Use of PSA nadir to predict subsequent biochemical outcome following external beam radiation therapy for T1-2 adenocarcinoma of the prostate

Purpose. This study assessed the ability of nadir prostate-specific antigen (PSA) to act as an early surrogate for subsequent freedom from biochemical failure following radiation therapy for T1-2 prostatic adenocarcinoma. Methods and materials. A retrospective analysis was performed on the biochemical outcome of 314 consecutive men with T1-2 disease treated by conventional external beam radiation at the Massachusetts General Hospital. Minimum follow up was 2 years, and failure was defined as three successive rises in serum PSA of greater than 10%. Kaplan-Meier actuarial analysis of outcome was employed. Results. The overall 5-year freedom from biochemical progression was 63%. For those who achieved a PSA nadir of ≤0.5 ng/ml ( n = 123) it was 90%, for 0.6–1.0 ng/ml ( n = 103) it was 55%, and for > 1.0 ng/ml ( n = 88) it was 34%. Multivariate analysis showed an undetectable PSA nadir to be independent of Gleason grade and initial PSA in predicting subsequent outcome ( P < 0.05). The likelihood of achieving an undetectable PSA nadir correlated strongly with the pretreatment value: 74% if this was below 4 ng/ml; 42% for those between 4.1 and 10 ng/ml; and 32% for those above 10 ng/ml. Conclusion. A PSA nadir of ≤0.5 ng/ml represents an early endpoint strongly predictive of a favorable outcome following radiation therapy which may be used for the rapid assessment of new radiation strategies.