Prostate-Specific Antigen Kinetics Following Various Radiotherapy Modalities: A Multi-Institutional Analysis

Several equally effective radiotherapy (RT) options are utilized in the definitive treatment of low- and intermediate-risk prostate cancer (PCa), including external beam radiation therapy (EBRT), brachytherapy monotherapy (BT) and combination therapy (EBRT + BT). Monitoring prostate-specific antigen (PSA) kinetics after RT plays an important role in patient management and counseling. We hypothesize that certain forms of RT may lead to lower PSA nadirs (nPSA) than others. Retrospective PSA data were analyzed for low- and intermediate-risk PCa patients from 8 different institutions treated between 1990 and 2017. RT modalities included dose-escalated conventionally fractionated (CF) EBRT (37-45 fractions (fx)), hypofractionated (H) EBRT (26-28 fx), low-dose-rate (LDR) BT, high-dose-rate (HDR) BT, EBRT + BT or stereotactic body radiotherapy (SBRT). Patients who had less than 12 months (mths) of PSA data or received androgen deprivation therapy were excluded. nPSA was defined as the lowest PSA after treatment and time to nPSA was defined as the time between nPSA and the radiation end date. The Phoenix definition was used to define biochemical failure (BF); although the ASTRO definition was used for some patients treated before 2009. The Kruskal-Wallis and chi-square test were used for statistical analyses. In total, 5345 patients were included in this study. PSA data for each RT modality are presented in Table 1 with relevant interquartile ranges in parentheses. Rates of achieving a nPSA ≤ 0.5 were 62%, 69%, 88%, 79%, 91% and 84% for CF-EBRT, H-EBRT, LDR BT, HDR BT, EBRT + BT and SBRT respectively. Fewer patients were able to achieve an nPSA ≤ 0.2 (rates of 26%, 42%, 78%, 63%, 79%, and 57% respectively). Of those patients, the BF rates were 4.4%, 7.5%, 3.7%, 3.1%, 5.2% and 1.4% respectively. Overall nPSA and percentage of patients able to achieve either nPSA threshold was statistically different across the various RT modalities (p<0.001), with LDR BT and EBRT + BT having the lowest nPSAs. The majority of patients in this multi-institutional, multi-modality analysis achieved a nPSA ≤ 0.5. Further, the majority of patients who received BT, EBRT + BT or SBRT were able to achieve a nPSA of ≤ 0.2. As expected, rates of BF were low across all modalities. A limitation of this study is the inclusion of low-risk patients that would be candidates for active surveillance in the modern era.Abstract 2650; Table 1CF-EBRTH-EBRTLDR BTHDR BTEBRT + BTSBRT# Patients17449612745121941525Low/Intermediate Risk (%)55/4531/6987/1347/538/9255/45Median Follow-up Time (mths)6510583627173BF Rates (%)9.614.67.84.77.75.0Median iPSA (ng/mL)5.4 (4.1 – 7.4)6.8 (4.5 – 10.7)5.4 (4.2 – 7.0)5.8 (4.6 – 8.0)5.9 (4.6 – 7.9)5.8 (4.6 – 7.9)Median nPSA (ng/mL)0.4 (0.2 – 0.7)0.3 (0.1 – 0.7)0.05 (0.0 – 0.2)0.1 (0.1 – 0.4)0.04 (0.0 – 0.2)0.2 (0.1 – 0.3)Median Time to nPSA (mths)29 (17 – 46)31 (21 – 52)51 (28 – 81)37 (17 – 58)44 (22 – 66)28 (19 – 48) Open table in a new tab