3260 - Is This the Right Fossa? Verification of Prostate Fossa Contouring Volumes Using 18F-Fluciclovine PET/CT

Purpose/Objective(s): Radiation therapy to the prostate fossa following prostatectomy, either due to high-risk features, biochemical, or radiological recurrence, is a commonly utilized curative treatment approach. Current recommendations in target delineation of the prostate fossa come from expert consensus guidelines developed prior to the widespread use of radiotracers to localize prostate cancer. PSMA PET/CT is the current standard of care imaging modality to localize disease in biochemical recurrence (BCR) of prostate cancer, but it is limited in its evaluation of the prostate bed due to its urinary excretion. Unlike PSMA, 18F-FACBC is not excreted in the urine and can help identify the location of recurrent disease in the prostate fossa. We examined 18F-FACBC scans of patients with prostate fossa recurrence who had negative PSMA scans to map out the location of these recurrences and compare them to guideline based prostate fossa target volumes.

Materials/Methods: This study analyzes a group of 14 patients with prostate fossa recurrences identified utilizing 18F-FACBC PET/CT. These patients are a subset of a larger combined prospective single-arm cohort study (NCT 05722925) and a retrospective cohort at a single institution investigating the role of 18F-FACBC in patients with a BCR and a negative PSMA PET/CT. The prostate fossa was divided into 3 main regions (VUA, bladder neck, and SV remnant) by anatomical landmarks to quantify the location of the identified recurrences. The VUA region was separated from the bladder neck region by the landmark of the mid-pubic symphysis on a sagittal CT. The VUA and bladder neck region were also separated into anterior and posterior subregions by the VUA. The site of recurrence in each case was also reviewed by two prostate specialized radiation oncologists at an academic medical center to determine if the site of disease would be covered by a standard RTOG prostate fossa target volume.

Results: 7 (50%) recurrences were identified within the VUA region, 4 (28.6%) were in the bladder neck region, and 3 (21.4%) were in the SV remnant region. Interestingly, all recurrences within the VUA and bladder neck regions were located posterior to the VUA landmark. On review of recurrence locations, all 14 were within a standard RTOG prostate fossa contour volume.

Conclusion: All identified prostate fossa recurrences were found to be posterior to the VUA, indicating that we may be able to decrease our anterior coverage and thereby decrease bladder radiation dose. All recurrences were within a standard RTOG prostate fossa volume.