Reducing False-Positives Due to Urinary Stagnation in the Prostatic Urethra on 18 F-DCFPyL PSMA PET/CT With MRI

Abstract

Purpose: Prostate-specific membrane antigen (PSMA)-targeting PET radiotracers reveal physiologic uptake in the urinary system, potentially misrepresenting activity in the prostatic urethra as an intraprostatic lesion. This study examined the correlation between midline 18 F-DCFPyL activity in the prostate and hyperintensity on T2-weighted (T2W) MRI as an indication of retained urine in the prostatic urethra.

Patients and methods: Eighty-five patients who underwent both 18 F-DCFPyL PSMA PET/CT and prostate MRI between July 2017 and September 2023 were retrospectively analyzed for midline radiotracer activity and retained urine on postvoid T2W MRIs. Fisher's exact tests and unpaired t tests were used to compare residual urine presence and prostatic urethra measurements between patients with and without midline radiotracer activity. The influence of anatomical factors including prostate volume and urethral curvature on urinary stagnation was also explored.

Results: Midline activity on PSMA PET imaging was seen in 14 patients included in the case group, whereas the remaining 71 with no midline activity constituted the control group. A total of 71.4% (10/14) and 29.6% (21/71) of patients in the case and control groups had urethral hyperintensity on T2W MRI, respectively ( P < 0.01). Patients in the case group had significantly larger mean urethral dimensions, larger prostate volumes, and higher incidence of severe urethral curvature compared with the controls.

Conclusions: Stagnated urine within the prostatic urethra is a potential confounding factor on PSMA PET scans. Integrating PET imaging with T2W MRI can mitigate false-positive calls, especially as PSMA PET/CT continues to gain traction in diagnosing localized prostate cancer.

Figure 1.

Selection process of final patient cohort. Notably, patients were excluded from the study if they had poor disease visualization or imaging obtained from outside institutions.

Figure 2.

Patient with F-18 DCFPyL radiotracer uptake in the kidneys, ureters, urinary bladder, and prostatic urethra (arrows) in coronal and sagittal projection on PET imaging (A). This could be misinterpreted as a midline prostate lesion, shown on axial PSMA PET/CT (arrow) (B). Corresponding post-void axial T2W MRI in this patient, showing hyperintensity within the prostatic urethra, indicative of stagnated urine, providing an explanation for PSMA PET positivity.

Figure 3.

Mean urethral dimensions on T2W MRI in the entire patient cohort (n = 85) showed a significant difference between the case and control groups in both axial (4.7 mm vs. 0.8 mm, ****p < .0001) and sagittal (7.1 mm vs. 1.6 mm, ****p < .0001) planes. In patients with no T2W hyperintensity on MRI, prostatic urethra dimensions were considered as 0 mm in both axial and sagittal dimensions. Error bars represent the standard error of the mean.

Figure 4.

Mean prostatic urethral lumen diameters on T2W MRI in subgroup analysis of only patients with urethral hyperintensity (n = 31) shows that there remained a significant difference in measurements between the case and control groups in both axial (6.6 mm vs. 2.8 mm, ****p < .0001, Mann Whitney U) and sagittal (9.9 mm vs. 5.4 mm, **p < .01) planes. Error bars represent the standard error of the mean.

Figure 5.

Axial F-18 DCFPyL PET/CT image showing a focal uptake area in the midline portion of the prostate (arrow) (A). Axial T2W MRI demonstrating a focal hypointense lesion in the midline to right periurethral transition zone (arrows) (B), which demonstrates restricted diffusion on the ADC map (arrow) (C) and b-1500 diffusion-weighted MRI (arrow) (D) and early focal contrast enhancement on dynamic contrast enhanced MRI (arrow) (E) within this lesion. The focal uptake area in the midline portion of the prostate corresponds to the midline to right periurethral transition zone seen at multiparametric MRI. This lesion underwent MRI/trans-rectal ultrasound fusion-guided and systematic biopsy and histopathology revealed Gleason pattern 4+5 PCa within this lesion.