I would not advise waiting, if one is going to do salvage.
"One of consistent variables associated with PSA relapse-free outcome has been PSA levels prior to salvage RT. Multiple studies examined various PSA cut-off levels and showed that the lower the PSA level at the time of salvage RT, the better the treatment outcome."
Richard Choo (Mayo Clinic)
Salvage Radiotherapy for Patients with PSA Relapse Following Radical Prostatectomy: Issues and Challenges
Cancer Res Treat. 2010 March; 42(1): 1–11
"The PSA level before SRT was a highly significant predictor of disease progression, with more favorable outcomes observed at low PSA levels. An estimated 48% of patients who received SRT alone at PSA levels of 0.50 ng/mL or less were free of progression at 6 years, compared with 26% for those treated at higher PSA levels. The ability to provide successful salvage treatment for approximately 50% of patients with "early" SRT is similar to the 52% to 57% relative risk reduction in the rate of PSA progression among high-risk patients randomly assigned to adjuvant radiotherapy versus observation after RP in two recent randomized trials. An important observation in our study is that the 6-year responses for those treated at PSA levels less than 0.50 ng/mL appeared to be durable. This evidence suggests that approximately 50% of patients with recurrent prostate cancer after RP may derive long-term benefit from SRT when it is administered at the earliest signs of recurrence."
Predicting the Outcome of Salvage Radiation Therapy for Recurrent Prostate Cancer After Radical Prostatectomy (Stephenson, et al)
Journal of Clinical Oncology, Vol 25, No 15 (May 20), 2007: pp. 2035-2041
Full text: http://jco.ascopubs.org/content/25/15/2035.full
"An undetectable PSA level following salvage radiation therapy is more frequently achieved in men with lower pre-radiation serum PSA and those without seminal vesicle or lymph node involvement."
Salvage radiation therapy for prostate specific antigen progression following radical prostatectomy: 10-year outcome estimates.
Pazona, Catalona, Han, Hawkins, Roehl
Journal of Urology 2005 Oct;174(4 Pt 1):1282-6.
Dx Feb 2006, PSA 9 @age 43
RRP Apr 2006 - Gleason 3+4, T2c, NX MX, pos margins
PSA 5/06 <0.1, 8/06 0.2, 12/06 0.6, 1/07 0.7.
Salvage radiation (IMRT) Jan-Mar 2007
PSA 9/2007 and thereafter <0.1pcabefore50.blogspot.com