Hi RH. The thing that I understand about
RT is that it typically doesn't so much kill the cells as it does alter the DNA of the cells to prevent their reproducing. Cancer cells are whiny butt babies, so they are more prone to being altered by the RT. Since they aren't actually dead, they will still be active and produce PSA until they try to reproduce. At that point, when the cancer cell tries to divide, it will then die. I believe what the Onc will be looking for in terms of your PSA activity is a relative stabization, then decrease over a few years.
The other hope is that the healthy cells will survive to some extent and reproduce. In that case, you may always have "some" PSA present, but on a much lower scale.
The RP guys who have SRT is a totally different kettle of fish. The Onc has a much more stringent expectation in terms of fast PSA reductions.