Brain tumors

[124I]CLR1404 PET/CT in High-Grade Primary and Metastatic Brain Tumors

PURPOSE

There is a continuous search for imaging techniques with high sensitivity and specificity for brain tumors. Positron emission tomography (PET) imaging has shown promise, though many PET agents either have a low tumor specificity or impractical physical half-lives. [¹²⁴I]CLR1404 is a small molecule alkylphosphocholine analogue that is thought to bind to plasma membrane lipid rafts and has shown high tumor-to-background ratios (TBR) in a previous pilot study in brain tumor patients. This study attempts to define the clinical value of [¹²⁴I]CLR1404 PET/CT (aka CLR124).

PROCEDURES

Adult patients with new or suspected recurrence of high-grade primary or metastatic brain tumors (N = 27) were injected with [¹²⁴I]CLR1404 followed by PET/CT at 6, 24, and 48 h. Standard uptake values (SUV) and TBR values were calculated for all time points. Uptake of [¹²⁴I]CLR1404 was qualitatively assessed, compared with magnetic resonance imaging (MRI), and correlated with clinical outcome. Final diagnosis (N = 25) was established based on surgically resected tissue or long-term follow-up.

RESULTS

Positive uptake with high TBR was detected in all but one patient with a final diagnosis of primary/recurrent brain tumor (12/13) and in less than half of patients with treatment-related changes (5/12). Concordance between [¹²⁴I]CLR1404 uptake and contrast enhancement on MRI was seen in < 40 %, with no concordance between T₂-hyperintensities and uptake. No significant difference in overall outcome was found between patients with and without [¹²⁴I]CLR1404 uptake.

CONCLUSIONS

The uptake pattern in these patients suggests a very high sensitivity of [¹²⁴I]CLR1404 PET/CT for diagnosing tumor tissue; however, tumor specificity needs to be further defined. Relative lack of concordance with standard MRI characteristics suggests that [¹²⁴I]CLR1404 PET/CT provides additional information about brain tumors compared to MRI alone, potentially improving clinical decision-making.

Improving long-term survival in diffuse intrinsic pontine glioma

INTRODUCTION

Diffuse intrinsic pontine glioma (DIPG) is an almost universally fatal pediatric brain cancer. There has been no improvement in event-free survival (EFS) or overall survival (OS) despite immense effort through a multitude of clinical trials to find a cure. Recently, there has been a surge in the knowledge of DIPG biology, including the discovery of a recurrent H3F3A mutation in over 80% of these tumors.

AREAS COVERED

The authors review the most recent approaches to diagnosis and treatment of DIPG including chemotherapy, biologics, surgical approaches, and immunotherapy.

EXPERT OPINION

The authors propose four main opportunities to improve long-term survival. First, patients should be enrolled in scientifically sound clinical trials that include molecularly profiling either via stereotactic biopsy or liquid biopsy. Second, clinical trials should include more innovative endpoints other than traditional EFS and OS such as MRI/PET imaging findings combined with surrogates of activity (e.g. serial liquid biopsies) to better ascertain biologically active treatments. Third, innovative clinical trial approaches are needed to help allow for the rapid development of combination therapies to be tested. Finally, effort should be concentrated on reversing the effects of the histone mutation, as this malfunctioning development program seems to be key to DIPG relentlessness.

One-day protocol for 18F-FDG and 13N-ammonia PET/CT with uptake decoupling score in differentiating untreated low-grade glioma from inflammation

PURPOSE

Accurate identification of low-grade gliomas (LGGs; World Health Organization grades I and II) and their differentiation from brain inflammation lesions (BILs) remains difficult; however, it is essential for treatment. This study assessed whether a one-day protocol for voxel-wise ¹⁸F-FDG and ¹³N-ammonia PET/CT with uptake decoupling analysis could differentiate LGGs from BILs.

MATERIALS AND METHODS

Twenty-eight patients with LGGs and 16 patients with BILs underwent ¹⁸F-FDG and ¹³N-ammonia PET/CT on the same day before any type of therapy. The decoupling score and tumor-to-normal tissue (T/N) ratio of ¹⁸F-FDG and ¹³N-ammonia were calculated at each location. Student's t-test was used to compare values, and ROC curve analysis was used to establish a cut-off value for the T/N ratio and decoupling score. Area under the curve (AUC) was calculated to evaluate differential efficacy.

RESULTS

Significant differences were observed in ¹³N-ammonia T/N ratio (p=0.018) and decoupling score (p=0.003) between LGGs and BILs; however, the ¹⁸F-FDG T/N ratio did not show any differences (p=0.413). Optimal cut-off values for ¹⁸F-FDG T/N ratio, ¹³N-ammonia T/N ratio, and decoupling score were 0.73, 0.97, and 2.31, respectively, with corresponding AUCs of 0.48, 0.68, and 0.77. The respective sensitivity, specificity, and accuracy parameters using these cut-off values were 53.6%, 62.5%, and 56.8%, respectively, for ¹⁸F-FDG; 50.0%, 75.0%, and 59.1%, respectively, for ¹³N-ammonia; and 60.7%, 93.8%, and 72.7%, respectively, for decoupling score.

CONCLUSIONS

¹⁸F-FDG/¹³N-ammonia uptake decoupling score can be used to discriminate between LGGs and BILs. Use of a decoupling map of these two tracers can improve visual analysis and diagnostic accuracy.

Evaluation of 11C-Methionine PET and Anatomic MRI Associations in Diffuse Intrinsic Pontine Glioma

The role of metabolic imaging in the diagnosis, treatment, and response assessment of diffuse intrinsic pontine glioma (DIPG) is poorly defined. We investigated the uptake of ¹¹C-methionine in pediatric patients with newly diagnosed DIPG and evaluated the associations of ¹¹C-methionine PET metrics with conventional MRI indices and survival outcomes. Methods: Twenty-two patients with newly diagnosed DIPG were prospectively enrolled on an institutional review board-approved investigational study of ¹¹C-methionine PET. All patients underwent baseline ¹¹C-methionine PET/CT, and initial treatment-response scans after chemotherapy or radiation therapy were obtained for 17 patients. Typical and atypical DIPGs were assessed clinically and radiographically and defined by multidisciplinary consensus. Three-dimensional regions of interest, reviewed by consensus between a nuclear medicine physician and a radiation oncologist, were delineated after coregistration of PET and MR images. Associations of ¹¹C-methionine uptake intensity and uniformity with survival, along with associations between ¹¹C-methionine uptake and conventional MRI tumor indices over time, were evaluated. ¹¹C-methionine PET voxel values within regions of interest were assessed as threshold values across proportions of the study population, and ¹¹C-methionine uptake at baseline was assessed relative to MRI-defined tumor progression. Results: ¹¹C-methionine uptake above that of uninvolved brain tissue was observed in 18 of 22 baseline scans (82%) and 15 of 17 initial response scans (88%). ¹¹C-methionine avidity within MRI-defined tumor was limited in extent, with 11 of 18 positive baseline ¹¹C-methionine PET scans (61%) showing less than 25% ¹¹C-methionine-avid tumor. The increase in total tumor volume with ¹¹C-methionine PET was relatively limited (17.2%; interquartile range, 6.53%-38.90%), as was the extent of ¹¹C-methionine uptake beyond the MRI-defined tumor (2.2%; interquartile range, 0.55%-10.88%). Although baseline ¹¹C-methionine PET intensity and uniformity metrics did not correlate with survival outcomes, initial ¹¹C-methionine avidity overlapped with recurrent tumor in 100% of cases. A clinical diagnosis of atypical DIPG was associated with borderline significantly prolonged progression-free survival (P  =  0.07), yet ¹¹C-methionine PET indices at diagnosis did not differ significantly between atypical and typical DIPGs. Conclusion: Most newly diagnosed DIPGs are successfully visualized by ¹¹C-methionine PET. Baseline ¹¹C-methionine uptake delineates regions at increased risk for recurrence, yet intensity and uniformity metrics did not correlate with treatment outcomes in children with DIPG in this study.

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.

Preclinical TSPO Ligand PET to Visualize Human Glioma Xenotransplants: A Preliminary Study

Current positron emission tomography (PET) imaging biomarkers for detection of infiltrating gliomas are limited. Translocator protein (TSPO) is a novel and promising biomarker for glioma PET imaging. To validate TSPO as a potential target for molecular imaging of glioma, TSPO expression was assayed in a tumor microarray containing 37 high-grade (III, IV) gliomas. TSPO staining was detected in all tumor specimens. Subsequently, PET imaging was performed with an aryloxyanilide-based TSPO ligand, [¹⁸F]PBR06, in primary orthotopic xenograft models of WHO grade III and IV gliomas. Selective uptake of [¹⁸F]PBR06 in engrafted tumor was measured. Furthermore, PET imaging with [¹⁸F]PBR06 demonstrated infiltrative glioma growth that was undetectable by traditional magnetic resonance imaging (MRI). Preliminary PET with [¹⁸F]PBR06 demonstrated a preferential tumor-to-normal background ratio in comparison to 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). These results suggest that TSPO PET imaging with such high-affinity radiotracers may represent a novel strategy to characterize distinct molecular features of glioma growth, as well as better define the extent of glioma infiltration for therapeutic purposes.

Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[(18)F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, a potent pyridazinoindole ligand for PET imaging of TSPO in cancer

A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from commercially available starting materials. Focused structure-activity relationship development about the pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14), a novel ligand of comparable affinity. Radiolabeling with fluorine-18 ((18)F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[(18)F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([(18)F]-14) in high radiochemical yield and specific activity. In vivo studies of [(18)F]-14 revealed this agent as a promising probe for molecular imaging of glioma.

Advances in PET imaging of brain tumors: a referring physician's perspective

PURPOSE OF REVIEW

To highlight the most recent advances in PET imaging of brain tumors, aiming at expanding the referring physician's knowledge in the field, the sine qua non for translating PET into the practice of neuro-oncology.

RECENT FINDINGS

The role of PET with amino acid tracers in the setting of brain lesions of unknown significance has been better defined, reducing the need for invasive procedures. The impact of PET-guided resection of high-grade glioma using ¹¹C-methionine (¹¹C-MET) has been strongly documented. [¹⁸F]Fluoroethyl-L-tyrosine is currently available for glioma management; advances in targeting glial tumor biopsy and monitoring response to standard chemoradiation of malignant glioma have been remarkable. 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-penta-fluoropropyl)-acetamide is a rationally designed radiotracer with potential for imaging hypoxia in glioblastoma. New insights regarding the predictive value of 3-deoxy-3-[¹⁸F]fluorothymidine in outcome of recurrent malignant glioma treated with bevacizumab/irinotecan have been provided. First steps are being made toward apoptosis PET imaging for early assessment of radiotherapy response in brain metastases.

SUMMARY

The use of ¹¹C-MET and ¹⁸F-labeled PET tracers is getting a more precise position in the management of brain tumors. Advances hold promises in routine decision-making and in the design and conduct of clinical trials.