Purpose: The role of prostate specific antigen (PSA) nadir in the definition of disease freedom after radiotherapy of prostate cancer is controversial. We evaluate post-irradiation PSA nadir in men apparently cured of this disease. Materials and Methods: From 1984 to 1993, 354 men with clinical stage T1T2N0 prostate cancer were treated with radioactive 125iodine prostate implant followed by external beam radiation. Median pretreatment PSA was 8.4 ng./ml. (range 0.3 to 188). Of these men 250 are disease-free and median pretreatment PSA was 6.5 ng./ml. (range 0.3 to 123). Treatment failure is defined as 3 consecutive PSA increases above nadir. Median followup is 7 years (range 5 to 14 years) for the 250 disease-free men and 6 years (range 0.5 to 14) for all 354 men. Results: PSA nadir 0.5 ng./ml, or less was achieved by 98% of all disease-free men (244 of 250) with minimum 5-year followup, including 87% (217) who achieved nadir 0.2 ng./ml, or less. All 27 disease-free men with minimum 10-year followup had a PSA nadir of 0.5 ng./ml, or less. PSA nadir significantly correlated with disease-free survival by receiver operator characteristics curve analysis (0.93 area under the curve) in all 354 men. Conclusions: PSA nadir is the fundamental measurement that determines possible cure after radiotherapy. Except for perhaps rare occasions, men must at least achieve a nadir of 0.5ng./ml. or less to be cured of prostate cancer by irradiation. However, the prognostic value of this nadir level depends on most men achieving a nadir of 0.2 ng./ml. or less. Disease freedom for radiotherapy, defined as achievement and maintenance of PSA nadir 0.5 ng./ml, or less, is reasonable.
