Background: To compare the outcomes of a modern cohort of patients with Gleason Score (GS) 9-10 prostate adenocarcinoma (CaP) following treatment with external beam radiotherapy (EBRT), extremely dose-escalated radiotherapy (as exemplified by EBRT with a brachytherapy boost [EBRT+BT]), and radical prostatectomy (RP). Methods: One-thousand-and-one patients with biopsy GS 9-10 CaP who received definitive treatment between 2000 and 2013 were included (347 treated with EBRT, 330 with EBRT+BT, and 324 with RP). Kaplan-Meier analysis and multivariate Cox regression compared 5- and 10-year rates of distant metastasis-free survival (DMFS), cancer-specific survival (CSS), and overall survival (OS). Prostate cancer-mortality (PCSM) rates were compared with a competing risk analysis. Results: The median followup periods were 4.8, 6.4, and 5.1 years among patients receiving EBRT, EBRT + BT, and RP. The median doses among EBRT and EBRT+BT patients were equivalent to 78 Gy and 90 Gy in 2 Gy fractions. Over 90% of patients treated with EBRT or EBRT+BT received ADT (median durations of 18 months and 12 months, respectively). Nearly 40% of RP patients received postoperative RT, primarily in the salvage setting. Five- and 10-year DMFS rates were significantly higher with EBRT+BT (91.6% and 81.3%) than with EBRT (79.6% and 65.8%; p < 0.0001) or RP (77.9% and 60.1%; p < 0.0001). Five- and 10-year PCSM rates were significantly lower with EBRT+BT (3.8% and 14.1%) than with EBRT (10.3% and 25.2%; 5- and 10-year hazard ratios of 0.38 and 0.47; p = 0.003) or RP (8.9% and 20.3%; 5- and 10-year hazard ratios of 0.39 and 0.55; p = 0.02). Overall 5- and 10-year OS rates were 85.7% and 64.7% and were similar between cohorts (p > 0.1). Conclusions: Extremely dose-escalated radiotherapy offered improved systemic control and reduced PCSM when compared with either EBRT or RP. Notably, this was achieved despite a significantly shorter median duration of ADT than in the EBRT arm. This is hypothesis generating as it suggests that improved local control via dose-escalation may have systemic control and survival implications even for patients with very high risk disease.