# Timing and Patterns of Potentially Salvageable Recurrences Following Stereotactic Body Radiation Therapy for Clinically Localized Prostate Cancer Assessed by Preferential Amino Acid Uptake

## TL;DR

This study examines when and where prostate cancer recurs after SBRT, showing that some recurrences can be detected and treated early using 18F-fluciclovine PET/CT scans.

## Contribution

The study identifies potentially salvageable recurrences before meeting the Phoenix criteria using 18F-fluciclovine PET/CT scans.

## Key findings

- Approximately 50% of patients had scan evidence of recurrence before meeting the Phoenix criteria, all potentially salvageable.
- 73% of scans after the Phoenix criteria showed recurrence, with 82% being potentially salvageable.
- The most common recurrence sites were pelvic and para-aortic lymph nodes (40%).

## Abstract

Purpose: 18F-fluciclovine is a radiolabeled amino acid analog 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 exist 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 prostate-specific antigens (PSAs) following SBRT.

Methods: Between 2017 and 2022, 50 consecutive patients underwent an 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). A total of 38% of the patients were high-risk, and 46% received androgen deprivation therapy (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 interventional radiology (IR) ablation).

Results: The median time from treatment was 39 months, and the median pre-scan PSA was 2.8 ng/mL. The overall scan positivity rate in our cohort was 34/51 (67%). The most common sites for initial disease recurrence were the prostate (22%), pelvic and para-aortic lymph node basins (40%), and bone (6%). A total of 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, 12 (57%) 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.

Conclusions: The diagnosis and management of recurrence following prostate SBRT continues to evolve. Approximately 50% of patients 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.