Danielle,
I am so sorry to hear that one of your dogs has been diagnosed with cancer....its a heartbreaking diagnosis, even when its an animal, and not a person, because they, too, are an important part of our families.
I also lost a dog to cancer a couple of years ago. Casey, our Golden Retriever, who was only 3 years old became deathly sick very suddenly, and was diagnosed with lymphosarcoma, so I know exactly what you're going through.
While I haven't had any firsthand experience with chemotherapy for dogs, I would advise against it. After researching the issue, and several lengthy discussions with my vet, I came to the conclusion that putting them through that, simply to keep them with us for a while longer when there is no chance of removing the cancer surgically, or for recovery, would just be selfish on my part.
My vet feels so strongly about it, that she won't even offer chemotherapy as an option for companion animals when the cancer is all-invasive, such as in the lymph nodes or the bloodstream. In her opinion, chemo for the types of cancer that are very invasive, and spread very rapidly, is just a way that veterinarians can make more money, and it doesn't benefit the animal at all, and just prolongs the pain.
From what she told me, in most cases giving them chemotherapy may keep them alive for a bit longer, but at what cost? Most all of them become very sick from the chemo itself, and it actually will make them feel even worse during the time that they have left. Many of them stop eating, and those that don't are usually very sick to their stomach throughout the entire course of treatment. The treatments also make the majority of them extremely weak and tired all the time. Personally, I wouldn't want to live like that myself, so I wasn't about to even consider it for one of my pets.
I think you need to consider which is most important, the quantity of life that he has left, or the quality of it. I would much rather have a dog that is living the remainder of his life normally, with as little change as possible, as opposed to one who feels miserable all the time, as I'm sure you would too....especially if there is no chance of an actual recovery from it.
Let him live his life normally, for as long as he can be kept reasonably comfortable, and let him enjoy the time with your family that he has left. When the time comes that life is just too painful for him, you'll know, and at that point, then love him enough to let him go.
I had six wonderful weeks with Casey, during which we packed in all the things that he loved to do, and I wouldn't have traded them for anything, and I don't think that he would have either. He was sick, but he knew that he was loved, and still felt well enough to enjoy the time he had left. Then one morning I awoke to his having difficulty breathing, and I knew thatit was time. Keeping him alive any longer would have simply been selfishness on my part, so I held him in my arms. while our vet put him to sleep. He's with the angels now, but there's no more pain and suffering, and he went knowing how much that he was loved.....please give your dog the same chance, and say no to the chemotherapy...it will just prolong the inevitable...let him enjoy the time he has left!