The Combination of Immunotherapy and Stereotactic Ablative Radiotherapy in Oligometastatic Gastrointestinal Cancer

Inclusion Criteria: 1. Aged 18-70 years old, regardless of gender; 2. Fully informed and willing to provide written informed consent for the trial; 3. ECOG performance status 0-1; 4. Expected survival time ≥ 6 months; 5. Has gastric carcinoma /colorectal carcinoma / hepatocellular carcinoma, confirmed by histopathology (or pathology consultation in our hospital) and measurable oligometastatic lesions on imaging (RECIST version 1.1); pathological diagnosis confirmation of oligometastatic lesions using biopsy tissue samples (e.g. obtained by hollow core needle, biopsy, excision, etc.) is recommended but not required; 6. Has undergone curative treatment on the primary lesion at least three months ago, without local progression; 7. Has received standard treatment prior to enrolment, except for any type of immunotherapy; 8. Has no more than three metastatic lesions detected on imaging in single organ (e.g. lung, liver, brain, bone, etc.), and the total number of metastases is no more than five; 9. Multiple sites of lesions can be safely treated by SABR; and the maximum diameter of each lesion for irradiation is no more than 5cm. 10. Contraindicated for surgery or the participant refuses to receive surgery. 11. Has adequate organ function to tolerate the regimen: 12. Bone marrow function: neutrophils ≥ 1.5 × 109/L, platelets ≥ 100 × 109/L, hemoglobin ≥ 90 g/L; 13. Liver and kidney function: serum creatinine ≤ 1.5 times the upper limit of normal; AST and ALT ≤ 2.5 times the upper limit of normal or the presence of liver metastasis ≤ 5 times the upper limit of normal; total bilirubin ≤ 1.5 times the upper limit of normal, or patients with Gilbert's syndrome ≤ 2.5 times the upper limit of normal; 14. Subject is willing and able to comply with the protocol for the duration of the study including undergoing treatment and scheduled visits and examinations including follow up 15. Non-lactating patients. Exclusion Criteria: 1. Pregnant or lactating women 2. Serious medical comorbidities precluding radiotherapy 3. Prior radiotherapy to a site requiring treatment 4. Malignant pleural effusion 5. Inability to treat all sites of active disease 6. Has clinical or radiologic evidence of spinal cord compression or tumor within 3mm of spinal cord on MRI. 7. Dominant brain metastasis requiring surgical decompression 8. Has prior treatment with cancer immunotherapy including, but not limited to immune checkpoint inhibitors. 9. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy at a dose of \>10 mg Prednisone daily or equivalent at time of trial treatment. 10. Has a known history of active Bacillus Tuberculosis 11. Has active autoimmune disease that has required systemic treatment in the past 2 years 12. Hypersensitivity to PD-1 inhibitor or any of its excipients. 13. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study day 1 or who has not recovered from adverse events due to a previously administered agent.