Purpose: We determined whether the prostate specific antigen (PSA) nadir achieved following external beam radiation therapy alone predicts biochemical disease-free survival in a large cohort of men with clinically localized prostate cancer.
Materials and methods: Between January 1986 and October 1993, 364 men with clinically localized, stages T1 to T3 adenocarcinoma of the prostate received definitive external beam radiation therapy with no prior, concomitant or adjuvant endocrine therapy. PSA was measured before treatment in 326 men (90%) and serial PSA was measured following treatment in all patients. All men were followed continuously for at least 24 months (median 44 months, range 24 to 90, mean 46). Biochemical failure after irradiation was defined as PSA of 1.5 ng./ml. or more and 2 consecutive serum PSA elevations.
Results: The 5-year overall biochemical disease-free survival rate for the entire group was 56%. PSA nadir was predictive of subsequent biochemical disease-free survival. The biochemical disease-free survival rate at 3 years was 93, 49 and 16% for PSA nadirs of 0 to 0.99, 1 to 1.99 and 2 or more ng./ml., respectively (p = 0.0001). In a multivariate analysis PSA nadir (0 to 0.99 versus 1.0 to 1.99 versus 2 or more ng./ml.) was an independent predictor of biochemical disease-free survival along with pretreatment PSA, central axis dose, Gleason grade and T stage.
Conclusions: PSA nadir after radiation therapy is an indicator of subsequent biochemical disease-free survival. Patients who achieve a nadir of less than 1 ng./ml. following external beam radiation therapy have a favorable biochemical disease-free survival rate, while those with a nadir of greater than 1 ng./ml. have a high subsequent failure rate. Strategies to improve results should focus on techniques to increase the likelihood of achieving a PSA nadir of less than 1 ng./ml.