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 an tigen (PSA) to act as an early surrogate for subsequent freedom from b iochemical failure following radiation therapy for T1-2 prostatic aden ocarcinoma. Methods and materials. A retrospective analysis was perfor med on the biochemicai outcome of 314 consecutive men with T1-2 diseas e treated by conventional external beam radiation at the Massachusetts General Hospital. Minimum follow up was 2 years, and failure was defi ned as three successive rises in serum PSA of greater than 10%. Kaplan -Meier actuarial analysis of outcome was employed. Results. The overal l 5-year freedom from biochemical progression was 63%. For those who a chieved a PSA nadir of less than or equal to 0.5 ng/ml (n = 123) it wa s 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 nad ir to be independent of Gleason grade and initial PSA in predicting su bsequent outcome (P < 0.05). The likelihood of achieving an undetectab le PSA nadir correlated strongly with the pretreatment value: 74% if t his 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 less than or equ al to 0.5 ng/ml represents an early endpoint strongly predictive of a favorable outcome following radiation therapy which may be used for th e rapid assessment of new radiation strategies.