ABSTRACT: 18F-THK5351 was initially developed to target tau aggregates in neurofibrillary tangles. However, it was also shown to bind to an enzyme, monoamine oxidase B, which is highly expressed in the outer mitochondrial membrane of astrocytes. In a case with glioblastoma, both l-[methyl-11C]-methionine PET and 18F-THK5351 PET showed increased uptake in the tumor. On the other hand, in another case with primary central nervous system lymphoma, l-[methyl-11C]-methionine PET showed increased uptake in the tumor, but 18F-THK5351 PET showed no abnormal uptake in the tumor. 18F-THK5351 PET might be helpful in differentiating between glioblastoma and primary central nervous system lymphoma.