DUAL PET IMAGING OF NEOPLASTIC AND INflAMMATORY BIOMARKERS IN HIGH GRADE GLIOMA

Abstract AIMS To characterize spatial distributions of 18kDa translocator protein (TSPO) and amino acid uptake as measured through dynamic [11C] (R)-PK11195 and [11C]-methionine PET in high-grade glioma. METHOD Twelve patients with newly diagnosed high-grade glioma underwent dual PET studies. [11C] (R)-PK11195 binding potential (BPND) maps were generated using simplified reference tissue model with grey-matter cerebellar time- activity-curve as tissue input-function. [11C]-methionine uptake was calculated as tumour-to-background ratio (TBR). Volumes of interest (VOIs) were defined on T1W post-contrast MRI as contrast-enhancing (CE) tumour and a ‘peritumoural’ region 5mm from the CE edge. Voxel-wise comparison of the tracers used Pearson’s correlation coeffcient. VOIs were furthermore subdivided into higher/lower TSPO (BPND > 0.17) and higher/lower methionine (TBR > 1.7). RESULTS Within the CE, voxel-wise comparison showed a strong correlation (r > 0.7) in eight subjects, moderate correlation (r > 0.5) in three subjects and weak correlation (r < 0.5) in one subject. Volume (mm3) with high BPND/high TBR accounted for 62±29% whereas high BPND/low TBR accounted for 13±16%. Within the peritumoral area, there was a small reduction in the proportion of subjects in the strong correlation group (6/12) but the pro- portion of voxels with higher TSPO binding and lower methionine uptake was substantially greater (35±16%). CONCLUSIONS Within high-grade glioma, there is a strong overall correlation between TSPO expression and amino acid uptake. However, in the peritumoural region there are areas of elevated TSPO expression without increased methionine uptake which could represent an inflammatory component or a discrete neoplastic cell population that may not be adequately defined with standard imaging biomarkers.