External validation of the novel International Society of Urological Pathology (ISUP) Gleason grading groups in a large contemporary Canadian cohort.

Introduction: We sought to test the discriminatory ability of the 2014 International Society of Urological Pathology (ISUP) Gleason grading groups (GGG) for predicting biochemical recurrence (BCR) after robot-assisted radical prostatectomy (RARP) in a large, contemporary, Canadian cohort. Methods: A total of 621 patients who underwent RARP in two major Canadian centres were identified in a prospectively maintained Canadian database between 2006 and 2016. Followup endpoint was BCR. Log-rank test, univariable, and multivariable Cox regression analyses were used. Results: Mean followup was 27.9 months. All five ISUP GGG independently predicted BCR. Statistically significant differences in BCR rates were found between GGG 2 and GGG 3 strata (p<0.001). No statistically significant differences in BCR rates were found between GGG 4 and GGG 5 strata (p=0.3). Relative to GGG 1, the GGG 2, GGG 3, GGG 4, and GGG 5 yielded a 1.10-, 3.44-, 4.18-, and 4.74-fold hazard ratio (HR) increment in BCR, respectively. Conclusions: This population-based Canadian cohort study confirms the added discriminatory property of the novel ISUP grading, specifically for GGG 2 and GGG 3 strata. No difference, however, was observed between GGG 4 and GGG 5, likely due to the lower number of patients in these groups. As such, after external validation, the 2014 ISUP GGG appears to retain clinical prognostic significance in a Canadian population.