Membranous urethral length measurement on preoperative MRI to predict incontinence after radical prostatectomy: a literature review towards a proposal for measurement standardization

Abstract

Objectives To investigate the membranous urethral length (MUL) measurement and its interobserver agreement, and propose literature-based recommendations to standardize MUL measurement for increasing interobserver agreement. MUL measurements based on prostate MRI scans, for urinary incontinence risk assessment before radical prostatectomy (RP), may influence treatment decision-making in men with localised prostate cancer. Before implementation in clinical practise, MRI-based MUL measurements need standardization to improve observer agreement. Methods Online libraries were searched up to August 5, 2022, on MUL measurements. Two reviewers performed article selection and critical appraisal. Papers reporting on preoperative MUL measurements and urinary continence correlation were selected. Extracted information included measuring procedures, MRI sequences, population mean/median values, and observer agreement. Results Fifty papers were included. Studies that specified the MRI sequence used T2-weighted images and used either coronal images (n = 13), sagittal images (n = 18), or both (n = 12) for MUL measurements. ‘Prostatic apex’ was the most common description of the proximal membranous urethra landmark and ‘level/entry of the urethra into the penile bulb’ was the most common description of the distal landmark. Population mean (median) MUL value range was 10.4–17.1 mm (7.3–17.3 mm), suggesting either population or measurement differences. Detailed measurement technique descriptions for reproducibility were lacking. Recommendations on MRI-based MUL measurement were formulated by using anatomical landmarks and detailed descriptions and illustrations. Conclusions In order to improve on measurement variability, a literature-based measuring method of the MUL was proposed, supported by several illustrative case studies, in an attempt to standardize MRI-based MUL measurements for appropriate urinary incontinence risk preoperatively. Clinical relevance statement Implementation of MUL measurements into clinical practise for personalized post-prostatectomy continence prediction is hampered by lack of standardization and suboptimal interobserver agreement. Our proposed standardized MUL measurement aims to facilitate standardization and to improve the interobserver agreement. Key Points • Variable approaches for membranous urethral length measurement are being used, without detailed description and with substantial differences in length of the membranous urethra, hampering standardization. • Limited interobserver agreement for membranous urethral length measurement was observed in several studies, while preoperative incontinence risk assessment necessitates high interobserver agreement. • Literature-based recommendations are proposed to standardize MRI-based membranous urethral length measurement for increasing interobserver agreement and improving preoperative incontinence risk assessment, using anatomical landmarks on sagittal T2-weighted images.

Membranous urethral length is the single independent predictor of urinary continence recovery at 12 months following Retzius-sparing robot-assisted radical prostatectomy

“…The AUC of the model stands at 78.2%, indicating a moderate level of accuracy. Several studies and meta-analyses [ 10 , 31 , 32 ] have also demonstrated that MUL is the most studied and reliable urethral parameter to integrate a prediction tool [ 21 ]. Our investigation similarly demonstrated the significant contribution of a longer MUL to UC recovery.…”

“…We acknowledge that MRI measurement might have moderate interrater agreement which is a bias for the generalization of our results based on a single rater [ 39 , 44 ], an issue which may soon be solved by automated MRI measurements [ 45 ]. We attempted to mitigate this limitation by following a standardized protocol for midsagittal MUL measurements [ 20 , 21 ] and as such we believe it does not affect our conclusions [ 39 ]. Other limitations include a pre-operative lack of assessment for the presence of lower urinary tract symptoms which could affect UC recovery, a single institution perspective and a limited number of patients.…”

“…MRI measurements were performed as previously described and standardized [ 9 , 20 , 21 ] in axial and sagittal T2 planes with simultaneous cross-coronal T2 demarcation ensuring measurements were performed in the midsagittal T2 plane as per protocol. These were recorded as follows: (i) PV (ml) = π/6 × maximum height of the prostate × maximal prostate width × maximal prostate length; (ii) maximum height of the prostate measured from base to apex (midsagittal T2-weighted image); (iii) maximal prostate width and maximal prostate length measured at the same axial level, at the level of the maximal prostate width (axial T2-weighted image); (iv) MUL (mm) = distance between the apex of the prostate to the upper border of penile bulb, at the dorsal side of the urethral lumen (sagittal T2-weighted image); (v) urethral width (mm) = maximal diameter of urethra, measured immediately below the caudal margin of prostate apex (axial T2-weighted image); (vi) MUV (ml) calculated as urethral length × π × (urethral width/2) 2 ; (vii) ILD (mm) = the narrowest distance from inner border of levator ani muscles immediately below the prostatic apex (axial T2-weighted image); (viii) OLD (mm) = distance from outer border of levator ani muscles measured at same level as ILD (axial T2-weighted image) and (ix) LAT (mm) calculated as (ODL-IDL)/2 (Fig.…”

What the urologist needs to know before radical prostatectomy: MRI effective support to pre-surgery planning

“…[ 45 ]. Boellaard et al [ 46 ] proposed new standardized recommendations to correctly measure MUL on MRI. More specifically, they suggest evaluating it in sagittal T2WI since coronal images typically lack parallel angulation to the MU and to identify the hyperintense urethral lumen and dorsal hypointense membranous structure on one of the midsagittal images.…”