Correlation of biological aggressiveness assessed by 11C-methionine PET and hypoxic burden assessed by 18F-fluoromisonidazole PET in newly diagnosed glioblastoma

Glioblastoma multiforme (GBM) is characterized by tissue hypoxia associated with resistance to radiotherapy and chemotherapy. To clarify the biological link between hypoxia and tumour-induced neovascularization and tumour aggressiveness, we analysed detailed volumetric and spatial information of viable hypoxic tissue assessed by F-18-fluoromisonidazole (FMISO) PET relative to neovascularization in Gd-enhanced MRI and tumour aggressiveness by L-methyl-C-11-methionine (MET) PET in newly diagnosed GBMs. Ten patients with newly diagnosed GBMs were investigated with FMISO PET, MET PET and Gd-enhanced MRI before surgery. Tumour volumes were calculated by performing a three-dimensional threshold-based volume of interest (VOI) analysis for metabolically active volume on MET PET (MET uptake indices of a parts per thousand yen1.3 and a parts per thousand yen1.5) and Gd-enhanced volume on MRI. FMISO PET was scaled to the blood FMISO activity to create tumour to blood (T/B) images. The hypoxic volume (HV) was defined as the region with T/B greater than 1.2. PET and MR images of each patient were coregistered to analyse the spatial location of viable hypoxic tissue relative to neovascularization and active tumour extension. Metabolically active tumour volumes defined using MET uptake indices of a parts per thousand yen1.3 and a parts per thousand yen1.5 and the volumes of Gd enhancement showed a strong correlation (r = 0.86, p < 0.01 for an index of a parts per thousand yen1.3 and r = 0.77, p < 0.05 for an index of a parts per thousand yen1.5). The HVs were also excellently correlated with the volumes of Gd enhancement (r = 0.94, p < 0.01). The metabolically active tumour volumes as defined by a MET uptake index of a parts per thousand yen1.3 and the HVs exhibited a strong correlation (r = 0.87, p < 0.01). On superimposed images, the metabolically active area on MET PET defined by a MET uptake index of a parts per thousand yen1.3 was usually larger than the area of the Gd enhancement and about 20-30% of the MET area extended outside the area of the enhancement. On the other hand, the surface area of viable hypoxic tissue with a T/B cutoff of a parts per thousand yen1.2 on FMISO PET did not substantially differ from the area of the Gd enhancement. The volumetric analysis demonstrates that the viable hypoxic tissue assessed by FMISO PET is related to the neovascularization in Gd-enhanced MRI and the tumour aggressiveness by MET PET in newly diagnosed GBMs. The spatial analysis shows that the metabolically active tumour may be substantially underestimated by Gd-enhanced MRI. Complementary use of MET and FMISO to Gd-enhanced MRI may improve the understanding of tumour biology and lead to the most efficient delineation of tumour volume and treatment strategy.