Here is what I've found for you about PSA readings. The Hopkins site says "After treatment with radiation, PSA levels rarely drops completely to zero. However, the PSA should level out at a low number, which is called the nadir. When PSA rises 2 points from its lowest value, the cancer is considered recurrent." If one has had surgery it says "After surgery, PSA levels should drop to zero. When PSA levels rise above 0.2 ng/mL, the cancer is considered recurrent." Scroll down the page to the section labeled Long-Term Prognosis and read from there on down. (The source for above info is https://www.hopkinsmedicine.org/health/conditions-and-diseases/prostate-cancer/prostate-cancer-prognosis)
The next site says "The pattern of the drop in PSA after radiation therapy is also different from after surgery. PSA levels after radiation tend to drop slowly, and might not reach their lowest level until 2 years or more after treatment." Next, read the section labeled After Radiation Therapy (source is https://www.cancer.org/cancer/types/prostate-cancer/treating/psa-levels-after-treatment.html)