Higher SBRT Dose Levels for Localized Prostate Cancer Are Associated with Improved Post-treatment Biopsy Outcomes

Abstract

To investigate predictors associated with post-treatment biopsy outcomes after stereotactic body radiotherapy (SBRT) for localized prostate cancer. 231 patients treated with prostate SBRT to dose levels of 32.5 Gy to 42 Gy in five fractions (median dose 40Gy) underwent a post-treatment biopsy performed approximately two years after therapy to evaluate local control status. The majority of the patients had intermediate risk disease, 38.5% unfavorable, (n = 89) and 33.3% favorable (n = 77) and the remaining 18.6% (n = 43) and 9.5% (n = 22) had low and high-risk disease. 22.9% (n = 53) were treated with neoadjuvant and concurrent androgen deprivation therapy on conjunction with SBRT for a median duration of 4.6 months. Logistic regression analyses assessed relationships between factors and post-treatment biopsy outcomes (positive vs. negative/treatment effect). The median follow-up time was 46 months (range: 19-116 months). The incidence of positive, negative, and treatment-effect and biopsies were 16.0%, 56.3%, and 27.7%, respectively. The incidence of a positive biopsy according to dose was 33.3% (n = 8/24), 21.4% (n = 6/28), 19.4% (n = 6/31), 11.5% (n = 17/148) for 32.5 Gy, 35 Gy, 37.5 Gy and > = 40 Gy, respectively. The incidence of a positive biopsy was 7% (n = 3/43), 16.9% (n = 13/77), 21.3% (n = 19/89), and 9.1% (n = 2/22) for low, favorable intermediate, unfavorable intermediate and high risk respectively. Patients treated with SBRT doses of <40 Gy (OR: 2.59, 95%CI: 1.19-5.64, p = 0.016) and those with an unfavorable intermediate/high risk (OR: 2.31, 95%CI: 1.07-5.00, p = 0.034) had higher odds of a positive biopsy outcome. The use of neoadjuvant and concurrent ADT did not impact on post-treatment biopsy outcomes. In this cohort representing the largest number of post-treatment biopsies performed on patients treated with prostate SBRT, higher dose levels were associated with improved post-treatment biopsy outcomes independent of prognostic risk group.