External Validation of a Prediction Model for Side-specific Extraprostatic Extension of Prostate Cancer at Robot-assisted Radical Prostatectomy

The aim of this study was to externally validate a nomogram for side-specific extraprostatic extension (EPE) of prostate cancer (PCa) at robot-assisted radical prostatectomy (RARP). A prospectively maintained cohort of 1170 consecutive patients with PCa who underwent RARP at two high-volume RARP centres between 2018 and 2021 was retrospectively evaluated. Biopsies and magnetic resonance imaging (MRI) scans were centrally reviewed. The side-specific probability of EPE was calculated for each prostate side using prostate-specific antigen density, ipsilateral highest biopsy Gleason score, and ipsilateral MRI tumour stage. Model discrimination and calibration were analysed using the area under the receiver operating characteristic curve (AUC), calibration in the large, and calibration curves. The rate of side-specific EPE was 30% among 2254 prostate sides; the mean predicted rate was also 30%. The discriminatory value of the model was good, with an AUC of 80.4% (interquartile range 78.4–82.3%). The predicted probabilities matched the observed probabilities well (intercept −0.02, slope 1.053). There was slight underestimation of the observed probabilities from 70% upwards. In conclusion, an easy-to-use nomogram for side-specific EPE at RARP was externally validated and can be applied to virtually all PCa patients. Patient summary: A prediction model used to decide whether to spare the neurovascular bundles during removal of the prostate can be applied to virtually all prostate cancer patients.