Timing and Patterns of Potentially Salvageable Recurrences Following SBRT for Clinically Localized Prostate Cancer Assessed by Preferential Amino Acid Uptake

Abstract

<h3>Purpose/Objective(s)</h3> ¹⁸F-Fluciclovine is a radiolabeled amino acid analogue that is preferentially taken up by prostate cancer cells. ¹⁸F-Fluciclovine PET/CT scans are approved for the detection of biochemically recurrent prostate cancer. Stereotactic body radiation therapy (SBRT) is increasingly offered for the treatment of localized prostate cancer. Limited data exists on the patterns of failure following prostate SBRT. The impact of scan timing before or after meeting the Phoenix Criteria is unknown. Here we characterize ¹⁸F-Fluciclovine defined recurrences for patients with rising PSAs following SBRT. <h3>Materials/Methods</h3> Between 2017 and 2022, 50 consecutive patients underwent ¹⁸F-Fluciclovine scan for suspected recurrence. All patients were treated on an institutional protocol with either SBRT (35-36.25 Gy) or SBRT boost (19.5 Gy) with intensity-modulated radiotherapy (IMRT). 40% of the patients were high-risk and 46% received ADT as part of their initial treatment. Patterns of failure were classified as PSA only, Local (prostate), Lymph Node (LN), Bone, Visceral or combined. Patients were considered salvageable if all evidence of disease could be safely treated with local therapy (radiation, surgery or IR ablation). <h3>Results</h3> The median time from treatment was 56 months and the median pre-scan PSA was 2.8 ng/ml. The overall scan positivity rate in our cohort was 33/51 (65%). The most common sites for initial disease recurrence were: prostate (25%), pelvic and para-aortic lymph node basins (43%) and bone (7%). 21/51 scans (41%) were performed prior to reaching the Phoenix definition (nadir + 2) at a median PSA of 1.14 ng/ml. Of these patients 11 (52%) had evidence of disease recurrence. All of which were potentially salvageable local or LN recurrences. The remaining 30/51 (59%) scans were performed after meeting the Phoenix definition (median PSA = 5.65 ng/ml). Of these 22/30 (73%) had disease recurrence and 82% were potentially salvageable. <h3>Conclusion</h3> The diagnosis and management of recurrence following prostate SBRT continues to evolve. Approximately 50% of patient in our cohort who had yet to meet the Phoenix definition had scan evidence of disease recurrence, all of which were potentially salvageable with additional local therapy. Additional research is needed to identify factors predictive of disease recurrence on ¹⁸F-Fluciclovine scans prior to reaching the Phoenix definition when they may be most curable.