⁶⁸Ga-PSMA PET/CT–Based Atlas for Prostate Bed Recurrence After Radical Prostatectomy: Clinical Implications for Salvage Radiation Therapy Contouring Guidelines

Abstract

The aim of this study was to analyze the patterns of prostate bed (PB) recurrence in prostate cancer patients experiencing prostate-specific antigen (PSA) persistence (BCP) or biochemical recurrence (BCR) after radical prostatectomy using 68 Ga-PSMA-11 PET/CT ( 68 Ga-PSMA PET) in relation to the Radiation Therapy Oncology Group (RTOG) clinical target volumes (CTVs). Methods: This single-center, retrospective analysis included patients with BCP or BCR after radical prostatectomy and PB recurrence on 68 Ga-PSMA PET. The PB recurrences were delineated by nuclear medicine physicians, the CTVs by radiation oncologists contouring guidelines on the 68 Ga-PSMA PET, respectively, masked from each other. The coverage of the 68 Ga-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 68 Ga-PSMA PET staging (miTNM). Mann-Whitney U tests, the chi-square test, and Spearman (r) correlation analysis were used to investigate associations between CTV coverage and 68 Ga-PSMA PET-based tumor volume, serum PSA levels, miTNM, and rectal/bladder involvement. Results: A total of 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 extrapelvic disease (miTrN0M1), and 37 had both pelvic nodal disease and extrapelvic disease (miTrN1M1). In the miTrN0M0 cohort, the recurrence involved the rectal and bladder walls in 12 of 127 (9%) and 4 of 127 (3%), respectively. The PSMA-positive PB recurrences were completely covered by the CTV in 68 of 127 patients (53%), partially covered in 43 of 127 (34%), and not covered in 16 of 127 (13%). Full coverage was associated with a smaller tumor volume (P 5 0.043), a lack of rectal/bladder wall involvement (P 5 0.03), and lower miTNM staging (P 5 0.035) but not with lower serum PSA levels (P 5 0.979). Conclusion: Our study suggests that 68 Ga-PSMA PET can be a valuable tool for guiding salvage radiation therapy (SRT) planning directed to the PB in the setting of postoperative BCR or BCP. These data should be incorporated into the redefinition of PB contouring guidelines.

ESTRO ACROP guideline on prostate bed delineation for postoperative radiotherapy in prostate cancer

“…Anatomical patterns of progression in the prostatic fossa on MRI and PET imaging have shown that recurrences at the cranial aspect of the posterior edge of the pubic bone are rare [36] , [37] . Two recent 68 Ga-PSMA datasets [38] , [39] lead to similar insights for developing new contouring guidelines. Together, more than 250 patients with local recurrences on the 68 Ga-PSMA PET/CTs after prostatectomy showed the importance of including the antero-lateral angles of the rectum, including more than 5 mm below the VUA; and the possibility of excluding the CTV at the cranial aspect of the posterior edge of the pubic bone.…”

The Current Landscape of Prostate-Specific Membrane Antigen (PSMA) Imaging Biomarkers for Aggressive Prostate Cancer

“…In patients with nodal or local recurrence after RP, PSMA PET/CT SUVMax prior to sRT may be prognostic for future recurrence but is only significant in the setting of local recurrence [ 40 , 50 , 51 ]. There is potential for SUVMax to further guide intensification or de-escalation of the treatment in patients undergoing sRT therapy in the future; however, there remains a paucity of evidence to support this practice [ 52 , 53 , 54 ].…”