PSMA PET-Positive Local Recurrences in the Prostate Bed Region and Their Coverage Based on Clinical Target Volume...

Accurate clinical target volume (CTV) delineation is crucial for effective salvage radiotherapy (sRT) following radical prostatectomy. Current delineation guidelines are largely based on expert opinion and conventional imaging, with limited incorporation of prostate-specific membrane antigen (PSMA) PET/CT data. As PSMA PET/CT enables sensitive detection of local recurrences (LR), this study aims to evaluate the benefit of implementing PSMA PET/CT imaging into CTV delineation guidelines by evaluating spatial distribution of PSMA PET/CT-positive LR and their coverage by 5 commonly used guidelines. Methods: We retrospectively analyzed 77 postprostatectomy patients with 79 LR, staged with PSMA PET/CT and treated with sRT between 2014 and 2023. Planning CT scans on which the LR were delineated as gross tumor volume were further analyzed. The gross tumor volumes were mapped into a standard patient using a modified 3-dimensional prostate bed template according to the PSMA PET-guided hypofractionated salvage prostate bed radiotherapy of biochemical failure after radical prostatectomy for prostate cancer (PERYTON) guideline. Coverage of 5 commonly used CTV guidelines, namely, Groupe Francophone de Radiothérapie Urologique (GFRU), European Society for Radiotherapy and Oncology-Advisory Committee on Radiation Oncology Practice (ESTRO-ACROP), PERYTON, Faculty of Radiation Oncology Genito-Urinary Group (FROGG), and Radiation Therapy Oncology Group (RTOG) was assessed. Coverage was categorized as fully (100% coverage), partially (100% < coverage > 0%), or not covered (0% coverage). Results: Most LR occurred in the vesicourethral anastomotic region (52/79, 66%) and predominantly (73/79, 92%) posteriorly to the midline of the bladder. Full coverage of LR varied across guidelines: RTOG (44/79, 56%), PERYTON (41/79, 52%), ESTRO-ACROP (34/79, 43%), GFRU (31/79, 39%), and FROGG (30/79, 38%). Partial coverage accounted for the remaining cases, except for 1 lesion each missed entirely by FROGG, GFRU, and PERYTON. The posterior, posterolateral, and lateral borders were the most frequent sites where recurrences exceeded guideline-defined CTVs. Conclusion: Although current CTV guidelines achieve acceptable overall coverage, especially concerning the inferior, anterior, and superior border, recurrences tend to extend beyond posterior, lateral, and posterolateral borders, indicating potential areas for refinement. Incorporation of PSMA PET/CT data into CTV delineation may improve robustness and reduce the risk of geographic misses in sRT.