My mother in law died of that. The survivability rate is very low.
Friend of mine is battling it now. Glioblastoma Multiforme Grade IV.... Basically, he's been told he'll be lucky to see Christmas. Seen a couple people taken out by that MFer... It's not a pretty end. Sucks worse is he's got twin daughters (mid-teenagers) who lost their Mom to cancer as well. And a 2-3 year old little boy with his new wife (who just legally adopted his twin girls).
McCain may deserve a lot of things. But cancer, especially brain cancer, is not one of them. Praying peace and comfort to him and his family.