my rat...

[elfy]

one of my rats has cancer. the bigger tumour is unter her right hind leg.
is as big as a small egg, a very small. maybe like a golfball.
she still eats and drinks, she can walk and it doesn't hurt her.
she became very skinny and even if the can walk and does it aktively, she is giddy.

what should i do?
just let her live on (what i would prefer, as long as it doesn't hurt)
or 'let' her die?

help ;_;