Abstracts

Lesional epilepsy is due to cerebral gliomas in 10-15% of cases. The therapeutic management and prognosis in such patients depend on the reliable distinction between high- and low-grade gliomas.
MRI is an excellent tool for tumor localization. When MRI is scored based on multiple criteria (heterogeneity, cyst formation or necrosis, hemorrhage, tumor crossing the midline, edema and/or mass effect, definition of border, flow void, degree of contrast enhancement, heterogeneity of contrast enhancement), the accuracy of the grading of gliomas can reach 88%.
FDG-PET is thought to be another useful tool for the evaluation of the degree of malignancy of cerebral gliomas. In one study, 86% of the patients with hypometabolic FDG-PET had low grade gliomas, and 94% of the patients with hypermetabolic FDG-PET had high-grade gliomas. Therefore, combining these two noninvasive neuroimaging techniques may be considered highly accurate for preoperative grading of gliomas. However, hypermetabolism may also be present if the scan is obtained during a seizure (ie, an ictal scan). We recently encountered two patients with tumors in our epilepsy surgery program who exhibited paradoxical findings on both MRI and FDG-PET scans. Both patients presented with medically intractable epilepsy. Both patients underwent MRI brain and FDG-PET scans, EEG-Video Monitoring, and Neuropsychological evaluations. Patient #1 presented with a 2 year history of temporal lobe epilepsy. MRI scan revealed a right medial temporal homogenous, nonenhancing lesion involving the right amygdala and uncus. FDG-PET showed hypometabolism in the right medial and lateral temporal lobe structures. Video EEG monitoring showed seizure onset in the right medial temporal region. The patient underwent a right temporal lobecotmy including amygdalohippocampectomy. Postoperative histopathology showed a glioblastoma multiforme (grade IV).
Patient #2 presented with a 7 year history of left sided sensory seizures. MRI scan revealed a 4cm intraaxial mass involving the right posterior parietal region. This tumor had minimal mass effect, heterogeneous signal on T1 and T2-weighted images, cyst formation, a large area of surrounding edema, and moderate enhancement with contrast. FDG-PET revealed increased glucose uptake in the region of the lesion, and simultaneous EEG was normal. Subsequently, the patient underwent intracranial subdural grid placement, motor/sensory mapping, and resection of the lesion as well as the surrounding epileptogenic cortex. Postoperative histopathology showed an oligodendroglioma (grade II) without anaplastic features. The combination of MRI and FDG-PET is a useful tool for the noninvasive preoperative grading of gliomas, but rare exceptions do occur. Caution should be taken when planning epilepsy surgery and when interpreting and discussing these studies with patients.