@rlpostrp Yes, a PSMA PET scan (no matter whether Gallium68, Pylarify, or Posluma) will have difficulty detecting prostate cancer when PSA is very low (see attached chart), which is why they sometimes wait until the PSA is 0.4+ (following prostatectomy) to be certain before doing anything aggressive. (But, waiting that long is tricky…..)

As it turns out, PSMA (prostate specific membrane antigen) is not really “prostate specific.” There are other organs, tissues, and fluids that naturally express PSMA (without being cancerous) and will show up as physiologic tracer uptake (i.e., “light up like a Christmas Tree”) on a PSMA PET scan - particularly in the lacrimal (tear) and parotid (salivary) glands, blood, liver, spleen, pancreas, ganglia, and more, as well as the kidneys, ureters, bladder, and urethra (as the body tries to quickly excrete the radioisotope that was injected).

At low (but concerning) PSA levels, with a PSMA PET scan they may be able to see if the body is simply expressing naturally-occurring background (“normal”) levels, or whether there is a recurrence somewhere. It takes a trained, experienced eye to see this, so choose the best.

PSMA PET scans typically do out-perform the older, pre-PSMA PET scans (F18-FDG, F18-NaF, Choline C-11, and Axumin). Yes, PSMA is that good of a marker.

However (as you allude) with lack of PSMA expression, PSMA PET is sometimes as blind as a bat. In those instances, it’s often beneficial to fall back and use one of those older pre-PSMA PET scans.

Mayo Clinic regularly falls back to Choline C-11 in those cases. Others use F18-FDG if they believe that the recurrence is advanced but PSMA expression too low to be detected or if the prostate cancer is PSMA-negative (which 15% of the time it is). (Insurance no longer covers FDG-NaF.)

The pre-PSMA PET scan that’s more often used in these instances is the F18-Fluciclovine PET/CT (trade name: Axumin® that was FDA approved in 2016).

Axumin doesn’t rely on PSMA. Axumin works by exploiting the fact that prostate cancers absorb amino acids at a much more rapid pace than normal cells. Axumin is made up of a radioactive tracer linked to an amino acid. Cancer cells absorb the amino acids more avidly than normal cells, so when Axumin is used, the radioactive tracer concentrates inside the tumor cells. When the patient is imaged, the areas that have a high concentration of the imaging agent signal the location of the cancer in the patient’s body.

Insurance still covers Axumin. In these instances it’s always worth discussing this with one’s medical team and see if they’ll consider it.