Lost my mom to brain cancer when I was 16. We never will know what caused it, if anything. She lived 2 years from her initial diagnosis. Long enough to be relatively at peace with what was happening, and finish up what she wanted to do in life. Two surgeries, several clinical trials, and a lot of chemo kept her around that long. At the end, they couldn't do a third surgery, because the cancer was so infused in her brain. That was a horrible two years of watching helplessly and waiting for the inevitable.
But, like I said, at least we knew it was coming and could prepare for it.