@robertov What external radiation (both photon and proton) does is to damage DNA at the cellular level. (See attached graphic.)

External radiation doesn’t simply “radiate the prostate,” it damages cellular DNA which prevents prostate cancer cells from multiplying. Cancerous prostate cancer cells do not have healthy DNA repair mechanisms, and over time they die off (which we observe as PSA decreasing).

Non-cancerous prostate cells are damaged as well, but they have healthy DNA repair mechanisms and can (usually) repair the radiation-induced DNA damage. They survive external radiation treatments and continue producing PSA (though at low “new normal” levels).

This also is why it’s helpful to maintain hormone therapy for some time after radiation treatments have ended. Radiation damage to prostate cancer cells’ DNA can continue long after the radiotherapy itself has been completed. So by keeping the androgen receptors inhibited or suppressed by hormone therapy, you can suppress that DNA repair mechanism for months. This is why ADT is a very important component to kill prostate cancer cells.