This subject is coming over and over into surface, so I decided to start a specific thread as I got bored in writing the same things over and over again:
Post RP patients with a rising PSA are discouraged to take a PSMA PET/CT imaging before deciding on sRT, on the grounds that it will detect "nothing" at their low PSA value, usually in the range of 0.1 to 0.2.
First important comment is that the pre-test PSA value is not the only and _certainly_ not the best metric for this. PSADT is more important, but the clumsy way by which PSADT is usually derived leads us to better leave it out of this discussion. A brief explanation to this is that PSADT is the mathematical/numerical first derivative of PSA. Elementary knowledge of math and their application to measurements, indicates that the accuracy of the first derivative of any variable is an order of magnitude inferior compared to that of the variable itself. Taking 20% as a ballpark for the PSA measurement accuracy, the thus derived PSADT mounts up to 200%. There are ways to do better for the former and consequently the latter, but this is not our current subject.
Now lets have a look at Table 4 of the paper entitled:
"Use of gallium-68 prostate-specific membrane antigen positron-emission tomography for detecting lymph node metastases in primary and recurrent prostate cancer and location of recurrence after radical prostatectomy: an overview of the current literature" (2019)
Three studies for the PSA range of 0.01 to 0.20 (cumulatively 121 cases) are recorded. Out of them 59 cases (48.8%) led to a positive detection. The second comment here is that the radio tracer used in all three studies was 68Ga while today there are its 18F counterparts that are considered of being more sensitive in the lower PSA scale. Also, since 2 years ago there are digital PET scanners that are equipped with more sensitive detectors. Consequently, this 48.8%, considered as "nothing" by the naysayers, is a conservative figure by today's standards.
But the most important part of this discussion comes from the break-down of the location of the detected lesions:
Recurrence fossa only: 17 cases -> 14%
Pelvic lymph nodes +/- fossa: 22 cases -> 18%
Distant metastases +/- fossa: 20 cases -> 16.5%
Now, if we make the assumption that a similar break down characterizes all the lesions that due to their low concentration (not size < 5 mm - another grossly silly statement) that evaded detection, it is clear to see why sRT has a ~65% roof in success score. The remaining ~35% failed because the cancer was distant or distant as well to the irradiated field.
Lastly, there is the ultimately silly argument: Why take the test if it is not going to change your treatment plan. Because, there is a myriad of studies concluding that after a positive PSMA detection 1/3 to 1/2 of all irradiation plans were modified slightly or heavily to the limit of excluding this 16.5% (distant metastases) as bringing nothing but late toxicities.
The only difficult thing about it is to find a competent RO among the sea of incompetence that wants to rule.