Objectives: To investigate whether the second-order kinetics model of prostate-specific antigen (PSA) increase would be more appropriate in some cases than the traditionally assumed first-order model. PSA levels are used to detect tumor recurrence after radical prostatectomy, and the rate of PSA increase is used to predict patients' prognosis.
Methods: Sequential PSA values from 147 patients showing detectable PSA after radical prostatectomy were analyzed by nonlinear regression analysis. The best-fitting model was chosen using the lowest sum of squares residual error.
Results: Of the 147 cases, 90 (61.2%) followed the first-order kinetics, and 57 (38.8%) were better described by second-order kinetics. The order of PSA increase correlated with the Gleason score of the tumor in the radical prostatectomy specimens. The first-order kinetics were associated with tumors with Gleason scores of 5 to 6, 7, and 9, and the second-order kinetics were associated with tumors with a Gleason score of 8 (P < 0.01). Our data did not show a correlation between the order of PSA increase and the clinical prognosis.
Conclusions: The presence of a group of tumors in which the increase in PSA follows second-order kinetics is significant. Such an increase may be associated with an autocatalytic mechanism assisting tumor growth. The existence of such a mechanism requires additional investigation.