My son had a pet hairless rat, Molly, when he was little. She was the coolest gal ever. She developed pneumonia and was taken to the vet as it came on so sudden I thought she was dying and wanted to have her PTS while he was in school. She was bleeding from the nose and everything. When I arrived at the vets, full of tears, they checked her out and said it was pneumonia. They could put her down or I could try antibiotics and an injection of steroids. I figured Molly deserved a chance and so did my son to say goodbye. She got better. A few days later she developed a tiny little pea size lump in about the same area as your Tia. I knew what it was and her age was unknown, as we got her as an adult and she was at least 1.5 years, so we were guessing 2-2.5. I did not take her back to the vet as the vet told me when she had pneumonia that their life expectancy is only about 3 years. I did not want to put her through surgery so we watched and waited for any sign of discomfort. The lump grew fairly rapidly. It was about the size of a quarter, but not perfectly round, and I noticed that it was starting to interfere with walking but she still did not appear to be in pain. I had to make the decision to put her down just previous to the holidays(for anyone who has children will understand, I could not bare to have my little boy wake one morning near Christmas and her be gone). It was a very difficult decision to make but that is what we are here for, to love them while we have them but to make the right decision to let them go when we know it is for the best. IMO I would not put her through the surgery. Molly had been a completely happy healthy girl up until the pneumonia and I often wonder if that was cancer to, maybe in her lungs(?). We bought her about 6 months of time after the pneumonia and then she went to the RB. If I have to do it again I would do the same thing(of course considering the age of the rat first). I wish you much luck and hope that Tia just had a benign tumor.