I offer my condolences... a very good neurologist and I had a discussion about glioblastomas a few months back up at work...
The way he explained it, with the glial cells being the scaffolding (gross oversimplification, bear with me)... a tumor involving the glial cells (glioma, glioblastoma, etc) is like a "fog" invading the brain. This characteristic makes it difficult to "get it all" surgically.
I do not claim to be an expert in any way on this matter, but have never heard much good news about this sort of cancer..