The thing is that the generic word "chemotherapy" is the same as saying antibiotics. Just as there are many different antibiotics, with their branded names as well as their content of the drug names, each used either alone, or in conjunction with others, for different infections, so with "chemo". Breast cancer is not just one specific "catch all" disease. There are many different types generally, and then within each type, there are differences of their aggression in growth. So both are biggish dogs, but a pitbull is more aggressive (mostly) than say, a golden labrador. Then added to that, your oncologist will also take into account your age, other medical conditions and so on in deciding which and it what order, your treatment plan will be.
