And likely most-importantly, make sure you have a good team of doctors!
My husband's old neuro-oncologist gave him too much Depakote and he declined quickly. It was clearly the med but the doc said "if he's declining this quickly he should go to hospice."
An ER doc looked at my husband, said "he's overmedicated," cut the dose in half and my husband improved fairly quickly. He starts seeing a new neuro-onc this week. (this one seems to actually give a damn...)
Your neuro-oncologist is so very, very important to not only survival but quality of life. Good luck to you...