The purpose of this study is to determine the efficacy and safety of pulsed radiotherapy given concomitantly with Tislelizumab and as maintenance therapy in participants with locoregionally recurrent head and neck squamous cell carcinoma ( HNSCC).

Inclusion Criteria:

• Confirmed squamous cell head and neck cancer, A karnofsky performance status≥70.
• Previous radical therapy (surgery ± chemoradiotherapy or radical chemoradiotherapy) and the records of radiation fields and dosage of previous treatment can be obtained.
• Inoperable or completely resectable under MDT consultation.
• Has evaluable tumor burden (measurable and/or non-measurable tumor lesions) assessed by computed tomography scan or magnetic resonance imaging, based on RECIST version 1.1
• Female participants of childbearing potential must have a negative urine or serum pregnancy test within 72 hours prior to receiving the first dose of study therapy
• Female and male participants of reproductive potential must agree to use adequate contraception throughout the study period and for up to 180 days after the last dose of study therapy

Exclusion Criteria:

• Pregnant or breastfeeding, or planning to become pregnant during the study period
• The patient had another malignant tumor expcet HNSCC
• Have an active autoimmune disease or immunodeficiency, including but not limited to myasthenia gravis, interstitial pneumonia, enteritis, autoimmune hepatitis, hypophysitis, vasculitis, nephritis, hyperthyroidism, test positive for HIV or have a history of any of these diseases, or have a history of organ transplantation;
• They had received reradiotherapy within 1 month prior to entering the study
• Receiving systemic immunosuppressive drugs within 2 weeks prior to commencing study treatment, or anticipating needing systemic immunosuppressive drugs during study treatment;
• having received systemic immune-stimulating agents (including but not limited to interferon or interleukin-2 [IL-2]) within 4 weeks prior to study treatment initiation or remaining within 5 half-lives (whichever is longer);
• A history of other malignancies within the past 5 years, except cured cervical carcinoma in situ, non-melanoma skin cancer, localized prostate cancer, and ductal carcinoma in situ;
• Severe cardiovascular disease (e.g., New York College of Cardiology heart disease class Ⅱ or greater, myocardial infarction, or cerebrovascular accident), unstable arrhythmia, or unstable angina within 3 months before commences study treatment;
• The subject has an active infection or infectious disease, or develops a fever of unknown origin (body temperature >38.5 ° C) during screening and before the first dose;
• had received therapeutic oral or intravenous antibiotics within 2 weeks prior to starting the study; Patients receiving prophylactic antibiotic therapy, such as prevention of urinary tract infection or chronic obstructive pulmonary disease, were enrolled.
• Untreated active hepatitis (hepatitis B: HBsAg positive with abnormal liver function and HBV-DNA≥104ml; Hepatitis C: HCV-RNA≥103/ml or antiviral therapy required during the study);
• Immunotherapy such as PD-1/L1 antibody or CTLA-4 antibody within 4 weeks before enrollment.
• Chemotherapy or targeted therapy within 4 weeks prior to enrollment;
• Subjects have participated in or completed other clinical trials within 4 weeks prior to enrollment;
• Subjects may need to receive other antitumor treatments during the study;
• Subjects may need to have been vaccinated during the study or within 4 weeks prior to enrollment.
• A patient who, in the investigator's judgment, has other comorbidities that seriously jeopardize the patient's safety or interfere with the patient's completion of the study.