system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when you remove a brain tumour? What grows in its place, if anything?

I started thinking about tumours and realised I have no idea what happens to the body when you remove them, especially somewhere like the brain.
gpt
Well two things are possible...

One, you removed all the tumor cells (harder than you think). Nothing grows there. The brain of an adult doesn't grow under normal conditions. 

Two, you didn't get all the cancer cells. A tumor will eventually recur there.