The importance of an exponential prostate-specific antigen decline after external beam radiotherapy for intermediate risk prostate cancer

Background: To study the influence of an exponential prostate-specific antigen (PSA) decline on biochemical failure after external-beam radiotherapy (EBRT). Methods: We analyzed 114 patients with intermediate risk prostate cancer (Gleason ≤ 6 and PSA 10-20 or Gleason 7 and PSA <10). Patients were randomized between EBRT doses of either 70.2 Gy or 79.2 Gy (1.8 Gy per day). All patients had a follow up of at least six PSA measurements post-EBRT. Exponential decline and PSA half life were included in a Cox regression analysis for factors associated with biochemical failure. Results: A total of 80/114 (70.2%) patterns were classified as having an exponential PSA decline. Both exponential decline (HR 0.115, 95%CI 0.03-0.44, p = 0.0016) and PSA half life ratio were statistically significant predictors (HR 1.03 (95% CI 1.01-1.06)) of biochemical failure. In the model predicting for exponential decline, none of the factors were significant. Conclusion: Patients with an exponential PSA decline show a better biochemical outcome in the long term.