PSMA PET/CT-based Atlas for Prostatic Bed Recurrence after Radical Prostatectomy: Clinical Implications for Salvage Radiation Therapy Contouring Guidelines.

Purpose: The aim of this study was to analyze the patterns of prostate bed (PB) recurrence in prostate cancer patients experiencing PSA persistence (BCP) or biochemical recurrence (BCR) after radical prostatectomy using 68 Ga-PSMA-11 PET/CT (PSMA PET) and evaluate the relationship between the RTOG clinical target volumes (CTVs) and PSMA PET-defined recurrence topography. Methods: Patients with BCP or BCR post radical prostatectomy and PB recurrence on PSMA PET were retrospectively included in the analysis. Two nuclear medicine physicians and 4 radiation oncologists manually delineated the PB recurrence and the CTV using the RTOG contouring guidelines on the PSMA PET/CT, respectively, blinded from each other. The coverage of the PSMA PET recurrence was categorized as: PSMA recurrence completely covered, partially covered, or not covered by the RTOG-based CTV. Further, we evaluated the differences in PSMA recurrence patterns among patients with different PSMA PET staging (miTNM). Mann-Whitney U-tests, Chi-squared test, and Spearman ρ correlation analysis were used to investigate associations between CTV coverage and miTNM, PSMA PET-based tumor volume and rectal/bladder involvement. Results: 226 patients were included in the analysis. 127 patients had PSMA recurrence limited to the PB (miTrN0M0), 30 had pelvic nodal disease (miTrN1M0), 32 had extra-pelvic disease (miTrN0M1), and 37 had both pelvic nodal and extra-pelvic disease (miTrN1M1). In the miTrN0M0 cohort, the PSMA-positive PB recurrences were completely covered by the CTV in 68/127 (53%) patients, partially covered in 43/127 (34%), and not covered in 16/127 (13%). The recurrence involved the rectal and bladder wall in 12/127 (9%) and 4/127 (3%), respectively. Full coverage was associated with smaller tumor volume ( P = 0.043), lack of rectal/bladder wall involvement ( P = 0.03) and lower miTNM staging ( P = 0.035), but not to lower serum PSA levels ( P = 0.979). Conclusion: Our study suggests that PSMA PET can be a valuable tool for guiding SRT planning directed to the prostate bed in the setting of postoperative BCR or BCP. These data should be incorporated in the redefinition of PB contouring guidelines.