system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do brain surgeons remove tumors/foreign objects from the brain without damaging the tissue directly surrounding it?

Especially in the case where a tumor is not at the top/surface regions of the brain. How can they possibly go passed brain tissue without damaging it? As I understand you cant simply regrow brain cells so wouldnt the patient lose significant functions of the brain or memories or abilities or something?
gpt
So there are a number of things to talk about here, but the very simple answer is that usually they damage it a little bit, but it's very minor. The brain can't regrow but it can adapt, and it can regain function that's lost due to small injuries.

In terms of access -- the surgeons will cut a line through the brain tissue and then sort of spread it apart. It's the same if you're having abdominal surgery. The surgeon there won't cut a big patch of skin off the top, they'll make an incision and then pull the skin apart. It's the same with brain tissue, so only a very small amount gets damaged. The surgeons can then insert their instruments (and often a camera) through the gap to cut out the tumour.

Unfortunately, some tumours are in very sensitive areas (like the brainstem), where even a small amount of tissue damage is likely to severely impact vital things like breathing or heart control.

In terms of avoiding damage to surrounding structures once you're there, there's a lot to consider. Firstly, some tumours (such as meningiomas) are big and chunky, like a single solid ball in the brain. They stop very suddenly, and there's a clear border between the edge of the tumour and brain tissue. The surgeons can very carefully cut along this border and take out all of the tumour with a very tiny loss of surrounding brain tissue.

Some tumours are much more nasty though, and spread out in a very 'diffuse' way. It's sort of like being at the beach and about to eat a sandwich, and the wind blows sand into it. The sand grains are everywhere -- no matter what you do, that sandwich is now crunchy. In these cases, the surgeon can 'debulk' the tumour, cutting out the biggest and nastiest bits, and then the remainder is treated with chemotherapy and radiation to try and kill the very small pockets.

Another interesting tool is called a CUSA (cavitronic ultrasound aspirator). It basically vibrates at a certain sound frequency that will destroy tumour cells (which are quite weak and poorly 'made'), but leaves normal brain cells mostly intact. This is useful for rapidly clearing out large tumours.

One final thing that can be done, and this is very cool, is what's called an 'awake craniotomy'. You may have seen youtube videos of this being done. Essentially, the patient is put to sleep, but then woken up during the surgery. The brain doesn't feel pain (just the 'wrapping' does, which is what hurts in a headache usually), so the patient can sit there. The surgeon uses a little zapper to deliver tiny jolts of electricity that 'turn off' bits of the brain temporarily. The patient is then asked to do something (play a tune, draw a picture, count backwards from 100 by 7s, etc), and if they can still do it okay, the surgeon removes the piece that they zapped. If the zap affects them too much, the surgeon won't cut that piece out at all.

Obviously brain surgery's more complex than the few simple points here, but the general idea is that removal of tumours is done by surgeons who have thousands and thousands of hours of practice, and the skill to take their time and go very carefully. A lot of it is done with the surgeon looking down a microscope as they work, because it's that finely detailed.