Association of bi-parametric MRI measures with continence after robot-assisted radical prostatectomy

OBJECTIVE: To investigate the association between pre- and postoperative magnetic resonance imaging (MRI) measurements of the membranous urethra and the prostate volume and continence following robot-assisted radical prostatectomy (RARP).

PATIENTS AND METHODS: From December 2018 to June 2022, 100 continent patients undergoing unilateral nerve-sparing or non-nerve-sparing RARP were included in this cohort study. Bi-parametric MRI scans were performed before and 12 months after RARP and measurements included the membranous urethral length (MUL) measured in cm (mMUL) and in the number of image slices (sMUL; 3 mm/slice), the membranous urethral diameter (MUD), and the prostate volume. Urinary function was evaluated by the International Consultation on Incontinence Questionnaire-Urinary Incontinence Short Form (ICIQ-UI SF) and continence, defined as the use of zero pads and the answer 'never' to the ICIQ-UI SF question regarding incontinence frequency or <8 g urine-loss per 24 h. Regression with robust variance estimates was used to analyse the association between measurements and outcomes.

RESULTS: At 12 months, continence and MRI data were available for 82 patients. The continence rate was 63% and the median (interquartile range) ICIQ-UI SF score was 4 (0-9). Both preoperative MUL measurements were associated with continence at 12 months. Every extra 5 mm of MUL increased the likelihood of being continent by 13 percentage points (P = 0.03) and every additional slice of sMUL increased it by 6 percentage points (P = 0.05). Both postoperative MUL measurements were associated with better continence and lower ICIQ-UI SF scores (P < 0.01). A larger prostate volume was associated with urinary incontinence at 12 months, with a small effect size. The MUD was not associated with continence.

CONCLUSION: Preoperative mMUL and sMUL are associated with continence at 12 months after RARP. The sMUL may be a useful measurement when only the axial plane is available, and the slice gap is known. Postoperative MUL measurements are strongly associated with continence, while MUD and prostate volume hold minimal prognostic value.