system
Given the following post, respond as if you're commenting on Reddit:
human
How does radiation or proton therapy get rid of cancer cells?

I've received radiation, along side chemotherapy, for a tumor I had in my head. It was one of the main reasons why the tumor went away and never came back in that area. Unfortunately, it spread to a different part of my brain and now we are looking at proton therapy, as it's more precise, for treatment. I'm curious as to how radiation and proton therapy work and how they help kill cancer cells. Thanks in advance.
gpt
In most radiation therapy, including protons, most of the cell killing is through a multi-step process like this:

1. If the treatment is with x-rays or gamma rays, they hit some electrons in the patient, which shoot off at high speed.

2.  Those electrons, or the protons or incident electrons if receiving one of those therapies, fly though the tissues very fast and in their wake they gently pull off many more electrons from their molecules.  This is mostly from water molecules because they are the most abundant.

3. The ionized water molecules form a variety of highly reactive molecules and elements such as hydrogen peroxide and free radicals.

4. Some of these encounter DNA strands and cause damage.

5. Often the cell can repair this damage, but if too much happens in one spot the DNA can be cut in two.  This is very hard to repair.

6. With unrepaired breaks, the cell will often die in one of a few ways.  For example it may undergo programmed cell death called "Apoptosis", or the next time it divides it could have a major failure and die.

7. If every reproducing cancer cell in the targeted area is killed then the tumor *in that area* is cured.  This is called "local control", and is the objective of much radiation therapy.