This article on Surgery in Rats
<http://www.ratfanclub.org/surgery.html> should help you know what to expect for your girl during her surgery and while she is recuperating. The following articles will also explain more about tumors in rats:

Tumor Removal and Baytril:
<https://www.rmca.org/Articles/tumor.htm>
Tumors in Rats:
<https://www.rmca.org/Articles/tumors.htm>

Lastly, a very graphic site showing a surgical removal of an ovarian tumor and some mammary tumors (not for the squeamish):
<http://www.lbah.com/Rats/rats.htm>

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.