At a PSA of 0.2, a PSMA PET scan will rarely detect any prostates cancers, which is why they often wait until PSA is much higher.

As for salvage radiation before knowing where the recurrence is, Dr. Kwon (of Mayo Clinic) indicates that only 1/3 of men who have recurrence following prostatectomy have recurrence only in the prostate bed, and that they should not get salvage radiation there unless they’re absolutely certain of the location of recurrence. He says to first confirm where the recurrence is. (See Dr. Kwon’s presentation about recurrence: https://youtu.be/Q2joD360_pI)