Phase II Study of Moderate-dose Hypofractionated RT Combined With Pembrolizumab for HCC With Diffuse Tumor Thrombosis

Inclusion Criteria: 1. Confirmed clinically or histopathologically as hepatocellular carcinoma, concurrently with portal vein thrombosis or hepatic vein thrombosis; 2. Age 18-90 years; 3. Liver-GTV volume\<700ml or the estimated volume of Liver-GTV receiving less than 5 Gy of irradiation\<300ml but the average dose of Liver-GTV needs to be \<18Gy; 4. Allowed previous treatment including TACE, RFA, surgery, chemotherapy, targeted therapy, etc., but not including ICIs such as anti-PD-1, anti-PD-L1, or anti-PD-L2 therapies; 5. ECOG performance status 0-2, expected survival greater than 1 month; 6. Allowing patients with distant metastases; 7. Child-Pugh A5, A6, B7 and B8; 8. ALT within 2.5 times the normal upper limit; AST within 2.5 times the normal upper limit; TBIL \<60umol/L. 9. No significant abnormalities in the electrocardiogram, no apparent heart failure, and no contraindications for anti-PD-1 treatment; 10. CRE, BUN within 2.5 times the normal upper limit; 11. Hb ≥ 50g/L, ANC ≥ 0.5 × 10\^9 /L, PLT ≥ 30 × 10\^9 /L; patients with a history of gastrointestinal bleeding must be controlled for more than 2 weeks before enrollment with Hb ≥ 60g/L and a significant rising trend; 12. Patients voluntarily participate in this clinical trial and sign an informed consent form. Exclusion Criteria: 1. Currently participating in other clinical trials; 2. Previously received abdominal radiotherapy or liver transplantation; 3. Individuals with severe chronic disease conditions affecting vital organs such as the heart, kidneys, or liver; 4. Severe ascites with noticeable symptoms, anticipated to be unrelieved after treatment. 5. Suspected or confirmed drug addiction, medicine abuse,or alcoholism 6. Pregnant or lactating women; 7. Severe mental or neurological disorders 8. Presence of other life-threatening malignancy within the last 3 years before the start of the study (excluding superficial skin cancer, localized low-grade malignant tumor and in situ carcinoma).