Alzheimer's Ruled Out. Tests Showed High GAD65 Antibodies, Lighted Area In Brain, Slow Brain Waves. Diagnosed To Be Dementia. Right?

My husband began having a fast developing dementia in October, and it has gotten very bad. I would characterize his symptoms as later stage of Alzheimer s Disease. He is nearly always incohert, his cognitive abilities are practically gone, and he thinks he isn t in his own home. He knows everyone--friends and family. He has no pain, no seizures. His takes very small steps, like an old man. He s had CT scans, MRI, 2 spinal taps, Pet scan , EEG . His neurologist has rule out Alzheimer s. The MRI had a lighted area in his brain, but no cancers cells in LP. EEG showed slow brain waves, but nothing definitive. PET scan did not suggest Alzheimer s. Last spinal tap showed a high level of GAD65 antibodies (.2 titer). Five years ago, he was diagnosed with Stage IV NSCLC . He s had chemo, the last type being Tarceva, which he no longer takes. His diagnosis presently is dementia. What are the doctors missing, if anything?