Inclusion Criteria:
* Age ≥ 18 years
* Eastern Cooperative Oncology Group (ECOG) performance status 0-2
* Histologically or cytologically confirmed stage II-IVA head and neck squamous cell carcinoma (AJCC 8th edition), including oral cavity, oropharynx, hypopharynx, larynx, or cervical lymph node metastasis from unknown primary
* R0 resection and standard neck dissection per surgical evaluation
* Negative surgical margins and no extranodal extension in metastatic lymph nodes on postoperative pathology
* Presence of at least one intermediate-risk factor: close margins (\<5 mm), pT3-4, pN2, lymphovascular invasion, perineural invasion, poor differentiation, or for oral cavity cancer T2 with depth of invasion \>5 mm
* Adequate organ function (without blood transfusion or growth factors within 14 days):
* White blood cell count ≥ 2.0 × 10\^9/L
* Absolute neutrophil count ≥ 1.0 × 10\^9/L
* Platelet count ≥ 80 × 10\^9/L
* Hemoglobin ≥ 90 g/L
* Willing and able to comply with study and follow-up procedures
* Voluntary participation with written informed consent
Exclusion Criteria:
* Prior radiotherapy or surgery to the head and neck region
* Primary head and neck cancer arising from other sites such as paranasal sinuses, nasopharynx, glottic larynx, skin, or salivary glands
* Concurrent other anti-tumor therapy
* Prior neoadjuvant drug therapy
* Evidence of tumor progression on imaging within 28 days before enrollment
* History of severe infection within 1 month before enrollment, including but not limited to infection complications requiring hospitalization, bacteremia, severe pneumonia; any active infection, or unexplained fever \>38.5°C during screening or before first dose
* Severe cardiovascular disease: grade II or higher myocardial ischemia or infarction, uncontrolled arrhythmia (including QTc interval ≥450 ms for males or ≥470 ms for females); New York Heart Association (NYHA) class III-IV heart failure, or left ventricular ejection fraction (LVEF) ≤50% on echocardiography
* History of other malignancy within 5 years (except adequately treated in situ cervical cancer or basal cell/squamous cell carcinoma of the skin)
* History of psychoactive substance abuse that cannot be abstained from, or mental disorders
