The goal of this phase II clinical trial is to explore the efficacy and safety of anti-PD1 combined with stereotactic body radiation therapy (SBRT) for patients with oligometastatic esophageal squamous cell carcinoma. Participants will receive anti-PD1 and SBRT to the metastatic lesions which are amenable to the delivery of SBRT after 4~6 cycles of systemic chemotherapy and anti-PD-1.

Inclusion Criteria:

1. Eastern Cooperative Oncology Group performance status ≤ 2;
2. Histologically confirmed squamous cell carcinoma of the esophagus;
3. Diagnosed with stage IVB disease (according to UICC TNM version 8) with less than five metastatic lesions within three organs;
4. Received 4 to 6 cycles of standard chemotherapy (fluoropyrimidine or taxane-based platinum doublet chemotherapy) and anti-PD1 treatment, and no progression disease was confirmed;
5. At least one metastatic lesions amenable to the delivery of SBRT;
6. Estimated life expectancy >4 months;
7. The function of important organs meet the following requirements: a. white blood cell count (WBC) ≥ 4.0×109/L, absolute neutrophil count (ANC) ≥ 1.5×109/L; b. platelets ≥ 100×109/L; c. hemoglobin ≥ 9g/dL; d. serum albumin ≥ 2.8g/dL; e. total bilirubin ≤ 1.5×ULN, ALT, AST and/or AKP ≤ 2.5×ULN; f. serum creatinine ≤ 1.5×ULN or creatinine clearance rate >60 mL/min;
8. Ability to understand the study and sign informed consent.

Exclusion Criteria:

1. Progression was confirmed after completion of 4 to 6 cycles of standard chemotherapy and anti-PD1 treatment;
2. Patients with intracranial metastasis disease at diagnosis;
3. History of thoracic irradiation;
4. Known or suspected allergy or hypersensitivity to monoclonal antibodies and the chemotherapeutic drugs: Capecitabine, paclitaxel, or platinum;
5. Patients have spinal bone metastases combined with spinal cord compression;
6. A history of malignancies other than esophageal cancer before enrollment, excluding non-melanoma skin cancer, in situ cervical cancer, or cured early prostate cancer;
7. Patients who cannot tolerate radiotherapy due to severe cardiac, lung, liver, or kidney dysfunction, or hematopoietic disease or cachexia;
8. Inability to provide informed consent due to psychological, familial, social, and other factors;
9. Female patients who are pregnant or during lactation;
10. Active autoimmune diseases, a history of autoimmune diseases (including but not limited to these diseases or syndromes, such as colitis, hepatitis, hyperthyroidism), a history of immunodeficiency (including a positive HIV test result), or other acquired or congenital immunodeficiency diseases, a history of organ transplantation or allogeneic bone marrow transplantation;
11. A history of interstitial lung disease or non-infectious pneumonia; Presence of active hepatitis B (HBV DNA ≥ 2000 IU/mL or 104 copies/mL), hepatitis C (positive for hepatitis C antibody, and HCV-RNA levels higher than the lower limit of the assay).