Background: Retrospective reports of PBT in hepatocellular carcinoma (HCC) demonstrate local control (LC) rates exceeding 85%. We prospectively replicate these findings and explore predictors of overall survival (OS) in pts with unresectable HCC receiving high dose, HF-PBT. Methods: Pts were enrolled on an NCI sponsored, multi-institutional, phase II study (NCT00976898). Key eligibility were unresectable HCC; Child’s A/B; ECOG PS 0-2; no extrahepatic disease; no prior RT. Maximum tumor size was 12 cm if solitary, 10 cm if 2 tumors, and 6 cm if 3. PBT was given in 15 fractions to a maximum total dose of 67.5 GyE. Sample size was calculated to demonstrate > 80% LC at 2 yrs (LC-2). Results: From 2009-2015, 44 patients were treated. Median age was 70 years (53-89) and 37 (84.1%) were male. 35 (79.5%) pts had Child A or no cirrhosis. 32 (72.7%) pts had 1 tumor, 12 (27.2%) had multiple tumors. Median longest tumor dimension was 5.0 cm (range 1.9-12.0). Median baseline AFP was 18.6 ng/mL (range 1.3-66081) and 29 pts (67.4%) had elevated AFP ( > 7.9 ng/mL). Median RT dose delivered was 58.0 GyE (range 40.5-67.5). 1 pt (2%) had grade 3 RT related toxicity (thrombocytopenia). With a median follow up 21.8 mo among 28 survivors, the LC-2 was 94.8% (95% CI 84.5-99.1%). mOS was 49.9 mo (95% CI 17.8 months- upper limit not reached) and mPFS was 13.9 mo (95% CI 8.4-49.9). OS did not differ by CLIP score, PS, prior treatment, vascular thrombus, baseline AFP, size, or dose. Median AFP change from baseline to 3 mo post treatment was a 32.8% reduction. Median time to AFP nadir in pts with elevated baseline levels was 3.9 mo (0-30.5). % decrease in AFP from baseline to 6 mo post-treatment was significantly associated with lower hazard of death. (HR = 0.993, p = 0.016). Conclusions: High dose hypofractionated proton beam therapy demonstrated a high local control rate for HCC with favorable safety profiles, supporting the ongoing evaluation of radiation in HCC in phase III studies. AFP decrease from baseline to 6 months post-radiation is associated with improved overall survival. Clinical trial information: NCT00976898