I think you are right that ADT is somewhat substitutable for radiation dose. It seems to me that the latest generation of linacs are so precise that there is little or no benefit anymore to reducing the dose by a few Gy. The AUA/ASTRO recommended salvage dose is 70 Gy.
To help with your post-hoc research...
I should qualify what I wrote above. Except for one, all the studies I've seen that found no benefit to ADT treatment in men with GS≤3+4 were for primary therapy rather than salvage RT.
The one retrospective study from UMich that showed no benefit associated with added ADT with salvage RT, showed it for all low and intermediate risk patients. In Table 2
, 39% of those men who had ADT progressed in spite of treatment vs 36% of men who didn't have ADT - no significant difference. Neither was there a significant difference in the time to progression. There was only a benefit in high risk men.Concurrent androgen deprivation therapy during salvage prostate radiotherapy improves treatment outcomes in high risk patients
For primary therapy, one could argue that, if anything, a diagnosis of GS3+4 is slightly more risky than a GS3+4 found at pathology after RP. The reason is because biopsies sometimes miss higher grade tumors, and pathology is more likely to upgrade than to downgrade scores.
A retrospective study from MD Anderson found that "Favorable intermediate-risk patients had no significant benefit from the addition of ADT to RT (Freedom From Failure, 94% vs 95%, respectively; P=.85), and FFF for favorable intermediate-risk patients treated with RT alone approached that of low-risk patients treated with RT alone (98%)."Is Androgen Deprivation Therapy Necessary in All Intermediate-Risk Prostate Cancer Patients Treated in the Dose Escalation Era?
In a prospective study done at Emory, they conclude "our study suggests a benefit in patients with percent positive cores greater than 50%, GS 4+3, or multiple risk factors."High-Dose Radiotherapy With or Without Androgen Deprivation Therapy for Intermediate-Risk Prostate Cancer: Cancer Control and Toxicity Outcomes
Finally, a recent analysis from Dana-Farber found that after a median of 14 yrs of f/u there were no pc-related deaths among any favorable intermediate-risk men. They conclude, "The lack of PC deaths among men with favorable intermediate-risk PC suggests that adding AST may not reduce their risk of Prostate Cancer Specific Mortality."The likelihood of death from prostate cancer in men with favorable or unfavorable intermediate-risk disease
I think the benefit of adding ADT to salvage RT for men with GS≤3+4 is a psychological one. If it fails, he can say he did everything he could, whereas if he doesn't add it he may be beset with "what ifs." The cost of no decisional regret may be, at worst case, lifelong ED, so it's a difficult trade off.