Impact of PSMA PET-CT Findings on Dose Prescription and Target Definition in Patients with Biochemical Relapse after Radical Prostatectomy

Purpose/Objective(s): Standard treatment for biochemical relapse (BCR) after radical prostatectomy (RPE) in patients with histologically negative lymph nodes consists of salvage radiotherapy to the prostate bed only. However, up to one third of these treatments do not result in long-lasting biochemical control. Using PSMA PET-CT to identify possible relapse locations prior to radiotherapy could greatly affect the salvage treatment approach. This study was conducted to report the proportion of patients with BCR showing positive findings on PSMA PET-CT, leading to substantial changes of radiation target definition and dose prescription.

Methods: We retrospectively analysed 97 patients with BCR after RPE referred to our clinic between 01/2021 and 12/2023 to receive salvage radiotherapy to the prostate bed. No patient had histologically verified node positive disease. All patients received a PSMA PET-CT prior to treatment planning. PET-CT findings were grouped in recurrences in the prostate bed, pelvic nodes, bone metastases or in multiple sites. Radiotherapy dose prescription and target definition were adjusted according to the findings. In case of PET negative disease standard salvage radiotherapy was performed.

Results: At time of initial diagnosis prior to RPE patients showed a mean PSA level of 7,7 ng/ml (2,2-226,2 ng/ml). ISUP score distribution was as follows, ISUP1: 12,4%, ISUP 2: 30,9%, ISUP 3: 26,8%; ISUP 4: 22,7%, ISUP 5: 7,2%. Most patients showed organ confined disease (T2a-T2c in 63,9%), and low rates of extracapsular extension (19,6%) or seminal vesicle involvement (16,5%).Mean pre-salvage PSA level was 0,5 ng/ml (0,04-11,02 ng/ml). Median duration between surgery and radiotherapy was 30 months (3-353 months). 80,4% of all patients showed evidence of recurrence on PET-CT. The proportion of recurrences in prostate bed, pelvic nodes, bone metastases or in multiple sites were 29,9%, 23,7%, 6,2% and 20,6% respectively. The rates of positive findings on PET-CT were related to the pre-treatment PSA. At PSA levels between 0,0-0,19ng/ml, 0,2-0,5ng/ml, 0,51-1ng/ml and >1ng/ml, we found metabolic evidence of disease in 75%, 64,1%, 91,3% and 100% of patients. The findings on PET-CT lead to no change in the therapeutic approach in 19,6%, to minor changes (dose escalation without adjustment of target definition) in 29,9% and to major changes (dose escalation and adaption of target definition) in 50,5% of all patients.

Conclusion: In this cohort of patients with predominantly moderate risk factors (mean ISUP 3, PSA 0,5ng/ml, and mainly T2a to T2c) molecular imaging lead to major changes in the treatment in most patients. Even low postsurgical PSA levels <0,5ng/ml result in a high number of pathologic findings. To address the key question whether these changes lead to a clinically meaningful reduction of recurrences and metastatic spread, this cohort will be followed for treatment outcome and relapse patterns.