A PSMA PET/CT scan can detect prostate cancer at very low levels. Indeed, when PSMA first became available, and not much was known about it, my doctor ordered the scan 'just to see what it shows' and picked up my recurrence at a PSA of 0.029.
The thing is that the lower the PSA, the more likely it is that prostate cancer will be missed.
Declan Murphy at Peter MacCallum Cancer Centre says that with regards to PSMA PET/CT for recurrent disease, his group’s systematic review and meta-analysis showed the accuracy of PSMA PET/CT in the biochemical recurrent setting stratified by PSA level:
0-0.19 ng/mL: 33%
0.2-0.49 ng/mL: 42%
0.5-0.99 ng/mL: 59%
1.0-1.99 ng/mL: 75%
>2.0 ng/mL: 95%
My interpretation: Looking to see if your cancer has come back:
If your PSA is 0.19 or lower, the Gallium-68 PET/CT scan will miss prostate cancer 67% (100-33) of the time.
If your PSA is 2 or more, the Gallium-68 PET/CT scan will miss prostate cancer 5% of the time. (No scan is perfect.)
Doctors will choose the level they are happy with before ordering a scan, depending on other factors like PSA doubling time, your age and other health issues. Extra scans will be avoided because each has a cost to your body.