PSMA PET/CT–Based Atlas for Prostatic Bed Recurrence of Prostate Cancer after Radical Prostatectomy: Clinical Implications for Salvage Radiation Therapy

Abstract

<h3>Purpose/Objective(s)</h3> Postoperative radiotherapy (RT) to the prostate bed (PB) is a potentially curative treatment after radical prostatectomy (RP). The PB target definition follows contouring guidelines mostly based on expert consensus, and does not account for anatomical patterns of recurrence on more contemporary imaging. Herein, we analyze the patterns of recurrence in the PB of prostate cancer (PCa) patients with biochemical recurrence after RP using ⁶⁸Ga-PSMA-11 PET/CT (PSMA PET). We also evaluate patterns of PSMA PET recurrence in relation to the RTOG-based clinical target volumes (CTVs) definition. <h3>Materials/Methods</h3> Patients with biochemically recurrent PCa after RP, without previous history of PB RT, were retrospectively included if their PSMA PET showed evidence of recurrence in the PB. Two nuclear medicine physicians manually delineated the PB lesions on the CT images of the PSMA PET/CT. Four radiation oncologists, masked to the PSMA PET component of the PET/CT study and to the PSMA-based delineations, contoured the RTOG-based CTVs on the same CT. Three-dimensional heat maps of the PSMA PET-positive PB lesions were generated to visually depict patterns of PSMA PET recurrences. The coverage of the PSMA PET recurrence by the RTOG-based CTVs was categorized in consensus by a nuclear medicine physician and a radiation oncologist as follows: fully within, partly covered or fully outside. <h3>Results</h3> 2,415 PSMA PET scans performed at UCLA between 11/2016 and 11/2020 were screened for inclusion criteria. 230/2,415 (10%) scans of patients with prostate bed recurrence on PSMA PET were included. 127 (55%) patients had miTrN0M0, 30 (13%) miTrN1M0, 34 (15%) miTrN0M1, and 39 (17%) miTrN1M. In the miTrN0M0 cohort, PSMA-positive recurrences were fully covered by the CTV in 68/127 (54%) of the patients, partly covered in 43/127 (34%), and fully outside in 16/127 (13%). Lesions with partial coverage or fully outside the CTV were located at posterior border in 21/127 (53%), postero-lateral in 15/127 (25%), postero-inferior in 2/127 (3%), anterior in 1/127 (2%), antero-inferior in 1/127 (2%), superior in 1/127 (2%) and inferior in 8/127 (14%) of patients. The median (IQR) PSA levels were 1.11 ng/mL (1.7), 1.10 ng/mL (2.17) and 0.84 ng/mL (0.49), and the volumes of PSMA PET recurrences were 0.57cc (0.77), 1.01cc (1.71) and 0.68cc (0.59) in patients with recurrences fully within, partially covered and fully outside the CTV, respectively. <h3>Conclusion</h3> To our knowledge, this is the largest study with a detailed mapping of recurrences in the prostate bed using PSMA PET imaging. In patients with recurrences exclusive to the prostate bed (miTrN0M0), the RTOG contouring guidelines showed suboptimal coverage of the disease in 46% of patients. Our study suggests that PSMA PET can be an invaluable tool for postoperative radiotherapy planning and should be incorporated into a redefinition of RT contouring guidelines.