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Prognostic factors for prostate cancer include tumor, node, metastases stage, pretreatment prostate-specific antigen, and pathology (via Gleason score [GS] or grade group). Of these, GS yields the largest effect on prostate cancer specific mortality. It was previously determined that those with cores with a mix of higher and lower GS at biopsy (which was termed a "ComboGS") had decreased risk for prostate cancer specific mortality after either surgical or radiation treatment. We validate the effect of ComboGS in an independent cohort of patients with prostate cancer treated with definitive dose-escalated radiation therapy (DE-RT) at 2 institutions.DE-RT was administered to 2539 men, of which 687 men had a ComboGS. To further ascertain the ComboGS effect we employed the modified Cancer of the Prostate Risk Assessment (mCAPRA) score. Rates of biochemical event-free survival and distant metastasis-free survival were compared across CAPRA scores, with and without modification, and the prognostic value of the CAPRA scores was compared using Harrel's concordance index.On univariate analysis in Gleason 7 to 10 patients the presence of ComboGS improved 10-year biochemical event-free survival from 76.6% to 82.4% (hazard ratio [HR], 0.75; confidence interval [CI], 0.59-0.96; P = .021), 10-year distant metastasis-free survival from 89.3% to 93.2% (HR, 0.57; CI, 0.39-0.85; P = .005), 10-year prostate cancer specific survival from 93.9% to 97.4% (HR, 0.39; CI, 0.21-0.7; P = .001), and 10-year overall survival from 65.7% to 75.6% (HR, 0.69; CI, 0.57-0.83; P.001). Multivariable analysis also supported that ComboGS is protective for biochemical failure (HR, 0.64; CI, 0.50-0.83; P.001), distant metastasis (HR, 0.42; CI, 0.28-0.63; P.001), death from prostate cancer (HR, 0.32; CI, 0.17-0.58; P.001), and overall mortality (HR, 0.65; CI, 0.54-0.79; P.001). Additionally, adjusting the mCAPRA score for ComboGS decreased the risk of biochemical failure by nearly 30% (HR, 0.70; 95% CI, 0.55-0.88; P = .003) and by 50% (HR, 0.54; 95% CI, 0.37-0.80; P = .002) for distant metastasis.ComboGS is a useful and readily available independent prognostic factor for all clinical endpoints evaluated. Moreover, the ComboGS can be used in conjunction with the extensively validated CAPRA scoring to better risk stratify patients being treated with definitive DE-RT for GS 7 to 10 disease.