Effect of urinary stagnation in the prostatic urethra on generating false-positive midline lesions on 18F-DCFPyL PSMA PET/CT.
Authors: Gelikman David G , Mena Gonzalez Esther , Lindenberg Liza , Yilmaz Enis C , Harmon Stephanie A , Azar William S , Schuppe Kyle , Rathi Nityam , Wood Brad , Gurram Sandeep , Choyke Peter L , Pinto Peter A , Turkbey Baris 
Published: January 2024

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Background: Radiotracers used in prostate-specific membrane antigen (PSMA) positron emission tomography (PET) imaging have high specificity for the detection of prostate cancer (PCa), and PSMA PET/CT imaging has seen increased use in the management of localized PCa. However, the diagnostic capability of PSMA PET can be limited by intense uptake in the ureters, bladder, and urethra due to physiologic urinary excretion. Specifically, radiotracer activity in the prostatic urethra can be misinterpreted as a midline intraprostatic lesion if not carefully evaluated with morphologic imaging. This study aimed to elucidate the correlation between positive 18F-DCFPyL activity in the midline prostate with hyperintense signal on T2-weighted MRI (T2W MRI) within the prostatic urethra as an indication of retained urine. Methods: Ninety-seven patients received both 18F-DCFPyL PSMA PET/CT and 3 Tesla prostate MRI post-void imaging for localized, high-risk PCa. Patients with prior prostate treatment (i.e., surgery, androgen deprivation therapy, or radiotherapy) or poor prostatic urethra visualization (i.e., PCa involving the whole gland, Foley catheter in situ) were excluded, resulting in a final cohort of 84 patients. Radiotracer activity in the midline region of the prostate was prospectively evaluated by two nuclear medicine physicians during PET/CT read outs. Post-void T2W MRI scans were retrospectively evaluated for the presence of residual urine noted as hyperintensity within the prostatic urethral lumen by one genitourinary radiologist blinded to PET/CT evaluations. The corresponding prostatic urethra diameters were measured on both axial and sagittal T2W MRI. Fisher’s exact test and Mann-Whitney U tests were used to compare MRI and PET/CT findings. Results: In 84 patients, midline activity on PSMA PET imaging was documented in 16 patients (the case group), whereas the remaining 68 patients without midline activity constituted the control group. In total, 75% (12/16) and 31% (21/68) of patients in the case and control groups, respectively, had urethral hyperintensity on T2W MRI (p < .01, Fisher’s exact test). In patients with a hyperintense urethral lumen at MRI, there was a significant difference in mean prostatic urethral measurements between the case and control groups in both axial (7.25 mm vs. 2.81 mm, p < .0001, Mann Whitney U) and sagittal (12.25 mm vs. 5.43 mm, p < .0001, Mann Whitney U) planes. Conclusions: Our findings suggest that PSMA uptake within the midline of the prostate is more commonly observed in patients with a prominently visible prostatic urethra at MRI, a finding that is likely associated with the stagnation of urine containing PSMA tracer. This midline uptake may be prone to misinterpretation as a potential PCa lesion on PSMA PET/CT, and the use of post-void MRIs may be useful in reducing the likelihood of false positive interpretations of PSMA PET/CT.