Most tumors on rats are benign-- but we've had our share of non-cancerous lumpectomies.
This article on Surgery in Rats
Lastly, a very graphic site showing a surgical removal of an ovarian tumor and some mammary tumors (not for the squeamish):
I know it's pretty scary for the rat's humans the first time you take a rattie in for surgery. Please write to rat-help if you have any more questions.