Unfortunately, this rat was euthanized due to complications five days after a second surgery. Other tumors I've seen include one, probably cancer, of the ovaries, one involving the urinary tract, and a malignancy that arose in a rat after I had earlier done two surgeries in the same location to remove benign mammary tumors. One very fast growing tumor in the thigh of one of my female lab rats was diagnosed as a fibrosarcoma. This type of tumor is most common on the face and head and should be suspected in any case of swelling in this area on a rat. There are only three ways to try to prevent tumors in your rats.
Many studies link obesity and a high fat diet with tumors.
Second, you can try to locate a breeder who has been selectively eliminating the tendency for tumors from their stock. This type of breeding can be difficult, because tumors usually occur in rats over a year old, after they can already have great-great grandchildren. Currently, the only tumor resistant rats I know of are in California. The third way, and probably the most reliable at this time, is to choose male rats over female rats. While males can also get tumors, the benign mammary tumors that are so common in females are relatively rare in males.