Ed, my PSA acted the same as you and John T. It dropped immediately due to the HT. One month after IMRT I asked my onco what the PSA would be and he said <0.1. I was ready to bet the farm that he was wrong as everything I had read about
RT showed a slower decline. Sure enough the next week it was <0.1.
I expect mine to stay undetectable as long as I am on HT. Then we will see what happens. I do expect some PSA as my rad onc says that some prostate remains and may produce a small amout of PSA. I don't think that the usual RT PSA bump is as likely to occur due to the HT. It appears that John T is at the time that the bump should occur and you are approaching it as well. It usually happens between 12-36 months after RT stops. My rad onc says don't be surprised if it occurs but he thinks the HT will negate the normal bump. One of the leaing BT practices is here in Atlanta and they look for a nadir of 0.2 and for it to stay there except for the bump period when BT and IMRT are used without HT.
Celebrate undectable, hopefully for a very long time. Your treatment combination has proven to be the most effective at long term control and cure. Trust that it will work and verify that it is working.