I am not familiar with salvage radiation and thus need to be careful about my comments about it. I wish I knew more and will reserach it. You asked a couple of questions about proton radiation.

I had to decide on photon or proton after three diffeent medical provider consulations and a lot of reserach. My cancer was at low risk level (Decipher test) and confined to prostate per PSMA and bone scan. I chose proton radiation. It is like a lot of posters emphasize is a decision to make specfically for what is best for you and your decision only.

John Hopkins has a short explanation about proton and photon differences. I went through a really long planning phase to get proton. They have a physic department that sets up the beam trajectory and where it stops. They make a specifially designed bed for you to lie in during a simulation test where the define the mesurements for your treatments. Mine was done with CT and contrast and took a log of time.

Prior to each treatment you lay in your specific bed to put you in the precise location for treatment and for me they did a low dose xray to look at markers put in my prostate to make sure I am positioned in exact position. The gantry goes over you from different directions and they keep doing the low dose xrays to make your prostate is in exact position for treatment. If not they move you and then do another xray.

I have attached below a John Hopkins explanation about how proton radiation works and how they control beam and how it is and why it can be programmed to stop at a precise location. Those mesurement are confirmed by a physic department and then the exact measurements are put into computer. I had pencil beam, 30 rouns, and my entire prostate was treated along with margins outside the prostate. The R/O wanted to treat entire prostate to make sure did not miss any cells during testing and also margins outside the prostate.

There is a lot of information out there about proton versus photon but would take pages to post all. This one below was pretty precise on got to the topic you asked about. I think the information below is very good explanation that proton and photon traditional radiation treatments cause damage to the cancer cells making it impossible for them to repair and keep growing and thus eventually die. But the radiation also damages good cells but unlike prostate (andrio) cancer cells normal cells can repair themselves and grow back healthy ones.

This is the John Hopkins Explanation. It was part of multiple page explanation. This information is also availabe on Mayo, Cleveland Clinic and may other speciality medical providers.
Proton Therapy Versus Photon and Other Radiation Therapies
Unlike traditional photon radiation therapy that uses radiation in the form of X-rays or gamma rays, proton therapy uses proton particle radiation. Both photon and proton radiation damage the tumor’s DNA, but researchers say damage to the tumor cells caused by protons is more direct and harder to repair.

More importantly, protons only travel a certain distance into the body before they stop, and they deliver the highest dose of radiation at the end of their pathway. This burst of energy can appear on a graph as what is called the Bragg peak. Radiation oncologists plan proton therapy treatments so the maximum dose hits the tumor cells. In this way, proton therapy reduces radiation exposure and potential damage to healthy tissue, especially in sensitive areas such as the brain, eyes, spinal cord, heart, reproductive organs, major blood vessels and nerves.

In regular radiation therapy, such as photon therapy or gamma knife treatments, the beam of high-energy gamma rays or X-rays goes into the body, through the tumor and out the other side. Photons release energy along the entire path they travel, which means they radiate healthy tissues beyond the tumor. It is estimated that 30% to 40% of the photon dose passes through the tumor. This “exit dose” of radiation can damage the DNA of healthy cells. Proton therapy generates virtually no exit dose.

While the treatment effects of photon and proton radiation may be similar, the precision of proton therapy often makes it a safer choice with fewer aftereffects for certain types of tumors.