Every rat (4 now) I've owned except one has died of cancer. Or at least, they died with cancer, I'm not a veterinarian. The one that didn't got the flu or something at 6 months and died of some respiratory issue.
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.
