Biochemical Recurrence (BCR) after prostatectomy is not uncommon and often defined as PSA greater than .2 (after recheck).
Treatment "sweet spot" is .2 - .4/.5
Absent imaging identification of cancer elsewhere, belief is that cancer cells remain in and around prostate bed and/or pelvic lymph nodes.

See SPPORT trial.

I had immediate postop BCR at .19 (retested .18)
Salvage treatment (tx) at Johns Hopkins: Whole pelvic and pelvic lymph node radiation and 4 mos ADT completed 6 mos ago. First post salvage tx PSA undetectable at < .02. Praying for undetectable to continue of course.
Clearly, your Radiation Oncologist/Medical Oncologist will guide you.
Best wishes.