Induction Chemoimmunotherapy Followed by Deescalated Definitive Radiotherapy in Head and Neck Cancer

Inclusion Criteria: 1. Patients with head and neck squamous cell carcinoma (HNSCC) who have completed 2-3 cycles of induction chemotherapy plus PD-1 inhibitor and achieved deep response. 2. Within 1 month of achieving deep response, subsequent concurrent chemoradiotherapy is determined by integrating MDT recommendation and patient's discretion. 2\. Age 18 to 75 years (including 75). 3. Histopathologically confirmed HNSCC (excluding nasopharyngeal carcinoma). 4. Clinical stage T1-2N2-3M0 or T3-4N0-3M0 (AJCC 8th edition). 5. HPV or P16 negative. 6. ECOG performance status 0 or 1. 7. No contraindications to immunotherapy or radiotherapy. 8. Adequate organ function as defined by: * WBC ≥ 3.0×10\^9/L, ANC ≥ 2.0×10\^9/L, PLT ≥ 100×10\^9/L, HGB ≥ 90 g/L (no transfusion or G-CSF within 14 days). * TBIL ≤ 2.0×ULN, ALT/AST ≤ 2.5×ULN, BUN/CRE ≤ 1.5×ULN or creatinine clearance ≥ 60 mL/min. * INR or PT ≤ 1.5×ULN (or within therapeutic range for anticoagulation). * Myocardial enzymes within normal limits. 9. For women of childbearing potential: negative pregnancy test within 7 days prior to enrollment and use of effective contraception during treatment and for 2 months after last dose. Male participants must agree to use effective contraception for the same period. 10\. Willing to sign informed consent and comply with follow-up. Exclusion Criteria: 1. Occurrence of ≥ G4 toxicities during induction therapy and not suitable to receiving subsequent concurrent chemoradiotherapy or immunotherapy. 2. History of another active malignancy within 5 years, except cured basal cell carcinoma of skin, cervical carcinoma in situ, papillary thyroid carcinoma, or superficial bladder cancer. 3. Active or history of autoimmune disease (e.g., interstitial pneumonia, uveitis, colitis, hepatitis, hypophysitis, vasculitis, myocarditis, nephritis, hyperthyroidism/hypothyroidism requiring hormone replacement). Exceptions: vitiligo, childhood asthma fully resolved without intervention; asthma requiring bronchodilator medical intervention is excluded. 4. Uncontrolled cardiovascular disease: myocardial ischemia grade II or higher, myocardial infarction, poorly controlled arrhythmia (including QTc ≥ 470 ms), NYHA class III-IV heart failure, LVEF \< 50%, or myocardial infarction within 1 year. 5. Active infection or unexplained fever \>38.5°C during screening (tumor fever allowed if judged by investigator). 6. Congenital or acquired immunodeficiency (e.g., HIV infection), active hepatitis B (HBV-DNA ≥ 10\^4 copies/mL), or active hepatitis C (HCV-RNA above lower limit of detection). 7. Prior treatment with any PD-1/PD-L1 inhibitor. 8. Allergy to cisplatin, macromolecular protein preparations, or any component of anti-PD-1 antibodies. 9. Major surgery for non-tumor reasons without full recovery of toxicity/complications before starting study treatment. 10. Pregnancy or breastfeeding. 11. Any other condition that, in the investigator's judgment, would interfere with study participation or data collection (e.g., severe psychiatric illness, abnormal laboratory values, family or social factors).