Hi! There's a vet in Riverdale that has removed lumps from at least a dozen of our rat friends if you want to check them out. I put the name of the place in the Vet Rolodex yesterday. they used to charge, I think, $65.
I don't know if I've ever had a buck with a mammarial cyst, but it sounds like it could be that.
Good luck. You're not alone in this.