3590 Background: Implantation of radioactive microspheres via the hepatic artery with Yttrium-90 (90Y) is termed “radioembolization, (RE)”. Resin microspheres were FDA cleared for use in colorectal liver metastases (mCRC) in 2002. Rapid worldwide acceptance of this therapy has resulted in greater than 30,000 procedures conducted in the past 10 years. This investigator-initiated study focuses on RE treatment outcomes in US-only patients since 2002. Methods: A retrospective multi-institutional study was designed to analyze the outcome of consecutively treated mCRC patients undergoing RE by experienced treatment centers in the USA using resin 90Y microspheres. IRB approval was obtained by each center with independent data collection and analyses. Primary endpoints: CTC 3.0ae toxicity, RECIST response and survival; baseline treatment parameters, prior chemotherapy, and liver directed therapies. Results: 506 patients at 10 institutions (193 M, 313 F) received 770 RE treatments; median age = 60.4 years (20.8 – 91.9 years); median number of RE treatments per patient = 1.0 (1-5 treatments). Active extrahepatic disease was present at first RE in 34.8% of patients. The majority (90%) of patients received prior chemotherapy, with 30.6% also having undergone prior hepatic surgery/ablative procedures. Median follow up after RE = 8.4 mo. (0.4 – 67.6 mo.) with median survival = 10.1 mo. (95% CI 9.1 – 12.0). For first RE treatment, median tumor volume was 146.0 mL (2.8 – 3228.0 mL). Median radiation activity delivered = 1.18 GBq (0.12 – 2.29 GBq), lung shunt median = 4.8 % (.02 – 45%). Total grade 1-3 events were 32% GI, 44% fatigue and 1% liver failure. Only 2.4% of all treatments required an overnight stay post-procedure. Conclusions: The modern USA experience of 90Y therapy for unresectable, heavily pretreated mCRC liver metastases is encouraging with a median survival of 10.1 months after first RE procedure. Toxicity was mild and of short duration in most patients. RECIST response is being analyzed currently as are data of additional patients.