Tislelizumab Consolidation After Liver-Directed Therapy for Hepatocellular Carcinoma

Inclusion Criteria: * Each patient eligible to participate in this study must meet all the following criteria: 1. Written informed consent 2. Primary diagnosis of HCC, planned to receive radiation, treatment naïve to systemic therapy for HCC, prior TACE permitted 3. Hepatocellular carcinoma diagnosis by histologic findings and/or imaging criteria of LI-RADS 5 4. Eastern Cooperative Oncology Group performance status score of 0-2 5. Age\>/=18 years 6. Child-Pugh class A liver function or B7, BCLC A-C or deemed not a candidate for surgery or liver transplantation 7. No extrahepatic metastasis detected on CT chest with or without IV contrast, abdomen and pelvis with IV and oral contrast (triphasic-if feasible based on kidney function), or MRI abdomen/liver and chest CT. 8. Females of childbearing potential must be willing to use a highly effective method of birth control for the duration of the study, and ≥ 6 months after the last dose of tislelizumab, and have a negative urine or serum pregnancy test ≤ 7 days of first dose of study drug 9. Non-sterile males must be willing to use a highly effective method of birth control for the duration of the study and for ≥ 6 months after the last dose of tislelizumab. Males must agree not to donate or bank sperm during treatment with tislelizumab and for \> 6 months after treatment stop. 10. Must have 1 target lesion measurable in 1 dimension according to RECIST 1.1. 11. Demonstrate adequate bone marrow and organ function as defined below: 1. Hematologic - Absolute neutrophil count (ANC) ≥ 1,500/mcL, Hemoglobin \> 8.5 g/dL, Platelet count ≥ 75,000/mcL 2. Renal - Serum creatinine OR calculated\* serum creatinine clearance (GFR can be used in place of creatinine or creatinine clearance) ≤ 1.5x upper limit of normal (ULN) OR ≥ 30 mL/min for participants with creatinine levels \> 1.5x institutional ULN * Calculate serum creatinine clearance using the standard Cockcroft-Gault formula. Urine protein Urine dipstick for proteinuria \< 2+ within 7 days prior to start of study treatment \*Participants with ≥ 2+ proteinuria on dipstick analysis at baseline should undergo a 24-hour urine collection which must demonstrate \< 1g of protein in 24 hours 3. Hepatic - Serum total bilirubin ≤ 3 mg/dL , AST (SGOT) and ALT (SGPT) ≤ 5x ULN , Alkaline phosphatase (ALP) ≤ 8x ULN Coagulation - International Normalized Ratio (INR) or prothrombin time (PT) or activated partial thromboplastin time (aPTT) ≤ 2.0x ULN \*This applies only to participants not receiving therapeutic anticoagulation; participants receiving therapeutic anticoagulation should be on a stable dose. Exclusion Criteria: <!-- --> 1. Prior radiotherapy to the region of the liver that would result in excessive doses to normal tissues due to overlap of radiation therapy fields 2. Prior selective internal radiotherapy/hepatic arterial Yttrium therapy, at any time 3. Severe, active co-morbidity as per investigator 4. More than five discrete intrahepatic parenchymal foci of definite HCC or left/right or main portal vein thrombus 5. Direct tumor extension into the stomach, duodenum, small bowel or large bowel 6. Measurable common or main branch biliary duct involvement with HCC 7. Extrahepatic metastases or malignant nodes (that enhance with typical features of HCC) \> 3.0 cm, in sum of maximal diameters (e.g. presence of one 3.4 cm metastatic lymph node or two 2 cm lung lesions). Note: benign non-enhancing periportal lymphadenopathy is not unusual in the presence of hepatitis and is permitted, even if the sum of enlarged nodes is \> 2.0 cm. 8. Prior liver transplant 9. HIV positive 10. Immunodeficiency requiring chronic systemic therapy or that may relapse 11. Participants who have received prior immunotherapy. 12. Participants with clinically meaningful ascites, defined as ascites requiring non-pharmacologic intervention (e.g. paracentesis) to maintain symptomatic control a. Note: Participants with ascites who require pharmacologic intervention (e.g. diuretics) to maintain symptomatic control and who have been on stable doses of diuretics for two months days prior to the first dose of study treatment are eligible. 13. Participants with clinically meaningful encephalopathy 14. Participants who have undergone prior solid organ or bone marrow transplant except for patients with prior renal transplant for whom dialysis may be employed in the event of graft rejection. 15. Patients must have documented hepatitis virology status. a. Participants with active hepatitis B virus (HBV) infection must have a viral load \< 500 IU/mL within 28 days prior to start of Tislelizumab and be on suppressive therapy (per local standard of care) for a minimum of fourteen days prior to start of study treatment and for the length of the study. b. Participants with co-infection with HBV and hepatitis C virus (HCV) are excluded. c. Participants with a history of HCV infection but with negative HCV RNA by PCR are considered non-infected with HCV and can enroll. 16. Participants with a history of autoimmune hypothyroidism on a stable dose of thyroid replacement hormone are eligible. 17. Participants with controlled Type 1 diabetes mellitus on a stable insulin regimen are eligible. 18. Participants with eczema, psoriasis, lichen simplex chronicus of vitiligo with dermatologic manifestations only are eligible provided: 1) rash covers \< 10% of body surface area (BSA), disease is well controlled at baseline and requires only low potency topical steroids (e.g., hydrocortisone 2.5%, hydrocortisone butyrate 0.1%, flucinolone 0.01%, desonide 0.05%, aclometasone dipropionate 0.05%). 19. Any malignancy ≤ 5 years before first dose of study drug except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated curatively (e.g. resected basal or squamous cell skin