@dhasper I had 28 sessions of proton radiation, but during our months of decision-making, my medical team discussed the post-treatment nuances of both radiation and RP with me. The way they described it to me —>

Following radiation, in time PSA should be < 0.1 (with “< “ meaning “less than,” i.e., undetectable). Over time, following radiation PSA should rise since there’s still a healthy prostate producing PSA at a new (low) normal. (I do agree that following radiation, testing to >0.00x is probably overkill; but, if they’ll offer it, I’ll take it!)

A few months (3-6 months) following RP, PSA should be well below 0.1 ng/mL since there’s no longer a prostate producing any PSA. (There may be remnants left in the bloodstream, which is why retesting at 6 months is helpful if PSA isn’t quite there at 3 months). But, you’ll never know how much “well below” 0.1 ng/mL it is if the equipment can’t test below that. In fact, following RP, if PSA begins to rise and approaches 0.1 ng/mL, that’s a heads-up that there might be a problem. But again, you’d only know that if the equipment could test PSA < 0.1 ng/mL. (At the very least, they should test to 2 decimal places.)

As always, discuss all this with your medical team.