Atezolizumab and Bevacizumab With Proton Radiotherapy for Unresectable Hepatocellular Carcinoma

Inclusion Criteria: * Participants must have diagnosis of HCC that is deemed unsuitable for surgical resection or transplant. Participants may have multiple lesions with a total maximal tumor dimension of \< 20 cm, and no one lesion \> 15 cm. Diagnosis should be confirmed by at least 1 criterion listed below: * Histologically or cytologically proven diagnosis of HCC. * Typical arterial enhancement and delayed washout on multiphasic CT or MRI. * Age ≥18 years at the time of signing informed consent document. * ECOG performance status 0-1. * Barcelona Clinic Liver Cancer (BCLC) stages Intermediate (B) or Advanced (C). * Child-Pugh score 5-6 liver function within 28 days of study registration. * Documented virology status of hepatitis B virus (HBV), as confirmed by screening HBV serology test. * Documented virology status of hepatitis C virus (HCV), as confirmed by screening HCV serology test. * Ability to understand and the willingness to sign a written informed consent document * Adequate bone marrow, liver, and renal function within 4 weeks before study registration * Hemoglobin ≥ 9.0 g/dL * Absolute neutrophil count (ANC) ≥ 1,000/mm3 * Platelet count ≥ 50,000/μL * Total bilirubin \< 2.5 mg/dL * Serum albumin \>2.8 g/dL * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3 × upper limit of normal (ULN) * Prothrombin time ≤ 6 seconds prolonged * Serum creatinine ≤ 1.5 mg/dL Exclusion Criteria: * Prior invasive malignancy unless disease free for a minimum of 2 years * Prior radiotherapy to the region of the liver that would result in overlap of radiation therapy fields * Prior selective internal radiotherapy/hepatic arterial yttrium therapy, at any time * Untreated active hepatitis B or hepatitis C * Moderate to severe or intractable ascites * Presence of distant metastases that cannot be encompassed by proton radiotherapy * Untreated or incomplete treated esophageal or gastric varices * Severe, active co-morbidity, defined as follows: * Unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months prior to registration * Myocardial infarction within the last 6 months prior to study entry * Acute bacterial or fungal infection requiring intravenous antibiotics within 28 days prior to study entry * A bleeding episode within 6 months prior to study entry due to any cause. * Thrombolytic therapy within 28 days prior to study entry. * Known bleeding or clotting disorder. * Uncontrolled psychotic disorder * Pregnancy or women of childbearing potential and men who are sexually active and not willing/able to use medically acceptable forms of contraception * Prior solid organ transplantation. * Prior or active autoimmune disease (AID) including autoimmune hepatitis, inflammatory bowel disease, myasthenia gravis, systemic lupus erythematosus, rheumatoid arthritis, antiphospholipid antibody syndrome, Wegener granulomatosis, Sjogren's syndrome, Guillain-Barre syndrome, and multiple sclerosis. * Prior or active thrombotic or bleeding disorders, hemoptysis, cerebral vascular accident, significant cardiac disease (ischemic or congestive heart failure), or gastrointestinal perforation. * Inability to treat all sites of disease by proton radiotherapy (such as extrahepatic metastases or massive liver tumors whereby the liver constraints \[ULV/SLV \>40%\] cannot be met for covering all sites of liver tumors using proton radiotherapy.) * Known HIV infection.