Oligodendroglial component complicates the prediction of tumour grading with metabolic imaging

Previous radiological investigations have generally shown the superiority of metabolic imaging in distinguishing high-grade from low-grade glioma, but the presence of an oligodendroglial component may affect the diagnostic accuracy. We investigated the diagnostic accuracy of PET imaging using ¹¹C-methionine (MET) and ¹⁸F-fluorodeoxyglucose (FDG) in distinguishing high-grade from low-grade glioma, in correlation with the oligodendroglial component.

The study population comprised adult patients who underwent preoperative PET imaging using both MET and FDG within 1 week and successful excision of the tumour tissue, which confirmed WHO grade II–IV glioma. We examined the tumour metabolic activity in terms of lesion-to-normal uptake ratios (L/N ratio) in both MET PET and FDG PET images. We assessed the correlation between the imaging results and the histological findings to determine the diagnostic accuracy of receiver operating characteristics (ROC) analysis in detecting high-grade tumours.

We studied 46 patients with glioma (13 low-grade and 33 high-grade), including 26 with an oligodendroglial components. The L/N ratios of the PET images showed significantly higher metabolic activities in high-grade gliomas than in low-grade gliomas for both MET (4.29 ± 1.22 and 2.36 ± 0.72, respectively; p < 0.0001) and FDG (1.72 ± 0.91 and 0.77 ± 0.26, respectively; p = 0.0007) images, although significant overlaps in L/N ratio were observed between high-grade and low-grade gliomas. Excluding the 26 patents with an oligodendroglial component improved the separation for both MET (4.62 ± 1.14 vs. 2.16 ± 0.63; p < 0.001) and FDG (1.76 ± 0.87 vs. 0.71 ± 0.14; p < 0.05) images. The ROC analyses demonstrated the clinical utility of the metabolic radiotracers in distinguishing high-grade from low-grade gliomas, showing similar AUC values for MET (0.91) and FDG (0.92). Excluding the 26 patents with an oligodendroglial component also further improved the diagnostic accuracy for both MET (AUC 0.98), and FDG (AUC 1.00) images. The metabolic radiotracers were significantly correlated with the MIB-1 labelling index (R = 0.52, p < 0.05 for MET; R = 0.52, p < 0.05, for FDG) only in gliomas without an oligodendroglial component.

For better characterization of gliomas and for risk assessment, the results of metabolic PET imaging should be revised after obtaining the pathological report, because oligodendroglial differentiation may positively influence the substrate metabolism and thus complicated the preoperative evaluation.