I wonder how the rat got brain tumor in the first place... Aspartame?
Rats tend to develop tumors for some reason. I have no idea why but they do.
This reminds me of our white laboratory rats that our family used to keep as pets. For some unexplained reason one of our favorite rat Splinter suddenly got a huge cranial swelling when he got older. I was young then so I did not understand that Splinter probably had brain cancer of some sort. Splinter did not die because of the brain cancer but got smashed up because their cage fell down after a very strong wind caused the shack where their cage was attached to collapse. Several of our pet rats died but a few of the younger ones survived.