Chemo and radiation work by different principles. Most systemic therapies, if they do work, become less effective over time, and that is generally a function of the surviving cancer cells being resistant and still being able to divide.
Imagine that you have a very effective treatment that kills 95% of the cancer cells, so the cancer shrinks dramatically. The 5% of remaining cancer cells are alive because cancer cells are generally chaotic, make bad copies of themselves, and create new mutations as they divide, some of which will confer resistance to a given treatment. You can imagine this rapid division making sloppy copies and new mutations accelerates evolution within a person's body. So only the cancer cells that are unusually "fit" survive, then divide, passing on resistance to their "daughter cells", which continue to grow and divide. Eventually, whether you kill 20%, 50%, 90%, or 99.9% of cancer cells, if you start out with a few billion cancer cells, even having 1 in 1000 survive is enough to eventually come roaring back as a visible, growing cancer that is now resistant to a treatment that was previously impressively effective.
Radiation doesn't always work, but we expect that it often will. Resistance is generally related to parts of a tumor having different levels of blood supply and oxygenation, and the hypoxic (low oxygen) parts of a tumor not being as vulnerable to radiation effects. the body doesn't tend to develop immunity to radiation the way it does to systemic therapies. Parts of the body that have never received radiation are unaffected when a different part of the body has received radiation and will generally respond as if no radiation has ever been given.
-Dr. West
