The differential diagnosis for abnormal mesial temporal signal without enhancement on MRI is broad and includes infarction, HSV encephalitis, status epilepticus, low grade astrocytoma, gliomatosis cerebri and limbic encephalitis. 

The marked FDG avidity on PET scan and right hilar mass however strongly favor the diagnosis of paraneoplastic limbic encephalitis. Small cell lung cancer (as was diagnosed in this case on transbronchial biopsy) is the classic cause of limbic encephalitis however many other tumors can also cause the phenomenon.