The diagnostics of recurrent gliomas using FDG-PET: still questionable?

For a number of years, FDG-PET was considered as a gold standard for the differential diagnosis of recurrent glioma and radionecrosis. Recently published papers have introduced a wealth of scepticism into this area. The aim of this work is to specify the added value of FDG PET, as compared to MRI, in terms of diagnostics of recurrent gliomas in the clinical setting of the PET Centre Prague.

MRI and FDG-PET were used to examine 29 patients for suspicious glioma recurrence, after 30 open neurosurgical operations or re-operations combined with chemo- and/or radiotherapy. The sensitivity, specificity and accuracy of both examinations were calculated with respect to their micromorphological findings (n = 28) or the clinical and radiological follow-up (n = 2).

MRI detected 23/24 tumour recurrences (sensitivity = 95.8%) and FDG PET only 15 of these (sensitivity = 62.5%). MRI specified only 3/6 radionecrotic lesions (specificity = 50.0%), while FDG PET identified 5/6 (specificity = 83.3%). Overall accuracy was 26/30 (86.7%) for MRI and 20/30 (66.7%) for FDG PET. In the subgroup of MRI positive or equivocal findings (n = 29) FDG PET was clearly positive in 15 cases. High-grade glioma recurrence was subsequently confirmed in all of them. On the other hand negative or equivocal FDG PET was associated in 5/14 cases (35.7%) with radionecrosis, in 3/14 (21.4%) with low-grade glioma and in 6/14 (42.9%) with high-grade glioma.

MRI is the method of choice for the detection of glioma recurrence but it is associated with a high rate of false positive results. FDG PET has significantly lower sensitivity; nevertheless it does help to specify MRI positive lesions. FDG PET positive lesions give a very high probability of high-grade glioma, but its equivocal and negative findings are of no clinical value.