Glioblastoma mimicking anti‐ <scp>NMDA</scp> receptor encephalitis: a case series

Anti-N-methyl-D-aspartate receptor (anti-NMDAR) GluN1-IgG antibodies are the hallmark of anti-NMDAR encephalitis, a common type of autoimmune encephalitis. We describe two men with seizures and asymmetric, mass-like temporal lesions initially treated as anti-NMDAR encephalitis: Case 1 had serum and cerebrospinal fluid (CSF) NMDAR-IgG antibodies and Case 2 had serum-only (post-treatment CSF negative) antibodies. Both showed poor immunotherapy response, fluorodeoxyglucose-avid infiltrative disease and biopsy-proven glioblastoma, isocitrate dehydrogenase wildtype. Antineuronal antibody positivity can be misleading when the clinical and imaging picture are discordant. The current cases highlight the need to apply strict criteria, use short-interval imaging and perform biopsy early in antibody-positive mimics of autoimmune encephalitis.