PURPOSE The prostate-specific antigen (PSA) nadir that reflects potential cure of prostate cancer by irradiation has not been established. This report attempts to demonstrate the PSA nadir goal for radiotherapy. MATERIALS AND METHODS From January 1984 through April 1994, 536 stage T1T2NO prostate cancer patients were treated with radioactive iodine 125 (125I) prostate implants followed by external-beam radiation. All were staged node-negative: 68% by pelvic node dissection and the remainder by computed tomographic (CT) scan. The mean pretreatment PSA level was 12.4 ng/mL (median, 8.4 ng/mL; range, 0.3 to 188 ng/mL). The median follow-up duration is 40 months (range, 12 to 138). An increasing posttreatment PSA level defined recurrence. RESULTS Patients who achieved a PSA nadir < or = 0.5 ng/mL had a 95% (+/- 4%) 5-year and an 84% (+/- 12%) 10-year disease-free survival rate, compared with a 5-year disease-free survival rate of 29% (+/- 30%) for those who reached a nadir of 0.6 to 1.0 ng/mL (P = .0001). All patients with a nadir greater than 1.0 ng/mL ultimately failed. Eighty percent of all 536 patients are projected to achieve a nadir < or = 0.5 ng/mL and 90% of patients who achieve this PSA level do so within 48 months of treatment (median, 18 months). Compared with pretreatment PSA level and histologic grade, the PSA nadir is the most significant factor associated with disease-free survival. CONCLUSION For most patients to be successfully treated for prostate cancer with radiotherapy, at least with this combination technique, the PSA nadir should become undetectable (< or = 0.5 ng/mL), similar to that after radical prostatectomy. A PSA nadir of < or = 0.5 ng/mL after radiotherapy for prostate cancer may be used as a reasonable indicator of 10-year disease-free survival.
