Concurrent NMDAR and GFAP autoimmune encephalitis with glioblastoma diagnosed by ¹⁸ F-FDG and ¹¹ C-methionine PET/CT

A 78-year-old male patient presented with limb weakness accompanied by intermittent seizures for 2 months. He was diagnosed with autoimmune encephalitis, as determined by cerebrospinal fluid (CSF) analysis and other examinations. The initial presentation on magnetic resonance imaging (MRI) mimicked the cerebrovascular disease. The 18 F-fluorodeoxyglucose ( 18 F-FDG) and 11 C-methionine ( 11 C-MET) positron emission tomography/computed tomography (PET/CT) images showed abnormal accumulation in the brain. A needle brain biopsy, performed because of symptomatic deterioration after treatment for encephalitis, revealed glioblastoma on immunohistochemistry. This case exemplifies the necessity of considering a tumor as a potential underlying cause, in addition to encephalitis for the oberseved hypermetabolism on 18 F-FDG PET imaging, when the treatment for encephalitis was ineffective. 11 C-MET PET could exhibit higher specificity and earlier detection of the lesion.