This is a prospective, open-label, single-arm Phase II study designed to evaluate the safety of postoperative hypofractionated radiotherapy in patients with Stage II/III thymic epithelial tumors after complete resection. After a screening period of up to 28 days, eligible patients will receive adjuvant hypofractionated radiotherapy to a total dose of 40 Gy in 15 fractions, delivered once daily at 2.67 Gy per fraction, 5 days per week over 3 weeks. The study will primarily assess treatment-related safety and tolerability, with preliminary evaluation of efficacy, economic burden, and quality of life.

Inclusion Criteria:

1. Patients who voluntarily agree to participate in this study, sign the informed consent form, demonstrate good compliance, and are willing to comply with scheduled follow-up visits.
2. Age 18-75 years, male or female.
3. Histopathologically confirmed thymoma or thymic carcinoma after R0 complete resection. Eligible disease stages according to the Masaoka-Koga staging system include Stage II-III thymic carcinoma, Stage IIA Type B3 thymoma, Stage IIB Type B2/B3 thymoma, and Stage III thymoma.
4. Eastern Cooperative Oncology Group performance status of 0-1.
5. Life expectancy of at least 3 months.
6. Adequate major organ function to meet the requirements of study treatment, including bone marrow, hepatic, renal, and coagulation function according to protocol-specified criteria.
7. Patients with reproductive potential must agree to use medically acceptable contraception during study treatment and for 6 months after completion of study treatment. Female patients of childbearing potential must have a negative pregnancy test before enrollment and must not be breastfeeding.

Exclusion Criteria:

1. Patients who do not meet the protocol-specified requirements for pathological type, disease stage, or histological subtype.
2. Patients who have not undergone surgical treatment, or who underwent R1 or R2 resection, or who have definite gross residual disease.
3. Prior thoracic radiotherapy.
4. Major surgery other than the tumor resection procedure or severe trauma within 4 weeks before the first radiotherapy treatment.
5. Uncontrolled pleural effusion, pericardial effusion, or ascites requiring repeated drainage.
6. Prior or ongoing non-protocol-specified antitumor therapy, including immunotherapy, targeted therapy, radiotherapy, or other investigational treatment.
7. Active autoimmune disease or a history of severe autoimmune disease that, in the investigator's judgment, makes the patient unsuitable for radiotherapy.
8. History of immunodeficiency, including HIV positivity, history of organ transplantation, or history of allogeneic bone marrow transplantation.
9. Uncontrolled severe cardiovascular disease, such as New York Heart Association Class II or higher heart failure, unstable angina, myocardial infarction within 1 year, or severe arrhythmia requiring clinical intervention.
10. Severe infection within 4 weeks before the first radiotherapy treatment, or active pulmonary inflammation indicated by baseline imaging.
11. History of interstitial lung disease, history of non-infectious pneumonitis, or Grade 3 or higher pulmonary dysfunction confirmed by pulmonary function testing.
12. Active pulmonary tuberculosis, active hepatitis B infection, or active hepatitis C infection.
13. Diagnosis of another malignancy before the first radiotherapy treatment, except for low-risk malignancies that have been adequately treated.
14. Any other condition that, in the investigator's judgment, may affect patient safety, compliance, or the collection of study data.