Reply to "[18F]-fluoro-ethyl-L-tyrosine PET: a valuable diagnostic tool in neuro-oncology, but not all that glitters is glioma" by Hutterer et al

Amino Acid PET in Neurooncology

For decades, several amino acid PET tracers have been used to optimize diagnostics in patients with brain tumors. In clinical routine, the most important clinical indications for amino acid PET in brain tumor patients are differentiation of neoplasm from nonneoplastic etiologies, delineation of tumor extent for further diagnostic and treatment planning (i.e., diagnostic biopsy, resection, or radiotherapy), differentiation of treatment-related changes such as pseudoprogression or radiation necrosis after radiation or chemoradiation from tumor progression at follow-up, and assessment of response to anticancer therapy, including prediction of patient outcome. This continuing education article addresses the diagnostic value of amino acid PET for patients with either glioblastoma or metastatic brain cancer.

Diagnostic and Prognostic Potential of 18F-FET PET in the Differential Diagnosis of Glioma Recurrence and Treatment-Induced Changes After Chemoradiation Therapy

Background

MRI-based differential diagnosis of glioma recurrence (GR) and treatment-induced changes (TICs) remain elusive in up to 30% of treated glioma patients. We aimed to determine ¹⁸F-FET PET diagnostic performance in this clinical scenario, its outcome dependency on established prognostic factors, optimal ¹⁸F-FET semi-quantitative thresholds, and whether ¹⁸F-FET parameters may instantly predict progression-free survival (PFS) and overall survival (OS).

Methods

We retrospectively analyzed 45 glioma patients treated with chemoradiation therapy (32 males; mean age: 51 years, glioma grade: n=26 WHO4; n=15 WHO3; n=4 WHO2) who underwent ¹⁸F-FET PET to resolve differential diagnosis of GR and TICs raised by MRI performed in the preceding 2 weeks and depicting any of the following changes in their radiation field: volumetric increase of contrast-enhancing lesions; new contrast-enhancing lesion; significant increase in T2/FLAIR non-enhancing lesion without reducing corticosteroids. ¹⁸F-FET PET outcome relied on evaluation of maximum tumor-to-brain ratio (TBRmax), time-to-peak (TTP), and time-activity curve pattern (TAC). Metabolic tumor volume (MTV) and total tumor metabolism (TTM) were calculated for prognostic purposes. Standard of reference was repeat MRI performed 4–6 weeks after the previous MRI. Non-parametric statistics tested ¹⁸F-FET-based parameters for dependency on established prognostic markers. ROC curve analysis determined optimal cutoff values for ¹⁸F-FET semi-quantitative parameters. ¹⁸F-FET parameters and prognostic factors were evaluated for PFS and OS by Kaplan-Meier, univariate, and multivariate analyses.

Results

¹⁸F-FET PET sensitivity, specificity, positive predictive value, negative predictive value were 86.2, 81.3, 89.3, 76.5%, respectively; higher diagnostic accuracy was yielded in IDH-wild-type glioma patients compared to IDH-mutant glioma patients (sensitivity: 81.8 versus 88.9%; specificity: 80.8 versus 81.8%). KPS was the only prognostic factor differing according to ¹⁸F-FET PET outcome (negative versus positive). Optimal ¹⁸F-FET cutoff values for GR were TBRmax ≥ 2.1, SUVmax ≥ 3.5, and TTP ≤ 29 min. PFS differed based on ¹⁸F-FET outcome and related metrics and according to KPS; a different OS was observed according to KPS only. On multivariate analysis, ¹⁸F-FET PET outcome was the only significant PFS factor; KPS and age the only significant OS factors.

Conclusion

¹⁸F-FET PET demonstrated good diagnostic performance. ¹⁸F-FET PET outcome and metrics were significantly predictive only for PFS.

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.

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.

Influence of Bevacizumab on Blood-Brain Barrier Permeability and O-(2-18F-Fluoroethyl)-l-Tyrosine Uptake in Rat Gliomas

Restoration of the blood-brain barrier (BBB) after antiangiogenic therapy of gliomas with bevacizumab may result in a decrease in contrast enhancement on MRI despite tumor progression. This so-called pseudoresponse is difficult to differentiate from a true tumor response with conventional MRI. Initial patient studies have indicated that PET using O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) may be helpful for solving this diagnostic problem. This study was performed to investigate the effects of bevacizumab on BBB permeability and ¹⁸F-FET uptake in a human xenograft model. Methods: Human U87 glioblastoma cells were implanted into the striatum of immunodeficient RNU rats. ¹⁸F-FET PET scans and ex vivo autoradiography were performed in animals receiving a single high dose of bevacizumab (45 mg/kg 2 d before PET; n = 9) or in animals receiving 2 lower doses (10 mg/kg 9 and 2 d before PET; n = 10) to evaluate short-term and long-term effects on the BBB, respectively, and in control animals without bevacizumab treatment (n = 8). Time-activity curves, slope, and tumor-to-brain ratios of ¹⁸F-FET uptake (18-61 min after injection) were evaluated using a volume-of-interest analysis. After PET scanning, Evans blue dye (EBD) was injected into animals, and cryosections of the brains were evaluated by autoradiography, by histology, and for EBD fluorescence to assess BBB permeability. Results: Compared with the control, short-term bevacizumab therapy resulted in a trend toward BBB restoration (P = 0.055) and long-term therapy resulted in a significant decrease (P = 0.004) in BBB permeability, as assessed by EBD fluorescence. In contrast, no significant differences in tumor-to-brain ratios or slope of ¹⁸F-FET uptake were observed in PET and autoradiography (P > 0.05). Conclusion:⁸F-FET uptake in glioblastomas seems to be largely independent of BBB permeability and reflects the viability of tumor tissue during antiangiogenic therapy more reliably than contrast-enhanced MRI.

Combined PET Imaging of the Inflammatory Tumor Microenvironment Identifies Margins of Unique Radiotracer Uptake

The tumor microenvironment is highly heterogeneous. For gliomas, the tumor-associated inflammatory response is pivotal to support growth and invasion. Factors of glioma growth, inflammation, and invasion, such as the translocator protein (TSPO) and matrix metalloproteinases (MMP), may serve as specific imaging biomarkers of the glioma microenvironment. In this study, noninvasive imaging by PET with [¹⁸F]DPA-714 (TSPO) and [¹⁸F]BR-351 (MMP) was used for the assessment of localization and quantification of the expression of TSPO and MMP. Imaging was performed in addition to established clinical imaging biomarker of active tumor volume ([¹⁸F]FET) in conjunction with MRI. We hypothesized that each imaging biomarker revealed distinct areas of the heterogeneous glioma tissue in a mouse model of human glioma. Tracers were found to be increased 1.4- to 1.7-fold, with [¹⁸F]FET showing the biggest volume as depicted by a thresholding-based, volumes of interest analysis. Tumor areas, which could not be detected by a single tracer and/or MRI parameter alone, were measured. Specific compartments of [¹⁸F]DPA-714 (14%) and [¹⁸F]BR-351 (11%) volumes along the tumor rim could be identified. [¹⁸F]DPA-714 (TSPO) and [¹⁸F]BR-351 (MMP) matched with histology. Glioma-associated microglia/macrophages (GAM) were identified as TSPO and MMP sources. Multitracer and multimodal molecular imaging approaches may allow us to gain important insights into glioma-associated inflammation (GAM, MMP). Moreover, this noninvasive technique enables characterization of the glioma microenvironment with respect to the disease-driving cellular compartments at the various disease stages. Cancer Res; 77(8); 1831-41. ©2017 AACR.

18F-FET PET Uptake Characteristics in Patients with Newly Diagnosed and Untreated Brain Metastasis

In patients with brain metastasis, PET using labeled amino acids has gained clinical importance, mainly regarding the differentiation of viable tumor tissue from treatment-related effects. However, there is still limited knowledge concerning the uptake characteristics in patients with newly diagnosed and untreated brain metastases. Hence, we evaluated the uptake characteristics in these patients using dynamic O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) PET. Methods: Patients with newly diagnosed brain metastases without prior local therapy and ¹⁸F-FET PET scanning were retrospectively identified in 2 centers. Static and dynamic PET parameters (maximal/mean tumor-to-brain-ratio [TBR_max/TBR_mean], biologic tumor volume [BTV], and time-activity curves with minimal time to peak [TTP_min]) were evaluated and correlated with MRI parameters (maximal lesion diameter, volume of contrast enhancement) and originating primary tumor. Results: Forty-five brain metastases in 30 patients were included. Forty of 45 metastases (89%) had a TBR_max ≥ 1.6 and were classified as ¹⁸F-FET-positive (median TBR_max, 2.53 [range, 1.64-9.47]; TBR_mean, 1.86 [range, 1.63-5.48]; and BTV, 3.59 mL [range, 0.04-23.98 mL], respectively). In 39 of 45 brain metastases eligible for dynamic analysis, a wide range of TTP_min was observed (median, 22.5 min; range, 4.5-47.5 min). All ¹⁸F-FET-negative metastases had a diameter of ≤ 1.0 cm, whereas metastases with a > 1.0 cm diameter all showed pathologic ¹⁸F-FET uptake, which did not correlate with lesion size. The highest variability of uptake intensity was observed within the group of melanoma metastases. Conclusion: Untreated metastases predominantly show increased ¹⁸F-FET uptake, and only a third of metastases < 1.0 cm were ¹⁸F-FET-negative, most likely because of scanner resolution and partial-volume effects. In metastases > 1.0 cm, ¹⁸F-FET uptake intensity was highly variable and independent of tumor size (even intraindividually). ¹⁸F-FET PET might provide additional information beyond the tumor extent by reflecting molecular features of a metastasis and might be a useful tool for future clinical applications, for example, response assessment.

Influence of blood-brain barrier permeability on O-(2-18F-fluoroethyl)-L-tyrosine uptake in rat gliomas

PURPOSE

O-(2-¹⁸F-fluoroethyl)-L-tyrosine (¹⁸F-FET) is an established tracer for the diagnosis of brain tumors with PET. This study investigates the influence of blood-brain barrier (BBB) permeability on ¹⁸F-FET uptake in two rat glioma models and one human xenograft model.

METHODS

F98 glioma, 9L gliosarcoma or human U87 glioblastoma cells were implanted into the striatum of 56 Fischer or RNU rats. Thereafter, animals were divided into a control group and a group receiving injections of the glucocorticoid dexamethasone (Dex). After 12-13 days of tumor growth animals received injection of Evans blue dye (EBD) to visualize BBB disturbance and underwent ¹⁸F-FET PET followed by autoradiography. Time activity curves, standardized uptake values (SUV) and Tumor-to-brain ratios (TBR) of ¹⁸F-FET uptake [18-61 min post injection (p.i.)] were evaluated using a volume-of-Interest (VOI) analysis. BBB disturbance was quantitatively evaluated by EBD fluorescence. The membrane gaps of blood vessel endothelial tight junctions were measured using electron microscopy to visualize ultrastructural BBB alterations in one untreated and one Dex treated F98 glioma. Data were analyzed by two-way ANOVAs.

RESULTS

In Dex treated animals EBD extravasation was significantly reduced in 9L (P < 0.001) and U87 (P = 0.008) models and showed a trend in F98 models (P = 0.053). In contrast, no significant differences of ¹⁸F-FET uptake were observed between Dex treated animals and control group except a decrease of the TBR in the 9L tumor model in PET (P < 0.01). Ultrastructural evaluation of tumor blood vessel endothelia revealed significant reduction of the cleft diameter between endothelial cells after Dex treatment in F98 model (P = 0.010).

CONCLUSION

Despite a considerable reduction of BBB permeability in rat gliomas after Dex treatment, no relevant changes of ¹⁸F-FET uptake were noted in this experimental study. Thus, ¹⁸F-FET uptake in gliomas appears to be widely independent of the permeability of the BBB.

Late Pseudoprogression in Glioblastoma: Diagnostic Value of Dynamic O-(2-[18F]fluoroethyl)-L-Tyrosine PET

PURPOSE

Pseudoprogression (PsP) is characterized by therapy-associated but not tumor growth-associated increases of contrast-enhancing glioblastoma lesions on MRI. Although typically occurring during the first 3 months after radiochemotherapy, PsP may occur later in the course of the disease and may then be particularly difficult to distinguish from true tumor progression. We explored PET using O-(2-[(18)F]fluoroethyl)-L-tyrosine ((18)F-FET-PET) to approach the diagnostic dilemma.

EXPERIMENTAL DESIGN

Twenty-six patients with glioblastoma that presented with increasing contrast-enhancing lesions later than 3 months after completion of radiochemotherapy underwent (18)F-FET-PET. Maximum and mean tumor/brain ratios (TBRmax and TBRmean) of (18)F-FET uptake as well as time-to-peak (TTP) and patterns of the time-activity curves were determined. The final diagnosis of true progression versus late PsP was based on follow-up MRI using RANO criteria.

RESULTS

Late PsP occurred in 7 patients with a median time from radiochemotherapy completion of 24 weeks while the remaining patients showed true tumor progression. TBRmax and TBRmean were significantly higher in patients with true progression than in patients with late PsP (TBRmax 2.4 ± 0.1 vs. 1.5 ± 0.2, P = 0.003; TBRmean 2.1 ± 0.1 vs. 1.5 ± 0.2, P = 0.012) whereas TTP was significantly shorter (mean TTP 25 ± 2 vs. 40 ± 2 min, P < 0.001). ROC analysis yielded an optimal cutoff value of 1.9 for TBRmax to differentiate between true progression and late PsP (sensitivity 84%, specificity 86%, accuracy 85%, P = 0.015).

CONCLUSIONS

O-(2-[(18)F]fluoroethyl)-L-tyrosine PET provides valuable information in assessing the elusive phenomenon of late PsP. Clin Cancer Res; 22(9); 2190-6. ©2015 AACR.