My SIL had a cat with a tumor but his was on his face above the eye. The vet told her that it could be removed but that it would be a lot worse than just letting it run it's course (medical reasons and I'm sure some financial reasons). It got worse and worse for several years.
I'm sorry to say that she finally had to put her cat down because it eventually took over a large part of his skull. He couldn't see or hear or eat. This progressed over 5 years or so, though. She couldn't afford surgery for the cat so it wasn't an option, not sure if it would have changed the outcome. Sorry for the bad news, maybe your situation is different or maybe surgery will correct your cat's tumor. Good luck.
I'm sorry to say that she finally had to put her cat down because it eventually took over a large part of his skull. He couldn't see or hear or eat. This progressed over 5 years or so, though. She couldn't afford surgery for the cat so it wasn't an option, not sure if it would have changed the outcome. Sorry for the bad news, maybe your situation is different or maybe surgery will correct your cat's tumor. Good luck.