Phase III Non-Inferiority Trial: Reduced-Target vs. Full-Target IMRT After Chemo in Immunotherapy-Treated Metastatic Nasopharyngeal Cancer

Inclusion Criteria: * Age: 18 years - 70 years. * The patient has signed the informed consent form and is willing and able to follow the visit schedule, treatment plan, laboratory tests and other research procedures as per the research plan. * Newly diagnosed distant metastasis patients who achieved complete response (CR) or partial response (PR) after receiving adequate first-line systematic therapy (platinum-containing doublet chemotherapy + PD-1 monoclonal antibody). * Nasopharyngeal carcinoma non-keratinizing carcinoma (differentiated or undifferentiated type, i.e., WHO type II or III), confirmed by histological and/or cytological examination, with metastatic lesions detected on imaging (biopsy of metastatic tissue is preferred but not mandatory). * Clinical stage: TanyNanyM1, stage IVB (AJCC 9th edition). * ECOG score: 0-1. * Female subjects with reproductive capacity must have a negative urine or serum pregnancy test within 7 days prior to enrollment, and must agree to take effective contraceptive measures during the study. * For male subjects, if the female partner still has reproductive capacity, the male subject must agree to take effective contraceptive measures during the study. Exclusion Criteria: * Patients with malignant pleural effusion or those with other malignant tumors * Patients who have received ≥2 prior lines of systemic therapy. * Patients with known or suspected autoimmune diseases, including dementia and epileptic seizures. * Patients with grade ≥ II coronary heart disease, arrhythmia (including QTc interval prolongation in males \> 450 ms, females \> 470 ms) and heart failure. * Patients who received systemic or local glucocorticoid treatment within 4 weeks prior to enrollment. * Patients with comorbidities requiring long-term use of immunosuppressive drugs or requiring systemic or local use of corticosteroids at immunosuppressive doses. * Patients with active pulmonary tuberculosis (TB), who are undergoing anti-TB treatment or have received anti-TB treatment within 1 year prior to screening. * HIV-positive individuals; HBsAg positive and HBV DNA copy number positive (quantitative detection ≥ 1000 cps/ml); chronic hepatitis C blood screening positive (HCV antibody positive). * Patients who received any anti-infective vaccine (such as influenza vaccine, varicella vaccine, etc.) within 4 weeks prior to enrollment. * At the time of randomization, the expected lifespan of the patients was less than 6 months. * Other patients deemed unsuitable for inclusion by the treating physicians.