SBRT and HDR brachytherapy produce lower PSA nadirs and different PSA decay patterns than conventionally fractionated IMRT in patients with low- or intermediate-risk prostate cancer.

PURPOSE: To compare patterns of prostate-specific antigen (PSA) response following stereotactic body radiation therapy (SBRT), high-dose-rate (HDR) brachytherapy, and conventionally fractionated intensity modulated radiation therapy (IMRT) in patients with low- or intermediate-risk prostate cancer (CaP). METHODS AND MATERIALS: Eligible study patients included 439 patients with low- or intermediate-risk prostate cancer who were treated with radiation therapy (RT) alone between 2003 and 2013, remained free of biochemical recurrence, and had at least 2 PSA values within the first year following RT. Of these, 130 were treated with SBRT, 220 with HDR brachytherapy, and 89 with IMRT. Multivariate regression analysis was used to compare PSA nadirs (nPSA), time to nPSA, and PSA bounce parameters among the 3 modalities. Indicator variable analysis was used to develop empirical models of PSA decay using the treatment modalities as indicator variables. RESULTS: Significantly more patients treated with SBRT or HDR brachytherapy achieved raw nPSAs of <0.5 ng/mL compared with patients treated with IMRT (76.2% and 75.9% vs 44.9%, respectively; P < .0001 for SBRT or HDR brachytherapy vs IMRT). On multivariate analysis, nPSA was significantly lower with SBRT and HDR compared with IMRT (P < .0001). Time to nPSA and bounce parameters was not significantly different among IMRT, SBRT, and HDR. Overall, SBRT and HDR brachytherapy caused significantly larger PSA decay rates (P < .001). When truncating follow-up at 1000 days, the corresponding decay rates were larger for all 3 modalities, with no significant differences between them. CONCLUSIONS: Stereotactic body radiation therapy and HDR brachytherapy produce lower nPSAs than IMRT. Within 1000 days of follow-up, the modalities produce similar rates of PSA decay; subsequently, decay continues (albeit at a slower pace) after SBRT and HDR brachytherapy but plateaus with IMRT. Because nPSA is a validated predictor of long-term outcome, these data not only suggest a distinct radiobiological effect with SBRT and HDR brachytherapy, but also predict for clinical outcomes that might equal or surpass those of IMRT.