• Untreated, histologically confirmed head and neck squamous cell carcinoma (oral cavity, oropharynx, hypopharynx or larynx), staging T3-4N0M0 or T1-4N1-3M0, III-IVB, according to the eighth edition of the AJCC staging system; within 6 weeks after receiving neoadjuvant therapy and radical surgery.

• Pathological evaluation of surgical specimens was pCR (pathologic complete response, no viable tumor cells) or MPR (Major pathologic response, residual viable tumor cells≤10%).

• Negative surgical margin.

• No extranodal extension.

• Aged ≥ 18 years and ≤ 70 years.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.

• Life expectancy of more than 6 months.

• Adequate organ function, based on meeting all of the following criteria (no blood components and cytologic growth factors were received within 14 days prior to the test):

∙ Hemoglobin ≥ 75 g/L; absolute neutrophil count ≥ 1.5 × 10\^9/L; and platelet count ≥ 100 × 10\^9/L;

‣ Serum albumin ≥ 25 g/L;

‣ Total bilirubin ≤ 1.5 × upper limit of normal (ULN); ALT and AST ≤ 2.5 × ULN;

‣ Serum creatinine ≤ 1.5 × ULN;

‣ Activated partial clotting enzyme time and international standardized ratio (INR) ≤ 1.5 × ULN (Patients on stable doses of anticoagulant therapy such as low molecular weight heparin or warfarin with INR within the expected treatment range of anticoagulants can be screened ).

• Women of childbearing age should agree to the use of contraception (e.g., intrauterine devices, birth control pills, or condoms) during treatment and for 3 months thereafter.

⁃ The regimen of neoadjuvant therapy can be determined by the clinician.

⁃ Subjects voluntarily join the study and sign an informed consent form, with good compliance.