Dear Miss Lilac,
First, let me express my condolences on your loss. It is never easy to lose a beloved pet.
Pancreatic cancer in dogs does metastasize (spread), but the brain would be an uncommon site to do so. More common would be liver, abdominal lymph nodes and other internal abdominal surfaces.
I can only surmise, but in light of the central blindness I think he had a second diagnosis and that the masses found on the pancreas were incidental to the blindness, even if they were truly cancerous (i.e. malignant). Alternatively the pancreatic masses could have possibly been pancreatic abscesses.
Brain tumors do not typically metastasize outside the skull, therefore while I have no doubt a central nervous system disease process existed and may well have been a form of central nervous system cancer, I still think he had two separate problems concurrently. This is not as uncommon as we once thought. Sometimes multiple forms of cancer may exist in the same animal, unfortunately. Another theoretical linkage between brain and pancreas could be infection. Perhaps a brain abscess occurred secondary to a pancreatic abscess. I might have expected more neurologic dysfunction than just blindness in such a case however.
Only pathologic analysis of the pancreatic masses and the brain could establish any linkage. Today MRI is sometimes used to evaluate the brain structure and any lesions in the skull. Unfortunately it is not available everywhere, requires general anesthesia and can be quite costly.
With all of our advancements and our current "high level" of medical understanding we unfortunately cannot fully comprehend or arrest every medical problem. We must unfortunately be satisfied with incremental improvements at times. I do wish your dog could have been helped and could have lived a longer life.