This clinical trial aims to evaluate whether oral supplementation with a hydrolyzed whey protein formula ("TeYiShu") can reduce the incidence of severe radiation-induced oral mucositis and improve prealbumin levels in patients undergoing radiotherapy for head and neck squamous cell carcinoma (HNSCC). The study will compare the protein supplement to an isocaloric placebo without protein, maintaining equal caloric intake across groups. Participants will receive either the protein supplement or placebo daily throughout the course of radiotherapy. The primary outcomes are the incidence of grade ≥3 oral mucositis, assessed using the Radiation Therapy Oncology Group (RTOG) Acute Radiation Morbidity Scoring Criteria, and changes in serum prealbumin levels. Secondary outcomes include the time to onset and resolution of mucositis, nutritional status changes, adverse events, and quality of life measured using the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30) and the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire-Head and Neck Cancer Module (EORTC QLQ-H\&N35).