Since I sadly have a little experience with what John McCain's family is going through I guess I'll say a little bit. First, cancer is bad, brain cancer is worse and a GBM is worse than that. Survival is usually measured in terms of months not years. The typical course of therapy is anti-seizure medication, since the first serious signs are usually a seizure or stroke-like symptoms; then surgery to determine what kind of cancer or whether it even is cancer (since you can't tell until you cut out the mass and send it to a pathology lab, which takes a few days); a week or so off to heal and maybe more if you require physical therapy; then six weeks of radiation (strapped to a table for 10 to 15 minutes a day wearing a freaky mask) from Monday through Friday along with a low dose of chemo; a month off (the radiation actually keeps working after it's finished) and then six months of a much higher dose of chemo. Unfortunately GBMs usually come back fairly quickly and when (not if) they do come back they usually don't respond to treatment anymore and are much more aggressive. Therapy doesn't always follow surgery, especially for older people, because it is very rough physically.
I saw someone on twitter say the tumor is why McCain did such and such in 2008 or whenever. This is stupid and bull. Because of its very aggressive nature, GBMs usually form over a matter of months. Any changes in his personality or thinking would have only occurred recently, and that's if the tumor was located in a part of the brain where it would have affected those things.
Some news report said something about how the family was weighing their options on whether to have further treatment. It makes it sound precautionary since they already removed the tumor and it appears that they found Sen. McCain's tumor early before he started to show serious symptoms. Sadly, that's not how GBMs work. A surgeon will remove as much of the tumor as they can but, as they say, the object is not to do more harm. You can't cut into the brain the way you can cut into a liver so there are always going to be left over cancer cells. The normal course of treatment is almost always surgery/radiation/chemo. The only reason you would hesitate is if the patient is too weak to undergo the treatment or if the cancer is too extensive for treatment to reasonably work.
and p.s. fuck cancer