Multiparametric MRI and 18F-DCFPyL PET/CT for Detection of Locally Recurrent Prostate Cancer After Low dose Rate Brachytherapy

Background and purpose: Both multiparametric magnetic resonance imaging (mpMRI) and prostate specific membrane antigen (PSMA)-targeting positron emission tomography (PET) imaging have shown promise in early localization of prostate cancer (PCa) recurrence after primary external beam radiotherapy. Detecting recurrence after brachytherapy for PCa using MRI is significantly hampered by susceptibility artifacts secondary to brachy seeds. Here, we compare the efficacy of 18F-DCFPyL (2-(3-{1-carboxy-5-[(6-18F-fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid) PET/CT versus mpMRI for detecting sites of local recurrence after low dose rate (LDR) brachytherapy for prostate cancer. Materials and methods: A total of 155 patients with a history of recurrent PCa who underwent mpMRI at 3 Tesla and 18F-DCFPyL PET/CT were retrospectively reviewed. Patients who underwent LDR brachytherapy for PCa and had subsequent biochemical recurrence (BCR) followed by mpMRI and 18F-DCFPyL PET/CT were included in this study. mpMRI was performed on a 3T scanner with endorectal and surface coils and images were prospectively read by a single expert radiologist. The 18F-DCFPyL PET/CT scan was prospectively interpreted by one of two nuclear medicine physicians. Patients underwent targeted biopsy when deemed clinically necessary and specimens were interpreted by an expert GU pathologist. Positivity rates (PR) from mpMRI and 18F-DCFPyL PET/CT were compared, and pathology results were used to calculate the positive predictive value (PPV) of each imaging modality for detecting PCa recurrence.Results: 14 patients who underwent LDR brachytherapy and had subsequent biochemical failure were imaged with mpMRI and 18F-DCFPyL PET/CT. 17 lesions were identified on at least one imaging modality. The PR for detection of intraprostatic lesions was 71% (10/14) for PET/CT and 64.3% (9/14) for mpMRI. 18F-DCFPyL PET/CT identified pelvic and extra-pelvic lymph nodes in 8 (57%) patients, while mpMRI noted positive lymph node findings in only 2 (14%) patients. A total of 14 lesions corresponding to 8 patients were targeted for biopsy and in these, 18F-DCFPyL PET/CT performed better than mpMRI for the detection of local recurrence with a PPV of 81.8% vs. 66.7%. For the detection of intraprostatic lesions, 18F-DCFPyL PET/CT and mpMRI had a PPV of 87.5% and 71.4%, respectively. Conclusion: Our findings suggest that 18F-DCFPyL PET/CT detects local and regional recurrent PCa after LDR brachytherapy at a higher rate than mpMRI and has a much higher detection rate of suspicious pelvic lymph nodes. Further studies are needed to validate these findings in larger cohorts.