While this is not a pleasant subject, we feel that it is one that needs to be addressed. Cancer is the leading cause of death in dogs. In fact, as many as 25% of unspayed females will develop mammary tumors, and intact dogs, male or female, are prone to many reproductive cancers (ovarian, testicular, uterine, etc.) as they age. This is why we recommend the spaying or neutering of all dogs not intended for breeding purposes. Some breeds seem to be more susceptible to a certain forms of cancer. If you are considering purchasing a puppy please discuss this issue with the breeders that you contact.
From many discussions with people that have been through it, and our own limited experience, we know that when it happens it can be heartbreaking. The decisions are tough and very personal. The treatment of cancer can be very expensive and while sometimes it does work and the dog will live many more happy productive years, sometimes it does not and the dog may spend it's last few months in pain and sick, either from the disease or the treatment. Only you can decide what to do for your situation. Some will go to all out extremes and others simply cannot afford the treatment or are unwilling to subject the dog to it. If the prognosis is poor they simply may not want to see the dog suffer when the result will inevitably be the same and will choose to make the dog comfortable until it is time to have the dog humanely euthanized. No approach to this disease or treatment is wrong, but only you can make the decision as to what lengths you are willing to go based on the whole picture. In the end you as the dog's owner and the people that love them the most must be comfortable with your decision.