MRI-Derived Transition Zone Index Is Highly Predictive of Urodynamic Bladder Outlet Obstruction Prior to Holmium Laser Enucleation of the Prostate

Introduction and objective: Urodynamic study (UDS) is required to diagnose bladder outlet obstruction (BOO) during evaluation of benign prostatic hyperplasia (BPH) but is seldom performed due to cost and invasiveness. Therefore, anatomic and clinical parameters to predict BOO have been proposed, including the prostate transition zone index (TZI) which is the ratio of prostate transition zone volume (TZV) to whole gland volume (WGV). Historically computed with ellipsoid volume estimation of prostate WGV and TZV from transrectal ultrasound measurements, controversy exists regarding the utility of TZI to predict likelihood of BOO on UDS and clinical outcomes following BPH surgery. Here, we aim to assess the association between MRI-measured TZI and BOO on preoperative UDS in a modern BPH cohort before holmium laser enucleation of the prostate (HoLEP).

Methods: A prospectively maintained institutional database of 944 consecutive HoLEP patients between 2018 and 2022 was reviewed to identify those with preoperative UDS and MRI within 1 year of surgery. UDS was used to measure bladder outlet obstruction index (BOOI). 3D WGV and TZV were estimated using clinically available software (DynaCAD). We used linear regression to assess the relationship between TZI, WGV, and BOOI and logistic regression to determine the association between TZI, WGV, and BOO (defined as BOOI > 40).

Results: 45/944 (4.8%) patients had both preoperative UDS and MRI within 1 year of HoLEP. Of these, 27 patients were obstructed (BOOI > 40) and 18 patients were not obstructed (BOOI ≤ 40) on preoperative UDS. Obstructed patients had larger prostate WGV, TZV, and TZI compared to non-obstructed patients but were otherwise similar with respect to preoperative characteristics. Univariate analysis showed a positive association between TZI and BOO (R² = 0.373, p < 0.001) and WGV and BOO (R² = 0.214, p < 0.001). Multivariable logistic regression showed that TZI was independently associated with BOO (OR 1.08, 95% CI 1.02-1.14, p = 0.013) while accounting for WGV. WGV was not independently associated with BOO while accounting for TZI (OR 1.00, 95% CI 0.98-1.01, p = 0.614). The Youden index was utilized to create an optimal cutpoint for TZI (0.528) above which urodynamic BOO was very likely on multivariate logistic regression while accounting for WGV (OR 25.0, 95% CI 3.40-183.58, p = 0.002). The generated cutpoint for WGV (61.5 mL) was not significantly associated with urodynamic BOO on multivariate logistic regression while accounting for TZI (OR 0.993, 95% CI 0.98-1.01, p = 0.452).

Conclusions: Noninvasive MRI measurement of prostate TZI was highly and independently predictive of BOO before HoLEP and superior to WGV alone. This suggests that MRI obtained in evaluation of BPH/LUTS patients may be used to calculate TZI and inform patient selection for invasive urodynamic study and surgical treatment.