My girl was only a little over a year old and still did not survive her surgery. Of course by then she had lost a LOT of blood, so I knew her chances were slim. It's always a tough choice to make between letting her be and having a risky surgery, but if the vet saw a tumor, it's going to be there until they remove it.
I'm not telling you what to do, but I will let you know that the longer you wait, the weaker she could get, making her chances even more slim.
My Relm seemed to improve greatly the week before she passed away, so sometimes that is not a good way to tell if she is truly "fine" or not.
I'm so sorry you're facing this decision. =/