Hmm..

The generally accepted definition of BCR is a PSA of .2 followed by a subsequent increase and a PSA of .3 or higher.

There is less consensus with USPSA about what constitutes BCR and the value of determining PSADT and PSAV.

Still, the clinical data you have, your pathology report and labs may indicate BCR.

Again, we generally know the earlier one begins salvage radiation treatment when there is BCR, the greater your "chances" are of either an elusive "cure" or a durable remission.

With that GS, you may have advanced PCa with micro metastatic disease outside the prostate bed.

As you say, the challenges are many:

The lower your PSA, the less likely the scan is to locate site(s) of your PCa.

Should the scan locate your recurrence, there may be micro metastatic disease that does not show up, too small.

If the scan shows nothing, how long do you have to wait for the next one?
Your medical team is suggesting mono therapy based on population based medicine that frankly, is outdated. As you can see from the attached clinical history chart, I had SRT to the prostate bed in March 2016. I brought up extending the treatment field to the pelvic lymph nodes and adding six months of ADT based on emerging data from clinical studies. They rejected my thoughts, miserable failure...

So, think about this. You may have advanced PCa with spread outside the prostate bed. If so, systemic therapy may be your best treatment choice. If that is the case, waiting for a few more labs where your PSA is >.5 may provide a target for your radiologist to use in building his or her treatment plan that is part of doublet or triplet systemic therapy. If you believe that systemic therapy is your best treatment decision then waiting a bit longer for actionable imaging data may not mean a change in the treatment decision but add more precision to it.

Kevin