Leg Bone Cancer My Doberman Has Bone Cancer On Hind Leg?

My doberman has bone cancer on hind leg? - leg bone cancer

was approximately 2 weeks, my dog is lagging behind on the left leg.
So we thought, she pulled a muscle causes increased so quickly.
So we have decided today to take him to the vet, she said, has bone cancer (tumor) in the knee.
He gave us the opportunity to give my dog for a biopsy.
He said the cancer has spread and is a great dog for an amputation.
or shelved. the only thing my dog insists on his leg looked happy and excited, but you do not know what to do: (

My dog is now 8 years

What is the best option