Prostate specific antigen five years following stereotactic body radiation therapy (SBRT) for low and intermediate risk prostate cancer: An ablative procedure?

Abstract

123 Background: Our previous work on early PSA kinetics following prostate SBRT showed that an initial rapid and then slow PSA decline may result in very low PSA nadirs. This retrospective study sought to evaluate the PSA nadir 5 years following SBRT for low and intermediate risk prostate cancer (PCa). Methods: 65 low and 80 intermediate risk PCa patients were treated definitively with SBRT at Georgetown University Hospital between January 2008 and October 2011. All patients were treated to 35-37.5 Gy in 5 fractions delivered via the CyberKnife Radiosurgical System. Patients who received androgen deprivation therapy were excluded from this study. Pre- and post-treatment PSA and total testosterone levels were obtained during routine follow up visits. Biochemical relapse was defined as a PSA rise > 2 ng/mL above the nadir and analyzed using the Kaplan Meier method. The PSA nadir was defined as the lowest PSA value prior to biochemical relapse or as the lowest value recorded during follow up. Prostate ablation was defined as a PSA nadir < 0.2 ng/mL. Univariate logistic regression analysis was used to evaluate relevant variables on the likelihood of achieving a PSA nadir < 0.2 ng/mL. Results: The median age at the start of SBRT was 72 years. These patients had a median prostate volume of 36 cc with a median 25% of total cores involved. At a median follow up of 5.8 years, 84% and 37% of patients achieved a PSA nadir ≤ 0.5 ng/mL and < 0.2 ng/mL, respectively. Five low and 8 intermediate risk patients experienced a biochemical relapse; those who did not experience a biochemical relapse, achieved a median PSA nadir of 0.2 ng/mL. There was no difference between the 5-year bRFS rate for low (96.6%) and intermediate risk (97.4%) patients and the median time to PSA nadir was 36 months. Initial PSA (p = 0.024) and a lower testosterone at the time of the PSA nadir (p = 0.049) were found to be significant predictors of achieving a PSA nadir < 0.2 ng/mL. Conclusions: SBRT for low and intermediate risk PCa is a convenient treatment option with low PSA nadirs and a high rate of early bRFS. Less than 40% of patients achieved an ablative PSA nadir. Thus, the role of further dose escalation is an area of active investigation.