@ozelli wrote "That said, there are no steadfast rules in regard to post radiation numbers. I think that is why they are so vague in their "2.0 points above a nadir meets the criteria for recurrence" rule. No clear numerical definition is ever given of what constitutes a nadir with radiation/no ADT patients."

Exactly. Again, layperson writing, but from the links I shared above, it appears that radiation kills only some of the healthy cells in the prostate (in addition to the cancer itself). Since healthy prostate cells also generate PSA at a lower level, your nadir would depend on exactly how and how much radiation hit your prostate and how the healthy cells reacted to it. It's not like a prostatectomy, where the prostate is gone completely so any PSA is probably coming from cancer cells outside it.

Note that the links I shared also said that while many hit their post-radiation PSA nadir after a couple of months, for some it can take as long as 2–3 years. (!!!)

Perhaps PSA after successful radiation therapy falls onto something like a bell curve. If a non-expert uses a small-enough sample (like sharing anecdotal evidence with a few dozen people in a discussion forum), they might hear *only* from people near the middle of the curve, and not realise that a wider spectrum of healthy results is possible. That's why I'm suggesting that the original poster not worry too much until they've talked with their actual oncologist.