289 Background: Salvage radiotherapy (SRT) may be employed as a potentially curative intervention for patients experiencing biochemical failure (serum prostate-specific antigen [PSA] ≥ 0.2 ng/mL) after prostatectomy for localized prostate cancer. Patients not showing a favorable response to SRT alone may require additional therapies and may benefit from earlier identification of this need. Methods: 131 consecutive patients received SRT during the timeframe of this study. 76 were deemed eligible based on the following criteria: prostatic adenocarcinoma diagnosis receiving SRT, no clinical evidence of metastasis, no hormone use prior to/during SRT, serum PSA measurement halfway through SRT, and minimum follow-up time of 3 months. Median follow-up time was 51.6 months. Eligible patients were divided into three groups based on PSA response by the midpoint of treatment: no change, decrease, or increase in PSA. The primary endpoint of the study was clinical failure (measured from SRT completion), defined as serum PSA value ≥0.2ng/mL above the post-radiotherapy nadir, initiation of androgen deprivation therapy, development of bone metastasis, or death from prostate cancer. Results: 13.1% experienced no change in PSA halfway through SRT (group 0), 68.4% of patients experienced a decrease (group 1), 18.4% experienced an increase (group 2). Four-year freedom from clinical failure was 60.0% for group 0, 58.3% for group 1, and 41.7% for group 2. Median time to clinical failure was 71.7 months for group 1, 26.8 months for group 2, and was not reached for group 0. Pairwise multiple comparison demonstrated a significant difference in four-year freedom from clinical failure between groups 1 and 2 (p = 0.036). Conclusions: These data strongly suggest that changes in PSA are apparent midway through SRT and are associated with 4-year freedom from clinical failure. Further study is warranted to determine whether mid-treatment PSA during SRT may be used to identify a subset of patients that may benefit from treatment intensification.
PD06-06 IMPORTANCE OF [-2] PROPSA MEASUREMENT IN A DIAGNOSIS OF PROSTATE CANCER: PROSTATE HEALTH INDEX TRIAL (PROPHET) - DIAGNOSTIC IMPACTS OF CLINICAL LABORATORY-BASED INDICES ON GLEASON GRADE GROUP ≥2 OR ≥3 CANCER