THE FALL AND RISE OF PROSTATE-SPECIFIC ANTIGEN - KINETICS OF SERUM PROSTATE-SPECIFIC ANTIGEN LEVELS AFTER RADIATION-THERAPY FOR PROSTATE-CANCER

Background. The serum kinetics of prostate-specific antigen (PSA) afte r radiation therapy for prostate cancer are not well characterized, an d the potential prognostic significance of serum half-lives and of ser um doubling times is unclear. This study was designed to address those issues. Methods. One hundred fifty-four patients with at least four s erial PSA determinations who received external-beam radiation therapy alone were analyzed to determine PSA kinetics and to correlate kinetic parameters with outcome. Nonlinear regression techniques were used to estimate PSA half-lives and doubling times. Results. The PSA data fit ted well to exponential models consistent with the hypothesis that PSA kinetics after radiation follow first-order (exponential) kinetics. T he mean PSA half-life was 1.9 months (range, 0.5 to 9.2 months). No si gnificant correlation existed between half-life and grade, stage, acid phosphatase level, serum testosterone level, or patient age. A weak c orrelation between half-life and pretreatment PSA level was observed: patients with low PSA levels tended to have longer half-lives. Half-li fe did not correlate with disease relapse or with the likelihood of de veloping a rising PSA profile. PSA doubling time in 37 patients with r ising values ranged from 1.6 to 53 months (mean, 12.5 months). Doublin g times were significantly longer than half-lives by an average factor of 6.5 and there was no correlation between half-life and subsequent doubling time. Doubling times were longer in low-grade tumors. Conclus ions. Serum kinetics of PSA in particular its rate of fall after radia tion provide little, if any, useful clinical information. It is possib le that serum kinetics of PSA are related to tumor cell kinetics but s uch relationships remain speculative. Correlative cell kinetic-PSA kin etic studies are needed to elucidate the mechanisms underlying the cha nges in PSA level after radiation therapy.