• Signed Informed Consent Form

• Age ≥18 years at time of signing Informed Consent Form

• Ability to comply with the study protocol

• Newly diagnosed, biopsy-proven hepatocellular carcinoma (HCC) that is histologically or cytologically confirmed

• Availability of a representative tumor specimen that is suitable for determination of PD-L1 status via central testing. A formalin-fixed paraffin-embedded (FFPE) tumor specimen in a paraffin block (preferred) or 10-15 slides (15 slides preferred) slides containing unstained, freshly cut, serial sections should be submitted along with an associated pathology report prior to study enrollment. If archival tumor tissue is unavailable or is determined to be unsuitable for required testing, tumor tissue must be obtained from a biopsy performed at screening. Availability of a representative tumor specimen for exploratory biomarker research. Newly diagnosed, biopsy-proven hepatocellular carcinoma (HCC) either outside of the Milan Criteria (MC), or within the MC, with high risk disease as defined by alpha-fetoprotein (AFP) ≥400 ng/mL, and also fulfilling the criteria below.

∙ Within MC with AFP ≥ 400 ng/ml

‣ single lesion (≤5cm) or 3 lesions (≤3cm)

⁃ Absence of vascular invasion or extra-hepatic disease based on cross-sectional imaging

⁃ Child-Pugh Score of A/B7 (without ascites)

‣ UNOS-DS Protocol

‣ HCC exceeding UNOS T2 criteria but meeting one of the following:

• Single lesion ≤ 8 cm

∙ 2 or 3 lesions each ≤ 5 cm with the sum of the maximal tumor diameters ≤8 cm

⁃ Absence of vascular invasion or extra-hepatic disease based on cross-sectional imaging

⁃ Child-Pugh Score of A/B7 (without ascites)

‣ Beyond UNOS-DS Liver Only Protocol a. HCC exceeding UNOS-DS criteria by any of the following:

⁃ HCC tumor number

• HCC tumor size

• Total HCC tumor diameter b. Absence of vascular invasion or extra-hepatic disease based on cross-sectional imaging c. Child-Pugh Score of A/B7 (without ascites)

• Measurable disease, or non-measurable but evaluable disease, per RECIST v1.1 criteria

• Must meet institutional standards for proteinuria (Urinalysis (pH, specific gravity, glucose, protein, ketones, and blood); dipstick permitted

• Eligible for treatment with Y\^90 and atezolizumab plus bevacizumab

• ECOG performance status of 0-1

• Life expectancy \> 6 months

• Adequate hematologic and end-organ function, defined by the following laboratory test results, obtained within 14 days prior to initiation of study treatment:

‣ ANC 1.5 ≥ 10\^9/L (1500/µL) without granulocyte colony-stimulating factor support, with the following exception:

⁃ Participants with benign ethnic neutropenia (BEN): ANC \< 1.3 x 10\^9/L (1300/μL) BEN (also known as constitutional neutropenia) is an inherited cause of mild or moderate neutropenia that is not associated with any increased risk for infections or other clinical manifestations. BEN is referred to as ethnic neutropenia because of its increased prevalence in people of African descent and other specific ethnic groups.

• Lymphocyte count ≥ 0.5 x 10\^9/L (500/µL)

∙ Platelet count ≥ 100 x 10\^9/L (100,000/µL) without transfusion

∙ Hemoglobin ≥ 90 g/L (9 g/dL)

⁃ Participants may be transfused to meet this criterion.

• AST, ALT, and alkaline phosphatase (ALP) ≤ 2.5 upper limit of normal (ULN)

∙ Serum bilirubin ≤ 1.5 x ULN with the following exception:

⁃ Participants with known Gilbert disease: serum bilirubin ≤ 3 x ULN

• Serum creatinine ≤ 1.5 x ULN

∙ Serum albumin ≥ 25 g/L (2.5 g/dL)

∙ For participants not receiving therapeutic anticoagulation: INR or aPTT ≤ 1.5 x ULN

∙ For participants receiving therapeutic anticoagulation: stable anticoagulant regimen

• Absent or controlled HIV, HCV, and HBV o Participants with a positive HIV test at screening are eligible provided they are stable on anti-retroviral therapy, have a CD4 count ≥ 200/μL, and have an undetectable viral load

• Negative serum pregnancy test within 14 days prior to the initiation of study treatment for participants of childbearing potential.

• Since adequate studies have not been performed in animals to determine whether Y\^90 affects fertility in males or females has teratogenic potential or has other adverse effects on the fetus, this product should not be administered to pregnant or nursing women unless it is considered that the benefits to be gained outweigh the potential hazards. Ideally the use of this radioactive device in women of childbearing capability should be performed during the first few (approximately 10) days following the onset of menses.

• Agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods