Oncology (SIOPE) Brain Tumour Group and the Response Assessment in Paediatric Neuro-Oncology (RAPNO) working group. They summarize also the views of the Neuroimaging and Oncology and Theranostics Committees of the EANM and reflect recommendations for which the EANM and other societies cannot be held responsible.

Radionuclide-Based Imaging of Breast Cancer: State of the Art

Simple Summary

Breast cancer is one of the most commonly diagnosed malignant tumors, possessing high incidence and mortality rates that threaten women’s health. Thus, early and effective breast cancer diagnosis is crucial for enhancing the survival rate. Radionuclide molecular imaging displays its advantages for detecting breast cancer from a functional perspective. Noninvasive visualization of biological processes with radionuclide-labeled small metabolic compounds helps elucidate the metabolic state of breast cancer, while radionuclide-labeled ligands/antibodies for receptor-targeted radionuclide molecular imaging is sensitive and specific for visualization of the overexpressed molecular markers in breast cancer. This review focuses on the most recent developments of novel radiotracers as promising tools for early breast cancer diagnosis.

Abstract

Breast cancer is a malignant tumor that can affect women worldwide and endanger their health and wellbeing. Early detection of breast cancer can significantly improve the prognosis and survival rate of patients, but with traditional anatomical imagine methods, it is difficult to detect lesions before morphological changes occur. Radionuclide-based molecular imaging based on positron emission tomography (PET) and single-photon emission computed tomography (SPECT) displays its advantages for detecting breast cancer from a functional perspective. Radionuclide labeling of small metabolic compounds can be used for imaging biological processes, while radionuclide labeling of ligands/antibodies can be used for imaging receptors. Noninvasive visualization of biological processes helps elucidate the metabolic state of breast cancer, while receptor-targeted radionuclide molecular imaging is sensitive and specific for visualization of the overexpressed molecular markers in breast cancer, contributing to early diagnosis and better management of cancer patients. The rapid development of radionuclide probes aids the diagnosis of breast cancer in various aspects. These probes target metabolism, amino acid transporters, cell proliferation, hypoxia, estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), gastrin-releasing peptide receptor (GRPR) and so on. This article provides an overview of the development of radionuclide molecular imaging techniques present in preclinical or clinical studies, which are used as tools for early breast cancer diagnosis.

The Role of PET in Supratentorial and Infratentorial Pediatric Brain Tumors

OBJECTIVE

This review aims to provide a summary of the clinical indications and limitations of PET imaging with different radiotracers, including 18F-fluorodeoxyglucose (18F-FDG) and other radiopharmaceuticals, in pediatric neuro-oncology, discussing both supratentorial and infratentorial tumors, based on recent literature (from 2010 to present).

METHODS

A literature search of the PubMed/MEDLINE database was carried out searching for articles on the use of PET in pediatric brain tumors. The search was updated until December 2020 and limited to original studies published in English after 1 January 2010.

RESULTS

18F-FDG PET continues to be successfully employed in different settings in pediatric neuro-oncology, including diagnosis, grading and delineation of the target for stereotactic biopsy, estimation of prognosis, evaluation of recurrence, treatment planning and assessment of treatment response. Nevertheless, non-18F-FDG tracers, especially amino acid analogues seem to show a better performance in each clinical setting.

CONCLUSIONS

PET imaging adds important information in the diagnostic work-up of pediatric brain tumors. International or national multicentric studies are encouraged in order to collect larger amount of data.

The use of multiparametric 18F-fluoro-L-3,4-dihydroxy-phenylalanine PET/MRI in post-therapy assessment of patients with gliomas

PURPOSE

To determine the utility of F-fluoro-L-3,4-dihydroxy-phenylalanine (F-DOPA) PET/MRI versus cross-sectional MRI alone in glioma response assessment and identify whether the two techniques demonstrate different tumour features.

METHODS

F-DOPA PET/MRI studies from 40 patients were analysed. Quantitative PET parameters and conventional MRI features were recorded. Tumour volume was assessed on both PET and MRI. Using dynamic susceptibility contrast perfusion-weighted imaging, maps of cerebral blood flow (CBF) and cerebral blood volume (CBV) were obtained. Within volume of tumours of tumour features and normal-appearing white matter (NAWM) drawn on MRI, standardised uptake value (SUV)max, CBF and CBV were recorded. Presence of residual active tumour was assessed by qualitative visual assessment. Receiver operating characteristic analysis was performed univariately and on parameter combination to analyse ability to determine presence/absence of disease. Reference standard for presence of viable tissue was biopsy or clinical follow-up.

RESULTS

Median SUVmax was 3.4 for low-grade glioma (LGG) and 3.3 for high-grade glioma (HGG). There was a significant correlation between PWI parameters and WHO grade (P < 0.001), but no correlation with SUVmax. Median F-DOPA volume was 8216.88 mm for HGG and 6284.94 mm for LGG; MRI volume was 6316.57 mm and 5931.55 mm, respectively. SUVmax analysis distinguished enhancing and nonenhancing components from necrosis and NAWM and demonstrated active disease in nonenhancing regions. Visually, the modalities were concordant in 37 patients. Combining the multiparametric PET/MRI approach with all available data-enhanced detection of the presence of tumour (area under the curve 0.99, P < 0.01).

CONCLUSION

MRI and F-DOPA are complementary modalities for assessment of tumour burden. Matching F-DOPA and MRI in assessing residual tumour volume may better delineate the radiotherapy target volume.

Molecular Imaging in Pediatric Brain Tumors

In the last decade, several radiopharmaceuticals have been developed and investigated for imaging in vivo of pediatric brain tumors with the aim of exploring peculiar metabolic processes as glucose consumption, amino-acid metabolism, and protein synthesis with nuclear medicine techniques. Although the clinical shreds of evidence are limited, preliminary results are encouraging. In this review, we performed web-based and desktop research summarizing the most relevant findings of the literature published to date on this topic. Particular attention was given to the wide spectrum of nuclear medicine advances and trends in pediatric neurooncology and neurosurgery. Furthermore, the role of somatostatin receptor imaging through single-photon emission computed tomography (SPECT) and positron emission tomography (PET) probes, with reference to their potential therapeutic implications, was examined in the peculiar context. Preliminary results show that functional imaging in pediatric brain tumors might lead to significant improvements in terms of diagnostic accuracy and it could be of help in the management of the disease.

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.

Advanced MR imaging and 18F-DOPA PET characteristics of H3K27M-mutant and wild-type pediatric diffuse midline gliomas

PURPOSE

The aim of this study was to investigate MRI-derived diffusion weighted imaging (DWI), ¹H-MR spectroscopy (¹H-MRS) and arterial spin labeling (ASL) perfusion imaging in comparison with ¹⁸F-dihydroxyphenylalanine (DOPA) PET with respect to diagnostic evaluation of pediatric diffuse midline gliomas (DMG) H3K27M-mutant and wild-type.

METHODS

We retrospectively analyzed 22 pediatric patients with DMG histologically proved and molecularly classified as H3K27M-mutant (12 subjects) and wild-type (10 subjects) who underwent DWI, ¹H-MRS, and ASL performed within 2 weeks of ¹⁸F-DOPA PET. DWI-derived relative minimum apparent diffusion coefficient (rADC min), ¹H-MRS data [choline/N-acetylaspartate (Cho/NAA), choline/creatine (Cho/Cr), and presence of lactate] and relative ASL-derived cerebral blood flow max (rCBF max) were compared with ¹⁸F-DOPA uptake Tumor/Normal tissue (T/N) and Tumor/Striatum (T/S) ratios, and correlated with histological and molecular features of DMG. Statistics included Pearson's chi-square and Mann-Whitney U tests, Spearman's rank correlation and receiver operating characteristic (ROC) analysis.

RESULTS

The highest degrees of correlation among different techniques were found between T/S, rADC min and Cho/NAA ratio (p < 0.01), and between rCBF max and rADC min (p < 0.01). Significant differences between histologically classified low- and high-grade DMG, independently of H3K27M-mutation, were found among all imaging techniques (p ≤ 0.02). Significant differences in terms of rCBF max, rADC min, Cho/NAA and ¹⁸F-DOPA uptake were also found between molecularly classified mutant and wild-type DMG (p ≤ 0.02), even though wild-type DMG included low-grade astrocytomas, not present among mutant DMG. When comparing only histologically defined high-grade mutant and wild-type DMG, only the ¹⁸F-DOPA PET data T/S demonstrated statistically significant differences independently of histology (p < 0.003). ROC analysis demonstrated that T/S ratio was the best parameter for differentiating mutant from wild-type DMG (AUC 0.94, p < 0.001).

CONCLUSIONS

Advanced MRI and ¹⁸F-DOPA PET characteristics of DMG depend on histological features; however, ¹⁸F-DOPA PET-T/S was the only parameter able to discriminate H3K27M-mutant from wild-type DMG independently of histology.

Fluorine-18 labeled amino acids for tumor PET/CT imaging

Tumor glucose metabolism and amino acid metabolism are usually enhanced, ¹⁸F-FDG for tumor glucose metabolism PET imaging has been clinically well known, but tumor amino acid metabolism PET imaging is not clinically familiar. Radiolabeled amino acids (AAs) are an important class of PET/CT tracers that target the upregulated amino acid transporters to show elevated amino acid metabolism in tumor cells. Radiolabeled amino acids were observed to have high uptake in tumor cells but low in normal tissues and inflammatory tissues. The radionuclides used in labeling amino acids include ¹⁵O, ¹³N, ¹¹C, ¹²³I, ¹⁸F and ⁶⁸Ga, among which the most commonly used is ¹⁸F [1]. Available data support the use of certain ¹⁸F-labeled AAs for PET/CT imaging of gliomas, neuroendocrine tumors, prostate cancer and breast cancer [2, 3]. With the progress of the method of ¹⁸F labeling AAs [4-6], ¹⁸F-labeled AAs are well established for tumor PET/CT imaging. This review focuses on the current status of key clinical applications of 18F-labeled AAs in tumor PET/CT imaging.

Grading and outcome prediction of pediatric diffuse astrocytic tumors with diffusion and arterial spin labeling perfusion MRI in comparison with 18F-DOPA PET

PURPOSE

The aim of this study was to investigate MRI-derived diffusion weighted imaging (DWI) and arterial spin labeling (ASL) perfusion imaging in comparison with ¹⁸F-dihydroxyphenylalanine (DOPA) PET with respect to diagnostic performance in tumor grading and outcome prediction in pediatric patients with diffuse astrocytic tumors (DAT).

METHODS

We retrospectively analyzed 26 children with histologically proven treatment naïve low and high grade DAT who underwent ASL and DWI performed within 2 weeks of ¹⁸F-DOPA PET. Relative ASL-derived cerebral blood flow max (rCBF max) and DWI-derived minimum apparent diffusion coefficient (rADC min) were compared with ¹⁸F-DOPA uptake tumor/normal tissue (T/N) and tumor/striatum (T/S) ratios, and correlated with World Health Organization (WHO) tumor grade and progression-free survival (PFS). Statistics included Pearson's chi-square and Mann-Whitney U tests, Spearman's rank correlation, receiver operating characteristic (ROC) analysis, discriminant function analysis (DFA), Kaplan-Meier survival curve, and Cox analysis.

RESULTS

A significant correlation was demonstrated between rCBF max, rADC min, and ¹⁸F-DOPA PET data (p < 0.001). Significant differences in terms of rCBF max, rADC min, and ¹⁸F-DOPA uptake were found between low- and high-grade DAT (p ≤ 0.001). ROC analysis and DFA demonstrated that T/S and T/N values were the best parameters for predicting tumor progression (AUC 0.93, p < 0.001). On univariate analysis, all diagnostic tools correlated with PFS (p ≤ 0.001); however, on multivariate analysis, only ¹⁸F-DOPA uptake remained significantly associated with outcome (p ≤ 0.03), while a trend emerged for rCBF max (p = 0.09) and rADC min (p = 0.08). The combination of MRI and PET data increased the predictive power for prognosticating tumor progression (AUC 0.97, p < 0.001).

CONCLUSIONS

DWI, ASL and ¹⁸F-DOPA PET provide useful complementary information for pediatric DAT grading. ¹⁸F-DOPA uptake better correlates with PFS prediction. Combining MRI and PET data provides the highest predictive power for prognosticating tumor progression suggesting a synergistic role of these diagnostic tools.

PET and SPECT studies in children with hemispheric low-grade gliomas

Molecular imaging is playing an increasing role in the pretreatment evaluation of low-grade gliomas. While glucose positron emission tomography (PET) can be helpful to differentiate low-grade from high-grade tumors, PET imaging with amino acid radiotracers has several advantages, such as better differentiation between tumors and non-tumorous lesions, optimized biopsy targeting, and improved detection of tumor recurrence. This review provides a brief overview of single-photon emission computed tomography (SPECT) studies followed by a more detailed review of the clinical applications of glucose and amino acid PET imaging in low-grade hemispheric gliomas. We discuss key differences in the performance of the most commonly utilized PET radiotracers and highlight the advantage of PET/MRI fusion to obtain optimal information about tumor extent, heterogeneity, and metabolism. Recent data also suggest that simultaneous acquisition of PET/MR images and the combination of advanced MRI techniques with quantitative PET can further improve the pretreatment and post-treatment evaluation of pediatric brain tumors.

Brain Tumors: An Update on Clinical PET Research in Gliomas

A previous review published in 2012 demonstrated the role of clinical PET for diagnosis and management of brain tumors using mainly FDG, amino acid tracers, and ¹⁸F-fluorothymidine. This review provides an update on clinical PET studies, most of which are motivated by prediction of prognosis and planning and monitoring of therapy in gliomas. For FDG, there has been additional evidence supporting late scanning, and combination with ¹³N ammonia has yielded some promising results. Large neutral amino acid tracers have found widespread applications mostly based on ¹⁸F-labeled compounds fluoroethyltyrosine and fluorodopa for targeting biopsies, therapy planning and monitoring, and as outcome markers in clinical trials. ¹¹C-alpha-methyltryptophan (AMT) has been proposed as an alternative to ¹¹C-methionine, and there may also be a role for cyclic amino acid tracers. ¹⁸F-fluorothymidine has shown strengths for tumor grading and as an outcome marker. Studies using ¹⁸F-fluorocholine (FCH) and ⁶⁸Ga-labeled compounds are promising but have not yet clearly defined their role. Studies on radiotherapy planning have explored the use of large neutral amino acid tracers to improve the delineation of tumor volume for irradiation and the use of hypoxia markers, in particular ¹⁸F-fluoromisonidazole. Many studies employed the combination of PET with advanced multimodal MR imaging methods, mostly demonstrating complementarity and some potential benefits of hybrid PET/MR.