Salvage radiotherapy after radical prostatectomy: outcomes and prognostic factors especially focusing on pathological findings

External beam radiotherapy is a potential salvage or adjuvant therapy after radical prostatectomy (RP). The purpose of this study was to investigate the treatment outcome of salvage radiotherapy (RT) following RP for clinically localized prostate cancer and to identify factors that may predict the outcome of salvage RT. Between 2000 and 2006, 41 patients received salvage RT because of increasing prostate-specific antigen (PSA) levels following an RP for clinically localized prostate cancer. All the patients received conformal radiotherapy to the prostate bed. The prescribed radiation dose was 60–70 Gy in 26–35 fractions. The overall 5-year biochemical disease-free survival rate was 38%. A multivariate analysis showed that the following pathological findings of the surgical specimen were significantly associated with biochemical failure following salvage RT: a high Gleason score, a negative surgical margin, seminal vesicle invasion, lymphatic vessel invasion and negative vascular invasion. Among these factors, lymphatic vessel invasion was the strongest predictor. In conclusion, the pathological features affected the outcome of salvage RT following RP. Lymphatic vessel invasion was strongly associated with the risk of biochemical failure despite salvage RT. Meanwhile, vascular invasion was not a significant hazardous factor.