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

OBJECTIVE

The 18-kDa mitochondrial translocator protein (TSPO) was reported to be upregulated in gliomas. F-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 F-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 F-GE-180 PET at initial diagnosis or recurrence. The PET parameters mean background uptake (SUV), maximal tumour-to-background ratio (TBR) and PET volume using different thresholds (SUV × 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 F-GE-180 PET and were depicted with extraordinarily high tumour-to-background contrast (median SUV 0.47 (0.37-0.93), TBR 6.61 (3.88-9.07)). F-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

F-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 F-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.