How did you diagnose the cancer?

There's nothing subtle about cancer in rats, especially the females. They're terribly prone to breast cancer and female rat has a lot of breasts. You'll see golf ball sized things in no time unless you're willing to pay for removals. Very common and any vet will spot it.

When they get a 'lump' it grows real quick. They make lovely, interactive pets, and very intelligent but they will break your heart and die too soon.