PP03 Presentation Time: 9:20 AM: Patterns of Failure for Salvage HDR Prostate Brachytherapy in the Prostate PET Imaging Era

Abstract

<h3>Purpose</h3> Although some patients who undergo prostate re-irradiation achieve long-term biochemical control, the majority of patients ultimately experience a biochemical recurrence. Furthermore, a small, but significant percentage of patients may experience significant toxicities. Prostate positron emission tomography (PET) may improve the management of radiorecurrent prostate cancer, by better delineating the systemic extent of disease prior to treatment. Furthermore, PET may allow for a more precise understanding of the patterns of failure after treatment. The purpose of this study is to analyze oncologic outcomes including patterns-of-failure for a contemporary cohort of patients who underwent salvage high-dose rate (HDR) prostate brachytherapy in the prostate PET imaging era. <h3>Materials and Methods</h3> We conducted a retrospective analysis of all patients, who underwent salvage HDR brachytherapy from 2017 onwards. 18F-fluciclovine and 18F-DCFPyL became available in 2017 and 2021, respectively. We utilized the Kaplan-Meier method to measure oncologic endpoints including biochemical progression-free survival (bPFS), clinical progression free survival (cPFS), and metastasis-free survival (MFS). We also implemented Cox regression analysis to identify factors associated with the development of bPFS and MFS. <h3>Results</h3> The median follow-up was 16.6 months. Of the 50 patients included in this study, 48 patients had 18F-fluciclovine (Axumin) imaging and two had 18F-DCFPyL (PSMA) imaging prior to treatment initiation. Six patients (12%) had oligometastatic disease prior to treatment and were treated with SBRT to those sites. Six patients (12%) had castrate-resistant prostate cancer at the time of salvage brachytherapy. The median brachytherapy dose was 1150 cGy in two treatments, and the entire prostate was treated. 43 (86%) patients were concurrently treated with androgen deprivation therapy. Six patients (12%) patients experienced a biochemical recurrence, and four patients (8%) experienced a local-regional recurrence by post-salvage PSMA or Auxmin imaging. Two patients (4%) experienced distant metastatic progression. No initial covariates were statistically associated with bPFS survival (Figure 1). However, increasing time to recurrence trended towards negative association (HR 0.77| CI 0.56 - 1.04, p = 0.091). The bPFS was 100% (CI 100% - 100%), 94% (CI 87% - 100%), and 90% (CI 81% - 100%) at 6 months, 12 months, and 18 months, respectively. <h3>Conclusion</h3> Patients undergoing HDR prostate brachytherapy to salvage recurrent disease in the prostate PET imaging era have durable bPFS. In this cohort, HDR salvage was equally effective regardless of initial treatment covariates (i.e. initial risk group), status of disease immediately prior to salvage (i.e. castrate resistance or oligometastases), or use of androgen deprivation therapy (ADT) during salvage. Therefore, prostate PET may identify candidates for whom HDR salvage will be effective.