system
A chat on 4chan
human
Radiation (ionizing radiation, the sort you are talking about) is not good for you. It destroys cells. How it works, when it works, as a cancer treatment is when you can expose cancer cells to more radiation than the rest of your body. You want to kill the cancer without quite killing the rest of the patient. So they do tricky things, like implant a radioactive 'bb' next to a tumor, or use beams of radiation that always pass through the tumor, but from different angles so over time the tumor gets a lot of exposure and any healthy tissue gets less.

In general, fighting off cancer is really hard because it is almost exactly like the rest of your body's cells. Anything that kills IT is also likely to kill YOU, unless you can target the cancer pretty exactly.
(Chemo targets the cancer in a different way -- for example, by using drugs that destroy cells as they are dividing. Most cells in your body divide rarely once you are an adult, but cancer cells divide constantly. So chemo drugs that target dividing cells preferentially kill cancer cells, plus those few other sorts of cells that divide a lot, such as those producing your fingernails and hair (which is why hair and nails tend to fall off during chemo.)
gpt
The best answer here.
human
Thanks, but I'm going to tip my hat to  for a good site-specific analogy.