TSPO PET for glioma imaging using the novel ligand 18 F-GE-180: first results in patients with glioblastoma

Abstract

Objective

The 18-kDa mitochondrial translocator protein (TSPO) was reported to be upregulated in gliomas. ¹⁸F-GE-180 is a novel 3rd generation TSPO receptor ligand with improved target-to-background contrast compared to previous tracers. In this pilot study, we compared PET imaging with ¹⁸F-GE-180 and MRI of patients with untreated and recurrent pretreated glioblastoma.

Methods

Eleven patients with histologically confirmed IDH wildtype gliomas (10 glioblastomas, 1 anaplastic astrocytoma) underwent ¹⁸F-GE-180 PET at initial diagnosis or recurrence. The PET parameters mean background uptake (SUV_BG), maximal tumour-to-background ratio (TBR_max) and PET volume using different thresholds (SUV_BG × 1.6, 1.8 and 2.0) were evaluated in the 60-80 min p.i. summation images. The different PET volumes were compared to the contrast-enhancing tumour volume on MRI.

Results

All gliomas were positive on ¹⁸F-GE-180 PET and were depicted with extraordinarily high tumour-to-background contrast (median SUV_BG 0.47 (0.37-0.93), TBR_max 6.61 (3.88-9.07)). ¹⁸F-GE-180 uptake could be found even in areas without contrast enhancement on MRI, leading to significantly larger PET volumes than MRI-based volumes (median 90.5, 74.5, and 63.8 mL vs. 31.0 mL; p = 0.003, 0.004, 0.013). In percentage difference, the PET volumes were on average 179%, 135%, and 90% larger than the respective MRI volumes. The median spatial volumetric correlation (Sørensen-Dice coefficient) of PET volumes and MRI volumes prior to radiotherapy was 0.48, 0.54, and 0.58.

Conclusion

¹⁸F-GE-180 PET provides a remarkably high tumour-to-background contrast in untreated and pretreated glioblastoma and shows tracer uptake even beyond contrast enhancement on MRI. To what extent ¹⁸F-GE-180 uptake reflects the tumour extent of human gliomas and inflammatory cells remains to be evaluated in future prospective studies with guided stereotactic biopsies and correlation of histopathological results.

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