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When protons (or other particles) or photons are used in radiation therapy to treat cancer patients, the main effect is for it to make DNA breaks that hopefully will make the cancer cell die eventually (At least that is the very easy explanation. There are a lot of biology I do not know).

Now, as I understand, photons interact with matter, and excites electrons from molecules, which then ionize for instance water molecules in order to create free radicals, which can then interact with the cancer cells (and other cells) and create DNA breaks (single or double). At least, that is the idea.

My question is: How does this work for particles ? I know that photons are indirect ionization, and protons for instance doesn't interact that greatly with matter until the Bragg Peak, where it then deposits most of it's energy. But how is this energy transferred in order to make the DNA breaks ? The Coulomb interaction ? If so, wouldn't that just create electrons again, making it indirect as well, or have I misunderstood something ?

Thanks in advance.

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    $\begingroup$ I know this is off-topic for Physics, but Google for "therapeutic ratio," and for "fractionated radiation therapy." Radiation is effective treatment because a cell often can repair its DNA, but not if the damage occurs while the cell is undergoing mitosis. Malignent cancer cells spend much more time in mitosis than healthy cells. Fractionated therapy delivers repeated sub-lethal (for healthy cells) doses of radiation, with time in between for the healthy cells to recover. $\endgroup$ – Solomon Slow Mar 31 '16 at 16:21
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Yes, it is essentially just the Coloumb interaction.

Sometimes, this will strip electrons from atoms. However, those electrons might have been responsible for a bond in a molecule, so this will have effectively destroyed the molecule.

Other mechanisms include having a nucleus recoil from a proton (perhaps imparting enough energy to break a bond), or causing a nuclear reaction to take place (that will change the molecule entirely).

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