Cemiplimab +/- Fianlimab Post Y90 Radioembolization in Patients With Hepatocellular Carcinoma

At 2-8 weeks after yttrium-90 (Y90) radioembolization, patients will be randomized to receive treatment with cemiplimab or with a fixed-dose combination of cemiplimab and fianlimab. If Y90 is administered in 2 treatments, the first dose of the study treatment must be given no more than 12 weeks after first Y90 treatment and at least 2 weeks after the second treatment. Patients will receive the assigned study treatment once per 3-week cycle and will continue treatment for up to 17 cycles (approximately 1 year) or until disease progression or intolerance.