Inclusion Criteria:
* Aged 18 to 65 years.
* ECOG performance status of 0-1.
* Newly diagnosed nasopharyngeal carcinoma confirmed by pathological examination of the primary lesion.
* Staging is classified as T4N1M0 or T1-4N2-3M0 according to the 9th Edition of the American Joint Committee on Cancer/Union for International Cancer Control (AJCC/UICC) staging system.
* Must have received recommended radical radiochemotherapy and sintilimab: 3 cycles of gemcitabine combined with cisplatin and Sintilimab; radical intensity-modulated radiotherapy with a total dose of 6,996 cGy over 33 fractions, concurrently receiving 2 cycles of cisplatin at 100 mg/m².
* Completed the last radiation treatment within 2 weeks before randomization.
* neutrophil count ≥ 1.5 × 10⁹/L, hemoglobin concentration ≥ 90g/L, platelet count ≥ 100 × 10⁹/L.
* Total bilirubin ≤ 1.5 times the upper limit of normal; AST and/or ALT ≤ 2.5 times the upper limit of normal; creatinine clearance ≥ 50 mL/min.
* Participants must sign an informed consent form and be willing and able to comply with the study's visit, treatment regimen, laboratory tests, and other requirements.
* Female participants of childbearing potential and male participants with fertile partners must agree to use reliable contraception (such as condoms, or physician-recommended contraceptive pills) from screening to one year post-treatment.
Exclusion Criteria:
* Active, known, or suspected autoimmune diseases. Participants with Type I diabetes, hypothyroidism requiring only hormone replacement therapy, or skin diseases (e.g., vitiligo, psoriasis, or alopecia) that do not require systemic treatment can be included.
* Severe immune-related adverse events that occurred during the initial Sintilimab induction treatment, as judged by the investigator, preventing continued adjuvant treatment with Sintilimab.
* Received any non-diagnostic surgeries or other targeted, immune, or biological therapies for nasopharyngeal carcinoma except the recommended radiochemotherapy and Sintilimab treatment before randomization.
* factors affecting oral medication (e.g., swallowing difficulties, chronic diarrhea, bowel obstruction).
* Disease progression post-radical radiotherapy.
* Positive HBsAg with HBV DNA \> 200 IU/ml or 1000 copies/ml.
* Positive HCV antibody.
* Received systemic corticosteroids or other immunosuppressive therapy at equivalent doses \> 10 mg of prednisone/day within 28 days before informed consent. Patients on systemic corticosteroids ≤ 10 mg of prednisone/day or inhaled/topical corticosteroids can be included. Using corticosteroids to manage or prevent treatment-related toxicity is permissible for inclusion.
* Active tuberculosis. Patients with active tuberculosis within the past year are excluded, even if treated; patients with a history of active tuberculosis over one year ago are also excluded unless they can prove they have undergone standard anti-tuberculosis therapy.
* History of interstitial lung disease.
* Received a live vaccine within 30 days before informed consent or is scheduled to receive one.
* HIV infection.
* Current or previous malignancies, except for adequately treated non-melanoma skin cancer, cervical carcinoma in situ, or papillary thyroid carcinoma.
* Pregnant or breastfeeding women; pregnancy tests should be considered for sexually active women of childbearing potential.
* Other diseases or conditions assessed by the investigator that may jeopardize patient safety or compliance, such as unstable cardiac diseases, renal diseases, chronic hepatitis, poorly controlled diabetes, psychiatric disorders, severely abnormal laboratory results, or other family or social high-risk factors.
