Your doctor is giving you the best advice. After a prostatectomy, your PSA should hit Undetectable. Mine stayed undetectable for 3 1/2 years before it started rising, and I had to have salvage radiation.

You do need salvage radiation, but will have to wait until you surgery has fully healed. Going on ADT is really the best choice you can make. It’s not unusual for the PET scan to show nothing this early in treatment.

Just so you see what the recommendations are by The American Society of clinical oncology (ASCO)

From Ascopubs about what PSA to do salvage radiation.

≤0.2 ng/mL:
Starting at this level maximizes disease control and long-term survival. Patients treated at PSA < 0.2 ng/mL achieve higher rates of undetectable post-SRT PSA (56-70%) and improved 5-year progression-free survival (62.7-75%).
Delaying SRT beyond PSA ≥0.25 ng/mL increases mortality risk by ~50%.

0.2–0.5 ng/mL:
Still effective, particularly for patients with low-risk features (e.g., Gleason ≤7, slow PSA doubling time). The Journal of Clinical Oncology recommends SRT before PSA exceeds 0.25 ng/mL to preserve curative potential.

0.5–1.0 ng/mL:
Salvage radiation remains beneficial but may require combining with androgen deprivation therapy (ADT) for higher-risk cases.