This really hits home as I had a rat PTS yesterday due to a massive internal tumor.
He was only 6 months old, and the tumor grew VERY rapidly. We did an ultrasound and it was a solid mass, not fluid (we were thinking it could be fluid from an internal injury). The vet said that likely, the only explanation was cancer, especially due to his young age.
This sounds very much like what happened with this girl. I'm so sorry :sad3:
Seems we are seeing more and more of these types of growths in our rats in Perth. I have no idea why but they all seem to be fairly quick to grow and can't be removed. A couple of rats that have had surgery for them have passed during surgery or just after.