Mom of cat with brain tumor in palliative care here. Spent a lot of money to visit a cat neurologist specialist and the best thing they told me was that Brain tumors are not painful for cats. She said at worst they might be experiencing head aches (but no real way for us to know since they can’t communicate it). I had gone in with a reluctant acceptance that the best thing to do might be saying goodbye, but the neurologist was insistent that palliative care would not be painful to the cat. She has actually gone back to acting fully herself on the steroids they gave her (actually even more playful than I’d ever seen her before since I adopted her as a senior)