Why don't they use radiation for brain tumors?

Radiation is everywhere so there's nothing you can do about it. But radiation is bad for cells because you get these high energy particles shooting through your body and through the cells. They damage the DNA directly and they generate these reactive species which further damage the DNA and cause mutations. Most mutations don't cause any problems, but very rarely you'll mutate a gene that's fundamental in regulating cell growth, survival, or where the cell moves and spreads, and then you're on the way to potentially forming a cancer. So in the right places, radiation is good because it can be used in the right places and the right doses to destroy cells. Radiotherapy is used for a variety of cancers, including some brain cancers. Part of the problem seems to be that a lot of the cancer cells, by the time they're presented as a clinical disease, pretty much don't care how much damage they have anymore. In order to become the tumour cell they have, they've had to throw off all the responsive mechanisms that would normally curtail the cell from growing, and you end up with replicating glass beads that you can't do very much about.