Sadly this species is really prone to having tumors.
This sounds more like an internal tumor growing outwards rather than a skin based tumor. Typically surgery for this is not successful.
What I try to do when making a euthanasia decision is to look at the pet's quality of life to see if they can still do the things they love to do or if they are uncomfortable and unable to enjoy life or if not much of their life is enjoyable.
I would say if she is not running on her wheel there is a level of discomfort. Its not so high that she is willing to miss out on a treat though. Its hard to figure just how uncomfortable they are as they can't really tell us. If she is not eating well, is losing weight or condition over her body it may be kindest to let her go.
I'm sorry you have to make this decision. Its always a hard one to make.
Hope this helps you!
Pet Health Care, Rescue,Train,Breed
30+yrs pet vet care & nursing, rescue, behavior & training, responsible breeding, small animal care