Real-World RAIRI-Guided Risk Stratification for Adjuvant Therapy Benefit After Chemoradiotherapy in NPC

Inclusion Criteria 1. Age 18 to 75 years, male or female. 2. Histologically or cytologically confirmed EBER-positive non-keratinizing nasopharyngeal carcinoma, including differentiated or undifferentiated subtype. 3. Non-metastatic nasopharyngeal carcinoma confirmed by multimodal staging, corresponding to stage I-III according to the AJCC 9th edition staging system, or stage I-IVA according to the AJCC 8th edition staging system. 4. Eastern Cooperative Oncology Group performance status of 0-1. 5. Availability of complete pretreatment high-quality contrast-enhanced MRI of the nasopharynx and neck, including functional MRI sequences such as diffusion-weighted imaging, and at least one measurable tumor lesion according to RECIST version 1.1. 6. Availability of pretreatment quantitative plasma cfEBV DNA measurement. 7. Adequate baseline laboratory function, defined as hemoglobin \>120 g/L, white blood cell count ≥4 × 10\^9/L, platelet count ≥100 ×10\^9/L, and liver and renal function parameters, including ALT, AST, total bilirubin, and serum creatinine, within 1.25 times the upper limit of normal; no severe clinically significant hearing impairment. 8. Ability to fully understand the nature and follow-up procedures of this observational study, and voluntary provision of written informed consent by the patient or the patient's legally authorized representative. 9. Agreement to allow the research team to use the patient's clinical data, routine diagnostic imaging data, and residual biospecimens, such as peripheral blood samples and pathological slides, for scientific research analyses. 10. Adequate major organ function, except for local compression or functional impairment directly attributable to nasopharyngeal carcinoma. 11. Good expected compliance with follow-up and reliable communication conditions, allowing completion of long-term survival follow-up. Exclusion Criteria 1. Absence of pretreatment plasma cfEBV DNA measurement, or missing key baseline clinical variables required for RAIRI model calculation, such as age, AJCC stage, lactate dehydrogenase level, or central liquefactive necrosis status. 2. Presence of distant metastatic disease, M1. 3. History of previous or concurrent malignancy, except for non-melanoma skin cancer or cervical carcinoma in situ that has been successfully treated and has remained disease-free for more than 5 years. 4. Pregnant or breastfeeding women, or participants of reproductive potential who are unwilling to use effective contraception during the study observation period. 5. Current participation in another interventional clinical trial involving an investigational drug or medical device. 6. Severe or uncontrolled comorbidities, including myocardial infarction within the past 6 months, severe unstable arrhythmia, severe cerebrovascular accident, active gastrointestinal ulcer, uncontrolled psychiatric illness, uncontrolled diabetes mellitus, active autoimmune disease, long-term systemic immunosuppressive therapy, active severe infection requiring systemic anti-infective treatment, known history of human immunodeficiency virus infection, hepatitis B surface antigen positivity with HBV DNA \>1 × 10³ copies/mL or \>200 IU/mL, or hepatitis C virus antibody positivity with abnormal viral load. 7. Any personal, social, geographic, or psychiatric condition that, in the investigator's judgment, would make it impossible for the participant to complete regular follow-up visits and assessments.