If your dog has cancer, you can guarantee that it is due to some sort of chemical exposure. Dogs that are in the wild do not get cancer. In fact, almost no animals do. The likely culprit is commercial dog food. In the very least, dog food should have meat as its main ingredient. Even that is rare nowadays, because meat is expensive when compared with corn and rice.
It is also possible that the dog consumed a chemical, was given a pharmaceutical that did this, came into contact with a chemical on somebodies lawn, was exposed to radiation, or was bathed in something particularly toxic (such as an anti-flea formula). Your dog being fixed could also be a contributing factor, due to the role of hormones in breast cancer. In fact, breast cancer was once called, "nun's disease", because women who breast feed do not get breast cancer. Of course, there is no reversing that decision.
As far as curing the cancer, that is harder to do in a dog because they are more acidic due to their high protein natural diets. Use your best judgment as to whether the tumor needs to be surgically removed, but be aware that cutting into cancer can cause it to spread. I have never heard of anyone using B17 on an animal, but our preferred anti-cancer regimen is the Budwig Protocol, which can be found here:
http://healthwyze.org/index.php/cancer-revisited-the-industry-suppressed-budwig-regimen-or-how-to-cure-cancer-with-cottage-cheese.html
Most of the dietary recommendations would not be appropriate for a dog, but the mixing of cottage cheese or yogurt with flax seed oil may help, and would be practical. Adding a small amount of hydrogen peroxide to your dog's water may also be beneficial. Animals produce their own vitamin C, unlike us, but dramatically boosting it with supplementation could be a good idea.