Metastatic disease is when a cancer spreads to multiple locations. This can make it difficult/impossible/impractical to treat effectively, especially surgically.

Whole brain radiotherapy works by killing everything a little bit in the hopes that the tumors die first (e.g. like chemo). There are good reasonswhy this tends to mostly-sort-of be true, but getting the balance right is hard and too much dose will definitely cause other problems.

SRS is a way of targeting radiation directly to locations to kill cells, with less effective dose (hence damage) to other parts of the brain.

It's all pretty harsh stuff, and you can die from the necrotic tissue caused by it, also.

Often with this kind of disease you know you aren't going to cure someone, but you can get rid of symptoms and make people more comfortable (palliative care).