A lot goes into answering this question. I have one friend who has traveled eight hours to a vet who specialized in the cancer her dog has. She paid for surgery and treatment. The dog is doing great (and I mean GREAT) a couple years after surgery, even though the diagnosis wasn't for such a great, long life. If this dog hadn't gone to the specialist, I'm sure she would have died two years ago.
I have another friend who traveled days several times to go to a vet who specialized in her dog's cancer. The dog also had surgery and treatment and lived much longer than expected.
However, not everyone has the resources to do this for their dogs. So the variables in what you do for a dog diagnosed with cancer has to include resources available to the owner. Spending thousands to give a dog extra years of great life may not be viable for some, and most parts of the country don't have the canine cancer specialists available to the general public.
This makes the decision on what to do for a dog with cancer so much more difficult. Owners without the resources feel guilty when their pet can't be helped but other pets with owners with time and money can. It's a dilemma for sure.
I had a cat develop fast spreading cancer that took him in about a week after diagnosis. My goal for that week was to make sure he wasn't suffering, and when he did, to be prepared to let him go. It was difficult, but he went just as the suffering became difficult. Of course, I miss him to this day.