Mammary tumors are one of the most common problems in female rats over one year of age. Luckily, they are usually non-cancerous, or benign.
A mammary tumor began to grow on Belle in July of this year, approximately one year after I got her. When she became a part of the family, she was already fully grown, so I am assuming she was at least a year and a half when her tumor popped up. Luckily none of my other females have had any mammary tumors, but they just reached the one year mark in September.
There are supplements that may halt the growth of or even make a tumor recede, however, surgery is the most surefire option for a rat in good health. If your female rat goes under anesthesia for any reason, I highly suggest getting her spayed at the same time. I do not necessarily agree with the idea of an animal as small as a rat having surgery as a preventative measure, but if it is required for something else, having her spayed at the same time may prevent mammary tumors in the future.
Some supplements that may help with mammary tumors, according to the Rat Fan Club, are: CLA and DHLEA, curcumin(tumeric), shark cartilage, and tamoxifen. I have not tried these so I can't say whether they work or not. High nitrate foods, like green, leafy vegetables, have also been linked to cancer so it is best to feed those sparingly.
Some rats like to pick at their sutures after surgery. Belle did. Our veterinarian wanted to keep her at the hospital for a few days, until they were ready to be removed, but I offered a better alternative. I printed out the Rat Fan Club's page on body casts and showed it to her, and she did an amazing job. We tried to do it at home and it was next to impossible for us.