This is a single-center, single-arm, open-label study that includes patients meeting the inclusion criteria (liver-GTV volume < 700ml or estimated liver-GTV V5 < 300ml) with hepatocellular carcinoma with diffuse tumor thrombosis involving both left and right lobes. All lesions receive moderate-dose hypofractionated intensity-modulated radiotherapy, with a gross tumor dose of 25Gy/5f, and a maximum dose of 35Gy/5f at the tumor center. One week before or during the radiotherapy, patients receive concurrent Tislelizumab at a dose of 200mg. Subsequently, Tislelizumab is administered intravenously every 3 weeks. Follow-up examinations are conducted 1-3 months post-radiotherapy. Lenvatinib 4mg may be used for maintenance therapy with Tislelizumab if there are no contraindications. Maintenance therapy is continued until disease progression or intolerance. The primary endpoint is median overall survival (mOS), and secondary endpoints include objective response rate (ORR), progression-free survival (PFS), and toxicity.

Inclusion Criteria:

1. Clinical or histological diagnosis of HCC with bilateral PVTT;
2. Estimated Liver-GTV volume < 700ml or the estimated volume of liver minus GTV volume receiving less than 5 Gy of irradiation < 300ml;
3. Age 18-90 years;
4. Eastern Cooperative Oncology Group (ECOG) score of 0 or 1;
5. Child-Pugh A5, A6, B7 and B8;
6. Alanine transaminase (ALT) and aspartate transaminase (AST) ≤ 2.5 times upper limit of normal (ULN); or ALT ≤ 1.5 times ULN and AST ≤ 6 times ULN; TBIL < 60umol/L.
7. Creatinine (CRE) and blood urea nitrogen (BUN) < 2.5 ULN;
8. Hb ≥ 50g/L, ANC ≥ 0.5 × 109/L, PLT ≥ 30 × 109 /L; patients with a history of gastrointestinal bleeding must be controlled for more than 2 weeks before enrollment with Hb ≥ 60g/L;

10. Voluntary to participate and sign informed consent.

Exclusion Criteria:

1. Participating in other clinical trials currently;
2. The history of abdominal irradiation;
3. The history of liver transplantation;
4. Known allergy to tislelizumab or lenvatinib;
5. Pregnant, breast feeding, or unwilling to use adequate contraception;
6. Serious myocardial disease or renal failure;
7. Other serious diseases, such as alcohol and drug abuse or mental illness;
8. Presence of other life-threatening malignancy within the last 3 years before enrollment (excluding superficial skin cancer, localized low-grade malignant tumor and carcinoma in situ).