PSMA PET imaging for evaluation of recurrent or persistent prostate cancer after primary prostate radiation.

Abstract

35 Background: PSMA PET is increasingly utilized for the evaluation of disease stage at biochemical recurrence. Here we evaluate the association of clinico-pathologic features with PSMA findings in biochemically recurrent patients after primary prostate radiation therapy (RT). Methods: We queried the Northwestern Medical System Electronic Data Warehouse between July 2021 and March 2023 to identify 122 men treated with primary RT for presumptive localized prostate cancer who subsequently underwent Gallium-68 or F-18 piflufolastat (DCFPyL) PSMA PET/CT for staging. Patient characteristics, including demographic, clinical, pathological, and imaging variables were obtained. PSMA positivity for suspicious nodal or metastatic disease was determined based on radiology interpretation, with equivocal or likely benign lesions considered as negative findings. Patients with incomplete clinical history (n=3) were excluded. Clinical variables were compared using a Wilcoxon rank-sum and Fisher’s exact tests. Regression analysis was performed to determine variables associated with PSMA positivity in biochemically recurrent patients. Statistical significance was determined if the p-value was less than 0.05. Results: Among the 119 men staged with PET PSMA after RT, median PSA was 3.18 (IQR 1.35, 5.55 ng/mL). 78% (93/119) were found to have suspicious PSMA positive disease. 39% had PSMA positivity in the prostate only (46/119), 6.7% had pelvic nodal disease (8/119), and 33% had extrapelvic disease (39/119). On univariable regression analysis, PSA at the time of PSMA was the only factor associated with PSMA positivity (OR 1.32 per 1 point increase, 95% CI 1.10, 1.67, p=0.01; Table). Higher PSAs at the time of scan were additionally associated with distant recurrence (73.3% of men with distant recurrence if scanned at PSA >10ng/mL versus 31.9% if scanned at PSA <4ng/mL p=0.029). 20% (9/46) of men with PSMA positivity within the prostate were biopsied identifying 78% (7/9) positivity, of which 67% (4/6) receiving treatment selected management with salvage treatments. Conclusions: Patients with prostate cancer initially treated with RT showed significant rates of PSMA positive disease even when scanned at low PSAs (including many below the nadir+2ng/mL definition). Evaluation at low PSAs favors the identification of localized disease only which can be cured by salvage therapy if confirmed by biopsy (i.e. prostatectomy, cryoablation, HIFU). [Table: see text]