http://www.oncologystat.com/journals/journal_scans/Achieving_an_Undetectable_PSA_After_Radiotherapy_for_Biochemical_Progression_After_Radical_Prostatectomy_Is_an_Independent_Predictor_of_Biochemical_Outcome_Results_of_a_Retrospective_Study.html
If you achieve an undetectable PSA after salvage radiation, this study shows your odds of being progression free 3.5 years later are 75%. But if your PSA doesn't fall that low, your odds are only 18%.
