I had an old dog who got lymphoma. By the time he developed the swollen lymph nodes, the cancer was all over. The vet and I had a serious talk -- he was 15 years old, and the cancer was very advanced. Buster the Awesomesauce Dog went on prednisone, pain killers, and a cheeseburger and ice cream diet. I took time off of work and took him on one last big road trip. A few short weeks later, his breathing changed and he didn't want his toy, so I let him go.
If Buster had been a young dog, I would have tried the chemo. As it was, he had lived a good long life.