I think the previous posters misunderstand the question --- or maybe I do. The .1 or .2 thresholds they mention in their replies are typically associated with the point after PRIMARY treatment (in your case, surgery) you initiate salvage treatment (your 37 sessions of IMRT).
I believe your question relates to at what point in PSA rise will doctors recommend hormone therapy after SALVAGE treatment "fails". At that point, the recurrence is no longer considered localized, and is systemic disease --- problem is, you don't know where the cancer cells are that are generating the PSA. The next step in the normal course of action is hormone therapy.
I discussed this with my doctor, and he said he'll let the psa "run up" some before initiating ht at this point (he never gave me a definite number) for a few different reasons. One, he's seen instances where the PSA might run up a little, but then stabilize. Two, at this point you really need to get an accurate reading on PSA velocity to gage how aggressive the recurrence might be. And finally, you have time. "PSA only" recurrence will often preceed actual clinical evidence of advanced PCa by YEARS in many cases. PSA doubling time will give your team some ideas in treatment options.
My doc also said not to panic at that point --- he's put patients on intermittent HT, and has seen cases of durable response to ht --- suggesting a curative result from a treatment not considered curative.
This is merely my understanding of the situation --- other members might have a better handle on this. I'd also recommend posting your question on the American Cancer Society Prostate Cancer bulletin board ---- all questions there are answered by experts and staff members. You can also call their 24 hour hotline and ask -- 1·800·ACS·2345.
Hope this helps -- good luck!
Age 47 - pre-surgery PSA 39
Open Radical Prostatectomy 6/9/2006
Pathological Stage T3a, Positive Surgical Margin
PSA rose to .24 in November of 2006
6 month hormone therapy initiated December 1. 2006
36 sessions of IMRT Ended Feb 1, 2007
PSA as of May 25, 2007 undetectable
PSA as of November 29, 2007 undetectable
PSA as of May 14, 2008 undetectable
Next PSA November of 2008