Index grade group is superior to composite grade group for prediction of biochemical recurrence following radical prostatectomy

The pathological grade of prostate cancer is the strongest predictor of recurrence. It is unclear whether the better predictor is the composite of all carcinomas within the prostate, or the highest grade lesion (index). The purpose of this study was to determine whether composite or index grade group better predicts biochemical recurrence (BCR). We undertook a retrospective analysis from a prospective institutional cohort study of men who underwent radical prostatectomy for localised prostate cancer between 2009 and 2020, in which an index and composite grade group was reported. The index grade in this study was defined as the highest grade of any tumour, usually with the highest stage, regardless of volume. Multivariate analysis and Kaplan-Meier plots were utilised.A total of 2024 men underwent radical prostatectomy during the study period; we analysed 1605 with composite grade group 2 or 3 prostate cancer. Median preoperative prostate specific antigen (PSA) was 5.9 ng/L, mean follow up was 56.8 months, 54% were pT2, 76% had multifocal disease and 16% had discordant index and composite grades.Patients with discordant index grade group had a higher risk of BCR [hazard ratio (HR) 2.22, p<0.0001]. The prevalence of BCR in the discordant group was higher at 1, 3, 5 and 7 years (4.7% vs 8.9%, 8.3% vs 18.1%, 14.5% vs 28.8% and 22.5% vs 49.5%, respectively). In cases of discordance, a higher index grade group is associated with increased rates of BCR after radical prostatectomy. Index rather than composite grade group should be used to counsel men post-operatively regarding prognosis and follow-up.