English, yours is a good question and concerned, and like most all things PC, there is never one straight sure answer.
From looking at your posted stats, you being a confirmed Stage III and a Gleason 8, your dr's urgency for further treatment might be considered typical.
You are quite right, adj radiation or even salvage radiation like a bunch of us have gone through, is no guarranty by any means to keep the cancer in remission. It is kind of hit and miss, and the overall odds for it working are less than 50% at best. With some stats that can drop to 20-40% range of working long term. I think all of us in that boat, had to come to our own conclusion of do it or not do it, and even with the not so great percentages, many, myself included, feel its worth a shot, being the last curative approach available right now to a surgery that didn't contain all the cancer.
In your case, some dr's would call for immediate adj. radiation, as a pre-emptive strike. Other doctors, would still want to wait for 3 PSA's exceeding .10 in an upward spiral, before doing any further action.
I agree with the above poster, this is a great time for a second and even third opinion if possible. You are correct too, there is no normal scan that's going to show up anything at this point.
Good luck and please keep your journey posted.
David in SC
57, 56 dx, PSA: 7/07 5.8, 7/08 12.3, 9/08 14.5, 10/08 16.33rd Biopsy
: 9/08 - 7/7 Positive, 40-90% Cancer, Gleason 4+3Open RP:
11/08, Rht nerves saved, 4 days in hospt, on catheters for 63 days, 5th one out 1/09Path Rpt
: Gleason 3+4, pT2c, 42g, 20% cancer, 1 pos margin
Incontinence: 1 Month ED: Non issue at any point post surgery, no problem post SRT
Post Surgery PSA: 2/09 .05,5/09 .1, 6/09 .11. 8/09 .16
Post SRT PSA: 1/10 .12, 4/8 .04, next one: July
Latest: 7/9 met 2 rad. oncl, 7/9 cath #6 - blockage, 8/9 2nd corr surgery, 8/9 cath #7 out 38 days, 9/9 - met 3rd rad. oncl., mapped 9/9, 10/1 - 3rd corr. surgery - SP cath/hard dialation, 10/5 - 11/27 IMRT SRT 39 sess/72 gys ,cath #8 33 days, Cath #9 35 days, 12/7 - Cath #10 43 days, 1/19 - Corr Surgery #4, Caths #11 and #12 same time, 2/8-Cath #11 out - 21 days, 3/2- Cath #12 out - 41 days, 3/2- Corr Surgery #5, 3/6 Cath #13 out - 4 days, Cath #14- 27 days, Cath #15 - 26 days, Cath #16 - 4/23 put in