Breast cancer has a nasty little habit of coming back without chemo and or radiation treatment. I had a dog that just had the surgery and was deemed all clear but 18mths later it was back with a vengeance and then it was too involved to go the surgery again plus chemo. I imagine it can be the same in people. If it comes back it is nastier and you definitely don't want that. Breast cancer is fed via the blood stream and during biopsies or surgery it is possible little microscopic bits of the cancer can break off and enter the blood stream and travel to other parts of the body. Under the right conditions it can then redevelop into a tumor. Doctors tend to hit an initial diagnosis with everything they have like surgery, chemo and radiation so hopefully it never comes back.
Maybe you will be one of those who doesn't have too much trouble with treatment side effects at all?