I just lost my last little rescue ratty Julie to mammary tumors. She was four years old, and we felt that we didn't want to put her through surgery. Most rat experienced vets will suggest having her spayed if you take her in for them to remove it. They typically can be removed, and the little ratties can live a full long life. They can recur though, and you must be vigelant about what they eat. The more fresh veggies the better, up to a point. You don't want to give them the runs, which commonly happens with small critters and fresh veggies. Mine loved oranges and tangerines, and I would give her 1/8 of an orange every other day. I give my degus oranges to supplement their vitamin c, so I always have them on hand. The extra vitamin c helps with their immune systems, and helps them from catching something else that could make the tumor worse.
Love her as much as you can, and do what your heart tells you to do. I would not put her down unless she begins loosing fur, looses her appetite, looses a substantial amount of weight, or it begins affecting her quality of life.
<<hugs!>>
Emily in NC