The 'prostate-muscle index': a simple pelvic cavity measurement predicting estimated blood loss and console time in robot-assisted radical prostatectomy

Abstract

This study was to show the impact of 'prostate-muscle index (PMI)', which we developed as a novel pelvic cavity measurement, in patients undergoing robot-assisted radical prostatectomy (RARP). We defined PMI as the 'distance between the inner edge of the obturator internus muscle and the lateral edge of the prostate at the magnetic resonance imaging (MRI) slice showing the maximum width of the prostate'. Seven hundred sixty patients underwent RARP at the University of Tokyo Hospital from November 2011 to December 2018. MRI results were unavailable in 111 patients. In total, 649 patients were eligible for this study. Median values of blood loss and console time were 300 mL and 168 min. In multivariate analysis, body mass index (BMI), prostate volume-to-pelvic cavity index (PV-to-PCI), PMI, and surgical experience were significantly associated with blood loss > 300 mL (P = 0.0002, 0.002, < 0.0001, and 0.006 respectively). Additionally, BMI, PMI, and surgical experience were also significantly associated with console time > 160 min in multivariate analysis (P = 0.04, 0.004, and < 0.0001, respectively). In conclusion, PMI may provide useful information to surgeons and patients in preoperative decision-making.

Figure 1

Definition of parameters in MRI measurements. (a) Axial section of MRI on T2 showing the maximum area of the prostate. A dashed-yellow line ‘BC’ indicates the maximum length of the width of the prostate at the section showing the maximum area of the prostate. The line is extended where it crosses the lateral edge of the obturator internus muscle. Point A and D were defined as the point crossing the lateral edge of the obturator internus muscle. ‘Prostate-muscle index (PMI)’ was defined as the sum of distance 'AB + CD' shown in a yellow line with arrowheads. (b) Sagittal section of MRI on T2 showing the pelvic cavity. ‘Anteroposterior diameter of the pelvic inlet (API)’, ‘pelvic depth (PD)’, and ‘anteroposterior diameter of the pelvic outlet (APO)’ were defined as the ‘diameter from the sacral promontory to the most superior aspect of the pubic symphysis’ (mm), ‘diameter from the sacral promontory to the most inferior point of the pubic symphysis’ (mm), ‘diameter from the inferior aspect of the pubic symphysis to the tip of the coccyx’ (mm). (c, d) Axial section of MRI onT2 showing the measurement of ISD and ASP. ‘Interspinous distance (ISD)’ was defined as ‘the narrowest distance between tips of the ischial spines’ (mm). ‘Angle of the symphysis pubis (ASP)’ was also measured. P: prostate, Fe: femoral bone, Pu: pubic bone, R: rectum, ObM: obturator internus muscle, Bl: bladder.