I've been in your shoes before, and I'm so sorry you are in them now. My beloved kitty was diagnosed with a cancer at 5, but it was already widespread at that point. It was slow growing, so I chose to keep him comfy, and he lived another 7 years before it got really bad -- but even then we were limping because I couldn't bear to put him down. Whatever decision you make, it will be the right one. Just get your vet in an honest, one on one conversation about what life would look like either way.

*hugs* Thinking about you.