INTRODUCTION: The translocator protein (TSPO) is a 18 kDa mitochondrial molecule associated with neuroinflammation and over-expressed in several brain diseases. 11C-(R)PK11195 is a specific ligand of TSPO for PET studies. We investigated the in vivo TSPO expression in high- and low-grade gliomas using 11C-(R)PK11195 PET, and applied it in guiding tumour biopsies. METHOD: 24 patients underwent volumetric MRI and dynamic 11C-(R)PK11195 PET scans. Reference tissue input function was obtained from the grey matter of cerebellum to determine the 11C-(R)PK11195 uptake in the tumours and the mirrored regions of interest (ROIs) in the contralateral hemispheres. Co-registered MR/PET images were used to guide tumour biopsies prior to surgical debulking, with high and/or low 11C-(R)PK11195 uptake foci defined as biopsy targets. Biopsy specimens were assessed for TSPO expression by immunohistochemical staining. RESULTS: In low-grade gliomas (14 cases), 11C-(R)PK11195 uptake within the tumours was lower than the contralateral ROIs (p = 0.001); sporadic high uptake foci were found in 8 tumours. In high-grade gliomas, 7 out of the 10 tumours showed higher uptake than the contralateral ROIs (p = 0.016), 4 of which displayed intensive 11C-(R)PK11195 signal despite only minor contrast enhancement on the MRI. There was low 11C-(R)PK11195 uptake and no contrast enhancement in 3 anaplastic asctrocytomas. Immunostains on tissue sections confirmed variable TSPO expression in neoplastic cells and activated microglia/macrophages. CONCLUSION: TSPO expression is decreased in low-grade gliomas and increased in most high-grade gliomas. TSPO expression within gliomas is inhomogeneous as detected by 11C-(R)PK11195 PET and confirmed by immunohistochemistry.
