This is a toughie...it would be great to know once and for all....

It's hopeful because he has done so well, and whatever is growing has gotten so much bigger and is not bothering Mr Meow.

If they would have to biopsy by going in - and if they could remove the whole lump - I personally would like the surgery to be as gentle as possible so that Mr Meow can have lots more time with you, even if it IS malignant. I wouldn't want you to have to decide to PTS while he is still under...that would be so brutal, especially since he is doing so well right now...

What is the least invasive way they could find out for sure? I mean - I don't expect you to know that, but maybe the vets could answer that question...