You are absolutely right; there is an abundance of discussion on standard vs ultra-sensitive PSA testing after primary treatment, not so much discussion (that I've seen) about which test to use after SRT.
The recommendation that I give to men after RP is dependent on whether they were categorized high-risk pre-RP, or whether they have any adverse pathological outcomes post-RP. A significant number of men are low- or intermediate-risk before surgery and have negative margins, no SVI, no EPE, etc.; and for these men I recommend the standard PSA test. All others, I recommend the ultrasensitive PSA test.
So, I’ll offer my opinion to your question below, plus my reason for this opinion. As a “bonus”, my reply also includes some additional discussion on things you probably already know, but might be beneficial to others less knowledgeable who might read this thread as a reference in the future.
Undetectable post-RP baseline (shortly after surgery) PSA test results which later become detectable and progressively rise to, or above, 0.2ng/mL (followed by a 2nd test which confirms this rise) suggests local recurring PC (BCR) which exists in the prostate “bed” (a.k.a. local recurrence). This is the scenario you presented with after two years of undetectable PSA (standard test, with lower detection limit, LDL, of 0.1ng/mL). Salvage Radiation Therapy (SRT) which is focused on the prostate bed is the most common 2nd-line treatment for patients who likely have a local recurrence, and in fact SRT will only benefit those patients with local recurrence.
By contrast and for completeness of discussion only (albeit not your case), a detectable PSA level (and more specifically a detectable PSA level above the standard PSA test LDL of 0.1ng/mL) at post-RP baseline that also shows further progressive increases over time likely represents micro metastatic PC elsewhere…a progression which was likely present prior to RP. A systemic salvage treatment using hormone therapy (ADT) would likely be the appropriate 2nd-line treatment in this case.
Some recent studies have examined the effectiveness & benefits of SRT. A wide range of SRT successes/failures exists, so further studies broken-down factors that were predictive to SRT outcomes. For example, the Gleason 8 (you), 9 & 10 guys had predictably worse outcomes from SRT, as did guys with faster PSA doubling times (with 2-years less than 0.1, you’re good here).
You have at least one adverse factor. From what you reported, your highest standard PSA test was 0.2ng.ml. The standard test results would only yield 0.2, 0.1 or less than 0.1 (or higher results). To me, these are reasons for you to use the ultra-sensitive PSA test which can be useful to track further possible relapse.
My opinion only…