The purpose of this study is to determine whether stereotactic body radiotherapy (SBRT) combined with recombined human granulocyte-macrophage colony stimulating factor(rhGM-CSF) and Peginterferon alfa-2b is safe, effective in the treatment of patients with metastatic thymic epithelial tumors.

Inclusion Criteria:

1. Age ≥18 years;
2. Histologically proven Thymic epithelial tumors；
3. Stage IV according to UICC stage system(version 8,2017), at least with two evaluable abscopal lesions (≥1cm)（excluding intestinal metastasis）;
4. Evaluated as effective or stable disease after first-line chemotherapy or progressed after second-line chemotherapy (progression sites were no more than three sites);
5. ECOG performance status: 0-1;
6. Life expectancy ≥ 3 months.
7. Adequate baseline organ and marrow function: absolute neutrophil count greater than 1500 cells per μL, platelet concentration of greater than 50 000 per μL, total bilirubin less than 1•5 times the upper limit of normal (ULN), aspartate aminotransferase and alanine aminotransferase less than 2•5 times the ULN, and serum creatinine less than 1•5 times the ULN;
8. Female subjects have a negative urine or serum pregnancy test within 1 week prior to treatment if of childbearing potential;
9. Asymptomatic subjects with brain metastasis can be included, but the sites of brain cannot be considered as target sites;
10. Asymptomatic subjects with bone metastasis can be included, but the sites of bone cannot be considered as target sites.

Exclusion Criteria:

1. Having received immunotherapy within 4 weeks prior to inclusion;
2. Allergic to GM-CSF, INF-α2b or diagnosed with immune system disease, receiving immunosuppressant, such as prednisone, dexamethasone, methylprednisolone, methotrexate, hydroxychloroquine, cyclophosphamide, azathioprine and so one;
3. receiving treatment of other trials;
4. Any unstable systemic disease, including active infection, symptomatic congestive heart failure,myocardial infarction onset six months before included into the group, unstable angina, and severe arrhythmia, uncontrolled chronic lung disease;
5. unwilling to sign consent;
6. Women in pregnancy or lactation;
7. Other malignancy except for non-melanoma carcinoma of the skin or in situ carcinoma of the cervix.