Efficacy and Safety of a Third-Course of Neoadjuvant Immunochemotherapy Combined With SBRT in Locally Advanced Head and Neck Squamous Cell Carcinoma Patients With Stable Disease After Two Treatment Courses: A Single-Arm Exploratory Study

Inclusion Criteria: 1. Age ≥18 and ≤75 years on the date of signing the informed consent form, male or female. 2. Histologically and imaging-confirmed T3-4a or N+M0 stage III-IVb (AJCC 8th) HNSCC, patients have received 2-courses of platinum-based chemotherapy and tirilizumab immunotherapy, and whose efficacy is assessed as Stable Disease (SD). 3. Life expectancy is at least 3 months. 4. ECOG PS 0-1。 5. Haematological analysis： 5.1 Absolute neutrophil count (ANC) ≥1.5×10\^9/L without granulocyte colony-stimulating factor in the last 14 days； 5.2 Absolute T-lymphocyte value ≥ 0.5 times the lower limit of normal value； 5.3 Platelets ≥100×10\^9/L without transfusion or platelet-boosting drugs in the last 14 days； 5.4 Haemoglobin ≥90g/L without transfusion or erythropoietin use in the last 14 days. 6. Renal function: 6.1 Creatinine clearance\* (Ccr) ≥60 mL/min; \*Ccr will be calculated using the Cockcroft-Gault formula: Ccr = (140-age) × body weight (kg) / \[0.818 (0.85 for males, 0.85 for females) × blood creatinine (SCr, umol/L) \] or Ccr = (140-age) × body weight (kg)/ \[72 × blood creatinine (SCr, mg/dL) \]; 6.2 Creatinine ≤ 1.5 × upper limit of normal (ULN)； 6.3 Routine urinalysis suggests urinary protein ≤ +; 6.4 Quantitative 24-hour urine protein \<1.0g. 7. Liver function: 7.1 Serum total bilirubin (TBil) ≤ 1.5 × ULN; 7.2 AST and ALT ≤ 2.5 × ULN, ≤ 5 × ULN for liver metastases, and TBil ≤ 3 × ULN. 8. Coagulation: international normalised ratio (INR) and activated partial thromboplastin time (APTT) ≤ 1.5 × ULN. 9. Normal thyroid function, defined as thyroid stimulating hormone (TSH) within the normal range. If baseline TSH is outside the normal range, subjects may be enrolled if total T3 (or FT3) and FT4 are within the normal range. 10. Cardiac enzyme profiles within the normal range (enrolment is also permitted if the investigator's combined judgement is that it is a purely laboratory abnormality of no clinical significance). 11. Patients must be able to understand and voluntarily sign an informed consent form. Exclusion Criteria: 1. Hypersensitivity to any of the antineoplastic therapeutic drug components of this research. 2. Those who have previously suffered from other malignant tumours and have received radiotherapy. 3. have uncontrolled clinical symptoms or cardiac disease including, but not limited to, symptomatic congestive heart failure (Grade 2 and above as determined by the New York Heart Association's Functional Class), unstable angina pectoris, acute myocardial ischaemia, and poorly controlled cardiac arrhythmias. Past history of myocarditis and cardiomyopathy. 4. Active autoimmune disease requiring systemic therapy (e.g., use of disease-mitigating drugs, glucocorticoids, or immunosuppressants). Alternative therapies (e.g., thyroxine, insulin, or physiological glucocorticoids for adrenal or pituitary insufficiency) are not considered systemic therapy. 5. History of (non-infectious) pneumonia requiring steroids or current pneumonia. 6. Have active tuberculosis. 7. History of non-infectious pneumonia requiring glucocorticoid therapy within 1 year prior to first dose or current clinically active interstitial lung disease. 8. Active or uncontrolled infection requiring systemic therapy. 9. History of human immunodeficiency virus (HIV) infection (e.g., HIV-positive). 10. Liver disease such as cirrhosis, decompensated liver disease; known active hepatitis B (e.g., hepatitis B surface antigen (HBsAg) positive and HBV-DNA \> upper limit of normal in the laboratory of the research centre) or active hepatitis C virus infection (e.g., HCV antibody positive and HCV RNA level above the lower limit of detection). \*Note: Hepatitis B subjects meeting the following criteria may also be enrolled: 10.1 HBV viral load \<1000 copies/ml (200 IU/ml) prior to the first dose, and subjects should receive anti-HBV therapy throughout the study treatment period to avoid viral reactivation; 10.2 In subjects with anti-HBc (+), HBsAg (-), anti-HBs (-) and HBV viral load (-), prophylactic anti-HBV therapy is not required, but close monitoring of viral reactivation is needed. 11. Have an active bleeding disorder or other history of severe bleeding. 12. Allogeneic organ transplantation (except corneal transplantation) or allogeneic haematopoietic stem cell transplantation. 13. Pregnant or breastfeeding, or preparing to become pregnant during the trial period. 14. Medical, psychological, or social condition that may interfere with the subject's participation in the research or affect the assessment of the results; or other conditions that, in the opinion of the investigator, make enrolment inappropriate, or that, in the opinion of the investigator, present other potential risks that make participation in this research inappropriate.