Head and neck squamous cell carcinoma (include nasopharyngeal carcinoma) is one of the most common malignant tumor in China, and radiotherapy is the main treatment method. Radiation-induced dermatitis is one of the most common complications of head and neck squamous cell carcinoma patients during radiotherapy. Severe radiation dermatitis will cause secondary infection, severe pain, and even lead to the interruption of radiotherapy. On the one hand, it will affect the efficacy of tumor treatment, which may affect the long-term survival of patients, and on the other hand, it will also seriously affect the quality of life of patients. Tetrahydrobiopterin（BH4）, also known as sapropterin, has been shown not only to reduce the severity of acute radiation dermatitis but also to enhance the repair of skin injury in animal experiments. It may be a new approach and method for the prevention and treatment of radiation-induced dermatitis

Inclusion Criteria:

1. Histologically confirmed diagnosis of head and neck squamous cell carcinoma (including nasopharyngeal carcinoma).
2. Age ≥ 18 years old, ≤ 80 years old.
3. Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
4. Head and neck radiotherapy alone or concurrent chemoradiotherapy, radiotherapy using intensity modulated radiotherapy.
5. Blood routine examination: hemoglobin ≥ 100g/L, platelet count ≥ 75×10^9/L, white blood cell count ≥ 3.0×10^9/L, absolute neutrophil count ≥ 1.5×10^9/L. Blood biochemistry: total bilirubin≤1.5 upper limit of normal (ULN), alanine aminotransferase (ALT) and aspartate aminotransferase (AST)≤2.5 ULN, serum creatinine ≤1.5 ULN or creatinine clearance rate >=60ml/min.
6. Have signed informed consent form.

Exclusion Criteria:

1. Allergic to the components of Sapropterin dihydrochloride tablets or severe allergic constitution.
2. Poor compliance.
3. Pregnant or breastfeeding.
4. Any previous radiotherapy to the head and neck region.
5. Patients deemed unsuitable for the study by the investigator (concomitant with any other serious disease)