A standardized method to measure the membranous urethral length (MUL) on MRI of the prostate with high inter- and intra-observer agreement

Objectives: The membranous urethral length (MUL), defined as the length between the apex and penile base as measured on preoperative prostate magnetic resonance imaging (MRI), is an important predictor for urinary incontinence after radical prostatectomy. Literature on inter- and intra - observer agreement of MUL measurement is limited. We studied the inter- and intra-observer agreement between radiologists using a well-defined method to measure the MUL on the prostate MRI. Methods: Prostate cancer patients underwent a preoperative MRI and robot-assisted radical prostatectomy (RARP) at one high-volume RARP center. MUL measurement was based on well-defined landmarks on sagittal T2-weighted (anatomical) images. Three radiologists independently performed MUL measurements retrospectively in 106 patients blinded to themselves, to each other, and to clinical outcomes. The inter- and intra-observer agreement of MUL measurement between the radiologists were calculated, expressed as intra-class correlation coefficient (ICC). Results: The initial inter-observer agreement was ICC 0.63; 95% confidence interval (CI) 0.28–0.81. Radiologist 3 measured the MUL mean 3.9 mm (SD 3.3) longer than the other readers, interpreting the caudal point of the MUL (penile base) differently. After discussion on the correct anatomical definition, radiologist 3 re-assessed all scans, which resulted in a high inter-observer agreement (ICC 0.84; 95% CI 0.66–0.91). After a subsequent reading by radiologists 1 and 2, the intra-observer agreements were ICC 0.93; 95% CI 0.89–0.96, and ICC 0.98; 95% CI 0.97–0.98, respectively. Limitation is the monocenter design. Conclusions: The MUL can be measured reliably with high agreement among radiologists. Key Points: • After discussion on the correct anatomical definition, the inter- and intra - observer agreements of membranous urethral length (MUL) measurement on magnetic resonance imaging (MRI) were high. • A reproducible method to measure the MUL can improve the clinical usefulness of prediction models for urinary continence after RARP which may benefit patient counselling.