Non-invasive grading of oligodendrogliomas: correlations between in vivo metabolic pattern and histopathology

Several studies have shown that the prognosis of oligodendrogliomas is depe ndent on their histological grade. In order to identify a non-invasive meth od for the primary diagnosis and follow-up of these tumours, we investigate d the relationship between their in vivo metabolism, assessed by positron e mission tomography (PET), and their histological grade assessed at the same time. Forty-seven patients with histologically confirmed oligodendroglioma s were investigated. Conventional neuroradiological assessment by computed tomography and magnetic resonance imaging (MRI) was performed in all the pa tients. All the histology slices were reviewed by the same pathologist afte r referral from various pathology laboratories. The PET investigation inclu ded a carbon-11 methionine (C-11-MET) uptake study and, in the majority of cases, a fluorine-18 fluorodeoxyglucose (F-18-FDG) uptake study, in order t o investigate at the same time both amino acid metabolism and glycolysis. T he sampled tumour region of interest (ROI) was defined from the T1-weighted 3D MR scan matched with the PET scan. Tracer concentration in each voxel o f the tumour ROI was divided by the mean concentration in an ROI of the sam e size located in the healthy brain tissue. For each tumour and each tracer , we characterized the metabolic pattern on the basis of the mean and the m aximum tumour to healthy tissue concentration ratio, and also the standard deviation and range of the ratios, which indicate the degree of metabolic h eterogeneity of the tumour. The histological criteria for differentiating b etween high- and low-grade tumours were those of the WHO and, partially, of the Sainte-Anne-Daumas-Duport classification. Highly significant differenc es between high- and low-grade oligodendrogliomas (Mann-Whitney test: P<0.0 001) were observed for all the assessed parameters of C-11-MET uptake. On t he other hand, the pattern of F-18-FDG uptake showed only moderate differen ces between the two tumour groups.