[en] Purpose A new clinical pre-treatment quantity that closely approximates the true prostate cancer volume is defined. Materials and Methods The cancer specific prostate specific antigen, PSA density, prostate cancer volume (V_Ca) and the volume fraction of the gland involved with carcinoma (V_Ca fx) were calculated for 227 prostate cancer patients managed definitively with external beam radiation therapy. A Cox regression multivariate analysis was employed to test if any of these clinical pre-treatment parameters added significantly to PSA in predicting early post-radiation PSA failure. Cancer specific PSA = PSA - [PSA from benign epithelial tissue] V_Ca = Cancer specific/[PSA in serum per cm³ of cancer] V_Ca fx = V_Ca/ultrasound prostate gland volume Results The prostate cancer volume (p = .039) and the volume fraction of the gland involved by carcinoma (p = .035) significantly added to the PSA in predicting post-radiation PSA failure on multivariate analysis. Conversely, the PSA density and the cancer specific PSA did not add significantly (p > .05) to PSA in predicting post-radiation PSA failure. Conclusion The volume of cancer (V_Ca) and the resulting volume fraction of cancer both added significantly to PSA in their ability to predict for early post-radiation PSA failure. These new parameters may be used to optimize the selection of patients in prospective randomized trials that examine the efficacy of combining radiation and androgen ablative therapy in patients with clinically localized disease but who are at high risk for early post-radiation PSA failure