Went through this last April. My GSP Max had a knot come up on his hip and foot. He was 11 years old. Vet said we could biopsy but odds are it was cancer. We let it run its course.
When Max started not eating well and started losing weight, I made the call. No need to waste away. At that point, a few days or weeks of painful living was not worth it.
Vet made it easy and peaceful.
